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You may look at that person up above and think one of two things:
1. “Whoa, he’s huge.”
2. “Meh, other wrestlers are better.”
Either way you take it, the Great Khali (pictured above), a fairly well known wrestler, is only this huge because of a specific medical condition that he has: Acromegaly.
And that is what we would talk about today.
Acromegaly is directly a result of excessive Growth Hormone production by the body. It’s literally in the name right? Growth Hormone = Excessive Growth. And that is essentially what Acromegaly is.
Let’s start by reviewing how Growth Hormone is usually regulated:
Regulation of Growth Hormone
This is an excellent depiction of the simple way in which Growth Hormone is usually regulated. Growth Hormone is classically released by two different mechanisms:
1) Growth Hormone Releasing Hormone (GHRH)
2) Ghrelin
These are the two hormones that stimulate the production of Growth Hormone. GHRH is the far more classical of the two, and is a hormone that is released by the hypothalamus that directly stimulates the anterior pituitary to secrete Growth Hormone. GHRH is known to be released in a pulsatile manner - and thus Growth Hormone is released in a pulsatile manner. Take a look at the diagram below:
You can appreciate that the GH release is quite pulsatile indeed, and is highest between 1AM and 3AM in the night. This is reflective of the fact that GHRH is also released in a pulsatile manner.
The second hormone, Ghrelin is less understood (but honestly GHRH isn’t very well understood either). Ghrelin is released whenever the stomach is empty. Yep, it’s the hunger hormone. Somehow, this hunger hormone activates Growth Hormone release when it is very high. This is because Ghrelin is signalling that Growth Hormone release is necessary to allow the use of energy since the body is not achieving much energy from the food products it is consuming.
And this brings us to the functions of Growth Hormone:
Functions of Growth Hormone
The effects of growth hormone on the body can generally be described in one word: anabolic (building up). Some of its effects are catabolic, but the more important effects are generally anabolic.
It exerts its effects through two major mechanisms:
It promotes the activity of a pathway known as the MAPK/ERP Pathway, which causes chondrocyte proliferation and thus growth of cartilage.
It causes the release of this very important compound, IGF-1 (insulin like growth factor 1), that basically acts as a stimulant for everything, causing growth of every receptor it gets into contact with.
Other than this, the major catabolic effect of Growth Hormone is promoting lipolysis - hence the reason Ghrelin would activate GH. So that the body can use the free fatty acids for energy in a time when the body is starving.
Also note that you can get an increased blood glucose in the body because Growth Hormone inhibits the body from using up the glucose stores.
All in all, the functions can be summarised as:
Great, now let’s get into Acromegaly, shall we?
What is Acromegaly?
Acromegaly refers to a condition that occurs when there is excessive production of Growth Hormone after the closure of the epiphyseal plates. This means that Acromegaly CANNOT happen in children since their epiphyseal plates are still open. In children, excess Growth Hormone results in a condition called Gigantism (which is pretty much the same thing as Acromegaly, but a little different), that we will discuss at another point in time.
So what could the most common cause of excess Growth Hormone be?
Easy.
A Growth Hormone producing adenoma accounts for 98% of all cases of Growth Hormone excess. And the most common place this adenoma occurs is the Pituitary Gland - hence 98% of causes of Acromegaly can be accounted for by a Growth Hormone Producing Pituitary Adenoma (also called a Somatotrope Adenoma). This type of adenoma accounts for 10% of all pituitary adenomas.
The majority of these somatotrope adenomas produce growth hormone alone, but you should also remember that some (about 25% of all somatotrope adenomas also produce excess prolactin - and so you get symptoms of Acromegaly AND Hyperprolactinemia.)
The MRI below shows a Pituitary Adenoma, that when combined with a history of Acromegaly is pretty diagnostic for a Pituitary Adenoma.
But other causes DO exist, albeit VERY rare. You can probably think of one based on what I gave you above.
A GHRH producing tumour of the hypothalamus. I just bolded this, but honestly you probably would never see this unless you’re a superspecialist or something of the sort because it is pretty rare (like less than 1% kind of rare). They happen very rarely as a result of things like a hypothalamic hamartoma and pancreatic islet cell tumours, but don’t count on it.
Clinical Features of Acromegaly
So really and truly, we can say that Acromegaly exerts 3 major categories of clinical features.
Growth Related
Since acromegaly causes excess production of IGF-1 which literally causes the growth of a number of tissues.
The growth related effects can also now be divided into three further categories for even further simplicifcation:
Hard Tissue Growth
Arthralgia due to joint tissue overgrowth
Enlarged jaw (macrognathia)
Frontal Bossing
Widened space between lower incisor teeth
Increased hand and foot size.
Soft Tissue Growth
Increased heel pad thickness (and thus an increasing shoe size)
Increased soft tissue growth of tissues of the fingers (and hence rings feel tighter)
Coarsening of facial features.
Skin tags
Laryngeal Muscle Enlargement (which can result in sleep apnea and a hollow quality voice.)
Carpal Tunnel Syndrome.
Visceromegaly (Organomegaly)
Macroglossia
Hypertrophic Cardiomyopathy
This is the most common cause of death in patients with Acromegaly.
Fatal arrythmias and diastolic dysfunction leading to heart failure eventually develop.
Hypertension also occurs due to the increase in resistance virtually in every organ and vessel since everything is becoming thicker.
Thyromegaly
Colonic Polyps
It is for this reason that Growth hormone is associated with an increased risk of colon malignancy.
Hepatosplenomegaly (But this is less significant than the above 4).
Metabolic
We already know that growth hormone causes an increase in blood glucose, and an increase in lipolysis. Surely this would have some sort of metabolic effects, no? Let’s look at them:
Increased blood glucose level leading to diabetes mellitus.
25% of Acromegaly patients actually develop Diabetes Mellitus, and this is because remember, Growth Hormone is a counter-regulatory hormone that limits the effects of insulin and causes an increase in blood glucose - this is essentially insulin resistance!
Hyperhidrosis (excessive sweating)
Due to excessive growth and stimulation of the sweat glands.
Acanthosis Nigricans (Think Diabetes Mellitus)
Mass Effect
We cannot forget that Acromegaly is mainly caused by a pituitary mass, and this mass would of course invade into surrounding tissues and cause damage. This is known as parasellar invasion.
Headache
Superior Spread leads to compression of the Optic Chiasm leading to bitemporal hemianopsia.
Inferior Spread leads to sphenoid sinus invasion.
Lateral Spread leads to Cavernous Sinus invasion.
Despite all this, the clinical features of acromegaly are quite indolent, and will usually only appear about 10 years after the initial insult. Thus, in a case where a patient had Acromegaly AND Hyperprolactinemia due to a Somatotrope Adenoma that produces both, the symptoms of hyperprolactinemia would usually appear first and be much more prominent.
Diagnosis of Acromegaly
So let’s think about how we can diagnose Acromegaly. Let’s look at a little diagram:
(You may notice something there called somatostatin - I’ll get into that in a bit).
So seeing how GH works, let’s try to brainstorm a few methods of checking for Growth Hormone excess.
The most obvious is measuring growth hormone right? The problem is that Growth Hormone fluctuates throughout the day, so if we were to measure growth hormone we would have to make a standard of the normal growth hormone level for every second of the day, and even then, some people have different patterns. This also applies to Growth Hormone Releasing Hormone, which is also a fluctuating pattern. This means we cannot use these.
Hence, the best test to screen for acromegaly is serum age matched IGF-1. IGF-1 does not vary throughout the day, because when it is produced by growth hormone, the fluctuating effect is negated by the fact that IGF-1 has a long half-life and thus would remain in the system. Furthermore, IGF-1 is produced exclusively by Growth Hormone, and thus is very accurate in representing the effect of Growth Hormone.
But serum IGF-1 is still not sufficient enough to fully diagnose Acromegaly - however it is a very useful screening test.
The diagnosis of Acromegaly is made using a OGTT (Oral Glucose Tolerance Test). Let me explain. Remember that Growth Hormone is one of the counter-regulatory hormones. So when a person consumes a large amount of glucose, the body would want to store away that glucose using insulin, and thus would decrease Growth Hormone production, since Growth Hormone directly acts against insulin. In Acromegaly however, the GH is being produced so constantly that the body is not able to shut down it’s production when it is given a glucose load. So this means that in acromegaly, the growth hormone levels fail to decrease to a value that <0.4ug/L after being given a 75g dose of glucose. This means that the value of GH remains elevated >0.4ug/L after being given the 75g glucose.
After you have established that there is indeed acromegaly somewhere, you have to find out where is the acromegaly coming from? Naturally, since the most common cause is a pituitary adenoma, the next step after this is a pituitary MRI to confirm the GH producing pituitary adenoma.
This is quite unlikely, but if the MRI is normal, then you can do an abdominal and chest CT to examine for extra-pituitary sources of acromegaly, or do a GHRH level to examine for a hypothalamic etiology.
Do not forget that it may also be relevant to perform tests to monitor organs once the diagnosis of Acromegaly is made. These include little tests like Thyroid Function Tests for thyromegaly, Echocardiograms for the Hypertrophic Cardiomyopathy, Endoscopy for colonic polyps and HbA1c for monitoring Diabetes Mellitus (among many others).
Other laboratory abnormalities in acromegaly include:
Elevated triglycerides and free fatty acids
Due to increased lipolysis
Elevated prolactin (if the adenoma also produces prolactin).
Hyperphosphatemia
Treatment of Acromegaly
Now let’s think back about how we can treat acromegaly. The very first thought we can have is that acromegaly is produced as a result of (98% of the time) a pituitary growth hormone producing adenoma. The easiest solution?
But no, seriously speaking, transphenoidal resection of the pituitary adenoma is the first line treatment almost all cases of acromegaly.
The other option to think about is somehow inhibiting Growth Hormone Production. Thankfully, the body has a hormone that can do this. Remember how I mentioned something known as somatostatin above? Yep, somatostatin inhibits the production of Growth Hormone. This means that another viable alternative is to use somatostatin analogues, such as octreotide.
Other second line options include Growth Hormone Releasing Hormone Antagonists (such as pegvisomant) which would inhibit binding of the growth hormone to its receptor, and high dose dopamine agonists which are a last resort for reducing growth hormone production (such as bromocriptine and cabergoline -which are incidently the first choice method in treating a prolactinoma).
Radiation therapy is also an option, but these really are last line options that should come after Surgery and Octreotide.
That’s it for Acromegaly guys! Congrats on making it to the end! If there are any questions you can always PM me and I’ll reply.
Videos:
https://www.youtube.com/watch?v=1MJTkz02SaM
Questions:
1. A 23 year old man complains of a persistent headache. He has noticed gradual increase in his ring size and shoe size over the years. On physical examination he has a deep, hollow-sounding voice and a prognathic jaw. Visual studies suggest bitemporal hemianopsia. What initial study is indicated?
A. Serum IGF-1 and serum Prolactin
B. Morning Growth Hormone Levels
C. Dexamethasone Suppresion Test
D. Pituitary MRI
E. Oral Glucose Tolerance Test
2. A 24 year old female with an elevated IGF-1 and positive glucose tolerance test was studied using a pituitary MRI. However, the MRI revealed no adenoma. What is the next step?
A. Discharge the patient. She does not have acromegaly.
B. Counsel the patient on the normalcy of her physiological variant.
Inclusion bodies are those things you look at in cells and think, “Why are you even here?” Inclusion bodies are those things that just appear in cells and.. don’t really do anything. They’re just included within the cell, for some purpose.
Typically, inclusion bodies are nuclear or cytoplasmic aggregates of stainable substance, usually proteins.
The inclusion bodies in red blood cells are almost always indicative of some sort of pathology, and thus it is useful to understand each inclusion body that can occur within a red blood cell.
This diagram is a pretty comprehensive summary of most of (not all) the inclusion bodies we will come across. But as usual, let us go through each in detail:
Inclusion Bodies Within Red Blood Cells:
Howell-Jolly Body
Heinz Body
Hemoglobin H Inclusion
Pappenhemier Body
Basophilic Stippling
Cabot Ring
Howell-Jolly Body
Howell-Jolly bodies are histopathological findings on a blood smear. They appear as a very dark purple (described as a basophilic area) spot within the cytoplasm of the red blood cell. They are in fact, basophilic nuclear remnants, or remnants of DNA.
Typically, they are round to oval shaped, 1um across and are usually restricted to only 1 per cell, although there can be multiple amounts.
Typically, during the maturation of a red blood cell in the bone marrow, the nucleus must be expelled alongside all nuclear matter in order to become a mature red blood cell. However, in rare occasions, the nucleus is not appropriately removed and fragments of the DNA remain as these basophilic dark spots, and are called Howell-Jolly bodies. They are usually produced physiologically, but are quickly and easily removed by the spleen, that identifies the Howell-Jolly body and identifies the cell as defective.
Then, if Howell-Jolly bodies are present in high amounts, it must indicate that something is wrong with the spleen. If the spleen is malfunctioning in some way, then Howell-Jolly bodies are not removed and they remain in circulation.
Diseases Associated with Howell-Jolly Bodies:
Splenectomy
A splenectomy would leave an individual with no spleen to remove malformed red blood cells. Thus, even if red blood cells have Howell-Jolly bodies within their cytoplasm, the absence of a spleen means that these inclusion bodies will not be removed and are readily seen on a blood film.
Hyposplenism
Same as aforementioned explanation, except that the spleen is poorly functional and does not remove Howell-Jolly bodies readily.
Hyposplenism can occur either from direct trauma, or via autosplenectomy, where a disease such as sickle cell anemia damage the spleen to such an extent that it becomes non-functional.
Severe Megaloblastic Anemia
Notice the word severe. In severe megaloblastic anemia, it is possible for the buildup of chromosomal remnants as RNA synthesis continues unhindered. The mechanism for this is still under investigation however.
Severe Hemolytic Anemia
As blood loss occurs severely, the bone marrow is pressured to produce more red blood cells. Thus, there is a “left-shift” towards more erthyrcoytes being produced, even if they are immature. It is thus possible for red blood cells with whole nuclei being produced in this condition. Often however, the nuclei are still broken down into Howell-Jolly bodies, but are produced in such sheer numbers due to the left shift that the spleen cannot remove all of them.
Note that it is also possible for red blood cells to be entirely nucleated. A nucleated red blood cell, or NRBC, is a red blood cell (RBC) that retains nucleus. Normally, NRBCs are only found in the circulation of fetuses and newborn infants. After infancy, RBCs normally only contain an nucleus during the very early stages of the cell's life, and the nucleus is ejected before the cell is released into the bloodstream. Thus, if NRBCs are seen on an adult's peripheral blood smear, it suggests that there is a very high demand for the bone marrow to produce RBCs, and immature RBCs are being released into circulation. Possible pathologic causes include anemia, myelofibrosis, thalassemia, miliary tuberculosis, cancers involving bone marrow, and in chronic hypoxemia.
Heinz Body
Do you remember what “Bite cells” are from “Morphological Abnormalities of Red Blood Cells”? They are also referred to as Degmacytes, and occur when a portion of a red blood cell is phagocytosed due to the presence of an inclusion body within the cytoplasm of the cell. This inclusion body is the Heinz Body.
Heinz bodies are formed by damage and denaturing to the hemoglobin component of red blood cells, most commonly by oxidative stress, but also possibly by genetic abnormalities in hemoglobin.
Typically, during oxidative damage to hemoglobin, an electron is transferred from the hemoglobin to oxygen, resulting in the formation of a reactive oxygen species (ROS). This ROS can lead to severe damage within the cells, and can even cause lysis of the entire cell.
The ROS denatures portions of the hemoglobin, causing the to precipitate and produce Heinz Bodies, which becomes an antigenic agent. Thus, macrophages detect the antigen and remove the damaged portions of the cell, its damaged membrane and the denatured hemoglobin (now called the Heinz Body).
Diseases Associated with Heinz Bodies:
Heinz bodies are almost always associated with oxidative damage to the red blood cell, and only rarely to genetic causes. So obviously then, the cause of Heinz Bodies would be something that causes increased oxidative stress to increase the likelihood of production of ROS in the blood cells. So what are these conditions?
G6PD deficiency is the most important and most common cause of production of Heinz Bodies, and consequently Degmacytes.
In G6PD deficiency, the loss of the G6PD enzyme has serious consequences that increase oxidative stress.
Normally, G6PD as an enzyme is used in the pentose phosphate (or HMP Shunt) pathway.
G6PD converts glucose-6-phosphate into 6-phosphoglucono-δ-lactone and is the rate-limiting enzyme of this metabolic pathway that supplies reducing energy to cells by maintaining the level of the reduced form of the co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH).
The NADPH in turn maintains the supply of reduced glutathione in the cells that is used to mop up free radicals that cause oxidative damage.
The G6PD / NADPH pathway is the only source of reduced glutathione in red blood cells (erythrocytes). The role of red cells as oxygen carriers puts them at substantial risk of damage from oxidizing free radicals except for the protective effect of G6PD/NADPH/glutathione.
NADPH Deficiency
NADPH deficiency can cause a deficiency in glutathione peroxidase, which is an enzyme that can reduce hydrogen peroxide, a ROS, into water.
Chronic Liver Disease
Alpha Thalassemia
Normal adult hemoglobin is composed of two alpha and two beta chains. Alpha thalassemia patients have partial or complete defects in alpha globin production, leading to a relative abundance of beta globin chains in the cell. These excess beta globin chains aggregate to form HbH, which has decreased solubility and precipitates in the red blood cell cytoplasm. This is not direct damage to hemoglobin per se, but rather a perturbation in the quaternary structure of hemoglobin.
Thus, HbH is merely a moderate to severe form of alpha-thalassemia that consists of hemoglobin precipitating within the cytoplasm of the red blood cells in high amounts due to the insoluble nature of beta-chain heavy hemoglobin molecules.
Because the HbH aggregates are precipitated and are non-functional, HbH is considered a subtype of Heinz Bodies.
Observe a HbH aggregate below:
*Hyposplenism and Splenectomy
While not a direct cause of Heinz Bodies, the spleen helps with clearing the blood and thus removing Heinz Bodies from circulation. With a damaged spleen, more of these Heinz Bodies remain in circulation and are visible in a blood slide.
Observe a Heinz Body below:
Difference Between Heinz Body and Howell-Jolly Body:
Howell-Jolly bodies can only be seen with a special supravital stain (a supravital stain is a staining technique that involves using a stain on cells that are removed from the body), like the new methylene blue stain. On the other hand, the Howell-Jolly bodies are seen as very dark, basophilic spots on the cell.
Furthermore, the Heinz Bodies are usually found close to the inner surface of the RBC membrane, while Howell-Jolly bodies can be found in variable positions around the cell.
Pappenheimer Body
Pappenheimer bodies come next. We discussed hemoglobin deposits (HbH and Heinz Bodies) and nuclear inclusion bodies (Howell-Jolly Bodies), but the Pappenheimer Bodies are inclusion bodies of iron. They are deposits or iron that build up within the red blood cell.
Pappenheimer bodies are basophilic erythrocytic inclusions that are usually located at the periphery of the cell. They contain iron and stain with Prussian blue. Prussian blue is the stain that is used to identify that these Pappenheimer bodies are pure iron deposits, and not heme as in Heinz Bodies.
Cells containing Pappenheimer bodies can be confused with late reticulocytes. Prussian blue stain, which is not taken up by reticulocytes, is helpful in differentiating the two. Pappenheimer bodies can also cause a false elevation of platelet counts when performed with electronic counters. Pappenheimer bodies are also visible with a Wright and/or Giemsa stain, but Prussian blue is more useful to differentiate these bodies from Heinz Bodies.
Diseases Associated with Pappenheimer Bodies:
Splenectomy
Due to inability of spleen to remove pappenheimer laced cells.
Hemolytic Anemia
Sideroblastic Anemia
Megaloblastic Anemia
Hemoglobinopathies
The exact cause of Pappenheimer bodies in these diseases is unknown.
Basophilic Stippling
Basophilic stippling is also referred to as punctuate basophilia. In this condition, there are fine and coarse granules located at in a fairly evenly distributed manner . These small dots are actually accumulations of RNA and ribosomes.
Diseases Associated With Basophilic Stippling:
Basophilic Stippling can be divided into fine basophilic stippling and coarse basophilic stippling.
Fine basophilic stippling is associated with increased red cell production and is commonly seen when there is increased polychromatophilia.
Coarse basophilic stippling is seen in megaloblastic anemia and other forms of severe anemias, lead poisoning, and thalassemia. Coarse basophilic stippling indicates impaired hemoglobin synthesis, probably due to the instability of RNA in the young cell.
In fact, basophilic stippling is usually a very strong sign of lead poisoning.
Cabot Ring
Cabot rings. These things are really interesting. They are literally loops, rings or figure 8 type structures that are located within the cytoplasm of the RBC. They may look like beads on a string. They are typically colored red-purple under the Wright Stain.
They are quite rare, but they look really cool whenever you come across one. They are simply microtubular remnants of mitotic tubules that are involved in mitosis.
Their presence usually is indicative of some abnormality in the production of red blood cells.
Diseases Associated with Cabot Rings:
Myelodysplastic Syndrome
Megaloblastic Anemia
Both these conditions show abnormalities in production of red blood cells.
And here is a very unlucky cell with both Cabot rings and a Heinz Body:
That’s it for all the abnormalities guys! Hope this was useful as usual! You can combine this with “Morphological Abnormalities of RBCs” to figure out what pretty much anything strange about an individual red blood cell indicates. As usual, follow for more content!
Red blood cells, so nice and simple. It’s literally a red, blood cell. Amazing. 10/10 would study. Clearly we can all appreciate the normal shape of a red blood cell:
The key word for it is biconcave disc, meaning that it is a disc shaped (or circular) cell that is compressed at the centre in both directions. This allows an increased surface area: volume ratio, providing easier avenues for diffusion of oxygen.
They have a typical diameter of 7.2um, however the normal range is between 6.2um and 8.2um.
They have a typical width of 2.0um, although it can range between 2.0um and 2.5um.
They contain no nucleus, to maximize volume of Hb so as to maximize carriage of oxygen.
They are also very flexible, useful for squeezing into thin capillaries to deliver oxygen to tissues.
Great, that’s all gu- wait.. morphological abnormalities? ...Oh man, so much can go wrong with red blood cells. Oh boy, here we go.
Yep, all these things can go wrong with red blood cells. Poor things. There are actually more things than these, but let’s go through these in detail.
Variations in Size
We have mentioned under “General Features of Anemia” that the most important red cell index for discussing the variations in size of blood cells is the Mean Corpuscular Volume (MCV). Just to refresh, the MCV is the average volume of a red blood cell. It is reflective of the size of a cell, and thus, a large MCV indicates a large red blood cell, and a small MCV indicates a small red blood cell. Its typical reference value is 80fL - 95fL, though some sources use 100fL as the upper limit.
Any variation in size of the RBCs is known as anisocytosis, and the degree of anisocytosis in a sample of blood is known as the red cell distribution width (RDW).
Microcyte
A microcyte, is a red blood cell by definition, a small (micro-) mature cell (-cyte). Thank you for making it so simple, greek derivations. In terms of MCV, a microcyte has an MCV below 80fL. Since the reference range of MCV is 80-95, this should be easy to remember.
In terms of actual diameter, a microcyte is defined as any RBC with a diameter less than 5.0 microns. Compare this to the average of 7.2 microns.
Diseases Associated with Microcytes:
Microcytes indicate some problem with the manufacturing system of red blood cells, for example, by some deficiency. The diseases include:
Iron Deficiency Anemia
Deficiency of iron leads to a scenario where the red blood cell cannot be filled with Hb and thus is an overall smaller cell, since there is less Hb.
Sideroblastic Anemia
Condition where there the bone marrow releases immature red blood cells, sideroblasts, as opposed to mature red blood cells.
These sideroblasts have rings of iron around their nucleus, and thus the iron is used up on the sideroblasts.
Thus, for congenital forms of sideroblastic anemia, there is often a defiency of iron leading to red blood cells that do mature, becoming microcytes.
Thalassemia Minor
Beta thalassemias are a group of disorders in which the beta chain of Hb is usually missing or deformed.
They occur as a result of blockage to the HBB gene.
HBB blockage over time leads to decreased Beta-chain synthesis. The body’s inability to construct new Beta-chains leads to the underproduction of HBA. Reductions in HBA available overall to fill the red blood cells in turn leads to microcytic anemia.
Lead Poisoning
One of the main causes for the pathology of lead is that it interferes with the activity of an essential enzyme called delta-aminolevulinic acid dehydratase, or ALAD, which is important in the biosynthesis of heme, the cofactor found in hemoglobin. Lead also inhibits the enzyme ferrochelatase, another enzyme involved in the formation of heme.
The important thing to understand is that microcytosis in lead poisoning occurs due to lack of heme, which leads to a lack of Hb and thus a deficiency, causing smaller RBCs.
Some Hemoglobinopathies
Occasionally, chronic disease
Here is a microcyte below:
Macrocyte
A macrocyte on the other hand, is a large (macro-) mature cell (-cyte). Wow. In terms of MCV, a macrocyte has an MCV above 95, or above the reference range. It is thus an enlargement of red blood cells with a near constant concentration of hemoglobin.
Diseases/Conditions associated with Macrocytes:
Liver Disease
Patients with hepatic disease and obstructive jaundice have macrocytosis that is secondary to increased deposition of cholesterol or phospholipids on the membranes of circulating red blood cells (RBCs).
Megaloblastic Anemia
The most common cause of macrocytic anemia is megaloblastic anemia, which is the result of impaired DNA synthesis. Typically, although DNA synthesis is impaired, RNA synthesis is not, and RNA continues to be produced, increasing the nuclear matter within RBCs that is not being converted to DNA. Thus, the cell gradually enlarges due to increased nuclear matter, causing macrocytosis.
Vitamin B12 deficiency and Folate deficiency
Vitamin B12 and folate is very important in the synthesis of Thymidine and Purines, and thus DNA synthesis is impaired in these deficiencies, causing macrocytosis by the aforementioned mechanisms.
Aplastic Anemia
Mild macrocytosis is seen in recovery from aplastic anemia, which occurs when there is a bone marrow pathology and a deficiency of all 3 types of blood cells (WBC, RBC, platelets) occurs.
Neonates
Observe macrocytes below:
Notes about Red Cell Distribution Width (RDW)
RDW is the most important index relating to anisocytosis. The higher the variance in sizes of cells, the higher the RDW. Thus, a high RDW means that cells are of very different sizes, whereas a low RDW indicates that cells are more or less of the same size.
With this in mind, the normal reference range in human red blood cells is 11.5-14.5%, meaning that usually, only 11.5% - 14.5% of cells are of different sizes out of a sample of blood.
Implications of RDW:
Normal RDW in presence of Anemia: A normal RDW implies that most cells are of the same size. In cases of a normal RDW with anemia, the diagnosis is almost exclusively a thalassemia. This is since beta chains are gradually removed, and the cells become smaller, all the RBCs drop to smaller sizes, by the mechanism explained above. All the cells are affected since it is a genetic disorder. Since all the cells are now microcytic, they are expected to be of similar sizes, and thus the RDW remains normal.
Abnormally High RDW: An abnormally high RDW on the other hand means that cells are of a variety of different sizes. High RDWs are generally expected to occur in situations of deficiencies, since only some cells will be deficient and thus affected at a time. Notice that this is different from genetic conditions, where ALL cells are affected. High RDWs are usually indicative of:
Iron Deficiency Anemia: High RDW and Low MCV
Vitamin B12 and Folate Deficiency: High RDW and High MCV
Recent Hemorrhage: High RDW and normal MCV
Variations in Colour
Remember that the red blood cell consists of hemoglobin that can be in two states: Either oxygenated, in which case it is oxyhemoglobin, or deoxygenated, in which case it is deoxyhemoglobin. Well, the color of a red blood cell is... red. Yes. I don’t know where I was going with that.
To be more precise, oxyhemoglobin and deoxyhemoglobin are different shades of red. Oxyhemoglobin is a brilliant scarlet, while deoxyhemoglobin is a darker burgundy-red.
Hypochromia
Hypochromia is the most common disorder of color that occurs in red blood cells. Hypo-, of course, means “less” and -chromic means “color” so when we describe a cell as hypochromic, we say it has less color. Simple, right?
Hypochromic RBCs are much paler, and the reason for this is because they lack hemoglobin within them. Because of this, they lose their red colouration and appear much paler. Using this concept, recall that the unit to measure the amount of hemoglobin per cell is mean corpuscular hemoglobin, or MCH. The MCH is simply the average amount of hemoglobin in one red blood cell, from a particular sample. Its reference range is 27pg - 31pg, meaning that there is typically between 27pg and 31pg of hemoglobin in a single red blood cell.
The other unit for hemoglobin content of a red blood cell is mean corpuscular hemoglobin concentration (MCHC). The MCHC is reflective of the concentration of packed red blood cells (so blood excluding plasma) that is hemoglobin. It basically translates to the amount of hemoglobin present in the cellular component of blood, and thus excluding plasma. Its reference range is 32g/dL - 36g/dL.
There are two conditions that a RBC must satisfy in order to be classified as a hypochromic cell. These are:
The central zone of pallor of the RBC must be greater than 1/3 of the diameter of the cell.
The MCH must be below 27pg/cell and/or the MCHC must be below 32g/dL.
Diseases associated with Hypochromia:
Iron Deficiency Anemia
Recall that iron is essential in the production of heme, the centerpiece of hemoglobin.
Thus a deficiency in iron leads to a deficiency in heme and thus hemoglobin, resulting in lowered MCH values.
Also recall that iron deficiency anemia causes microcytosis, and thus iron deficiency anemia causes hypochromic, microcytic anemia.
Thalassemias
As discussed previously, thalassemia arises by a genetic defect in the HBB gene that codes for beta chains of hemoglobin, causing the absence of, or underproduction of, beta chains. As alpha chains production gradually slows down in the absence of beta chains, hemoglobin becomes less and less functional. Thus, less oxygen can be held by hemoglobin, and hemoglobin itself becomes destroyed due to destruction of the then non-functional red blood cells by thalassemia. Consequently, MCH decreases and hypochromic anemia results.
Notice that thalassemia also causes microcytosis as HBA production to replace HBB decreases over time and the red blood cell shrinks.
Sideroblastic Anemias
Recall that sideroblast production (very immature red blood cells), with rings of iron around their nucleus is the hallmark of sideroblastic anemias. These cells are known as ringed sideroblasts.
Because iron is used up in these non-functional rings, proper red blood cells are not formed, and thus proper hemoglobin is not formed adequately. This results in hypochromic cells, that are again, also microcytic.
Lead Poisoning
Lead poisoning impairs the functioning of the enzyme, ALAD, and the enzyme ferrochelatase, both important in the production of heme.
This impairment in the production of heme leads to decreased hemoglobin in red blood cells, thereby reducing MCH and MHCH and causing hypochromia and microcytosis.
Some cases of chronic inflammation
Polychromasia
Polychromasia is a medical condition in which there is an abnormally high amount of immature red blood cells being released into the bloodstream. The most significant of these is the reticulocyte, the immediate precursor to the red blood cell. The only difference between the reticulocyte and the red blood cell is the presence of a meshwork of RNA within the reticulocyte, when viewed with special stains such as the new methylene blue stain, that must be removed before the reticulocyte can be called a red blood cell.
Polychromasia literally translates to “many colors,” and the reason for this is because the many immature red blood cells being released into the bloodstream are all different shades of a bluish grey.
Man, that looks like a great color for hipst-, ahem, anyways.
Diseases Associated with Polychromasia:
Polychromasia is usually a sign of bone marrow stress as well as immature red blood cells. Polychromasia occurs in conditions which call for premature release of red blood cells into the bloodstream, such as in conditions where RBC levels in the bloodstream are severely low. These include:
Acute and Chronic Hemorrhage
During hemorrhage, as blood loss from the body occurs, there is a constantly lowered level of RBCs in the bloodstream. Thus the bone marrow opts to increase the RBC levels by increasing production of red blood cells, and increasing release of red blood cells, even if they are immature or not fully mature. In this way, immature cells such as reticulocytes may appear in the bloodstream, causing polychromasia.
Hemolysis
In hemolysis, as red blood cells are destroyed, the body attempts to replace them by increasing production and release of red blood cells, and this results in the release of immature red blood cells such as reticulocytes into the bloodstream.
Effective treatment for anemia
Neonates
Note that polychromasia is mainly associated with normocytic styles of anemia, as there is no change in MCH or MCV in any of these scenarios.
Variations in Shape of Cells
The term for variations in the shape of cells is poikilocytosis. Thus, any abnormally shaped cell is called a poikilocyte.
Poikilocytes can occur either due to membrane abnormalities or trauma.
The poikilocytes caused by membrane abnormalities are:
Acanthocytes (Spur Cells)
Codocytes (Target Cells)
Echinocytes and Burr Cells
Spherocytes
Stomatocytes (Mouth Cells)
Drepanocytes (Sickle cells)
Degmacytes (”Bite Cells”)
The poikilocytes caused by trauma are:
Dacrocytes (Teardrop cells)
Keratocytes
Microspherocytes and Pyropoikilocytes
Schistocytes
Semilunar Bodies
Let us explore each in detail.
Poikilocytes of Membrane Abnormalities
Acanthocytes (Spur Cells)
The word acantho- means thorns. This should give you a good idea as to what acanthocytes look like. The word literally means thorn cells after all. Thus, acanthocytes can be described as having a spiked cell membrane, due to irregular thorny projections that vary in width, length and number. Notably, they have no central area of pallor.
Acanthocytes typically arise via one of two mechanisms: Alterations in membrane lipids are seen in abetalipoproteinemia and liver dysfunction.
In liver dysfunction, apolipoprotein A-II deficient lipoprotein accumulates in plasma causing increased cholesterol in the RBC membrane. This causes abnormalities of membrane of RBC causing remodeling in spleen and formation of acanthocytes.
In abetalipoproteinemia, there is deficiency of lipids and Vitamin E causing abnormal morphology of RBC membranes.
Diseases Associated with Acanthocytes:
Abetalipoproteinemia
This is a rare autosomal recessive condition that affects the absorption of fat, and fat soluble vitamins from food.
Abetalipoproteinemia affects the absorption of dietary fats, cholesterol, and certain vitamins. People affected by this disorder are not able to make certain lipoproteins (hence the name, abetalipoproteinemia).
This leads to a multiple vitamin deficiency, affecting the fat-soluble vitamin A, vitamin D, vitamin E, and vitamin K. However, many of the observed effects are due to vitamin E deficiency in particular.
As explained above, the deficiency of Vitamin E and lipids results in an abnormal membrane layer of the RBC.
Abnormal lipid concentrations within the blood cause acanthocytosis primarily by inducing concentration gradients with the lipids in the red cell membrane, causing some portions of the membrane to extend outwards as lipids move in or out of them. This gradient is known as membrane stress, and in conditions of abetalipoproteinemia or hypobetalipoproteinemia, the RBC membrane is more vulnerable to membrane stress.
Vitamin E Deficiency
Severe Liver Disease
As explained above, cholesterol buildup in RBCs causes portions of the red cell membrane to extend out the RBC and form thorny extensions, thus producing acanthocytosis.
Splenectomy
A major function of the spleen is the clearance of opsonized, deformed, and damaged erythrocytes by splenic macrophages. If splenic macrophage function is abnormal or absent because of splenectomy, altered erythrocytes will not be removed from the circulation efficiently.
Malabsorption
Poor reabsorption of lipids and lipid-soluble vitamins has similar effects on RBCs as abetalipoproteinemia.
Hypothyroidism
Very rare cause of acanthocytosis.
Neuroacanthocytosis
This is a condition including 4 diseases, namely chorea acanthocytosis, McLeod Syndrome, Huntington Disease - like 2 (HDL2) and pantothenate kinase-associated neurodegeneration.
In summary, all neuroacanthocytoses affect the basal ganglia and brain and are a group of movement and neurological disorders.
All 4 conditions produce acanthocytosis (it’s in the name after all).
Notice the thorn like projections on each cell. They are irregular, and of varying distribution, length and width.
Codocytes (Target Cells)
Codocytes, also known as target cells, look like typical red blood cells, with a central area of pallor and appropriate size. However, they have a dark, red spot in the middle. For this purpose, they are also referred to as bullseye cells.
Yep, they look like bullseyes, don’t they? You can describe them of consisting of a central darkened area, surrounded by a white ring (or an area of achromia), in turn surrounded by a peripheral darkened stained area. For this reason, they are also called... Mexican Hat cells. ...Yeah. Unforgettable now isn’t it?
They occur due to a disproportional increase in the surface area:volume ratio of a red blood cell. You can say these cells have an abnormally high surface area for their volume. It is due to either increased red cell surface area (increased beyond normal), or else a decreased intracellular hemoglobin content (which may cause an abnormal decrease in cell volume without affecting the amount of membrane area).
As a result of this increased surface area, these cells are stronger in water, and have decreased osmotic fragility. This is since they have a higher surface area for diffusion of water, they can take up more water for a given amount of osmotic stress.
It should be noted however, that within the blood vessels itself, the codocyte is in fact thin and bell-shaped, and occasionally the central darkened spot is connected to the peripheral darkened ring. This leads to the central zone of pallor being C-shaped. It is only when a blood film is made that the bell-shape is squashed and appears as a target. Thus, within the blood cells, since they appear different from target cells and codocytes, they are sometimes referred to separately as leptocytes.
Diseases Associated with Codocytes:
Diseases associated with codocytes must obviously cause at least one of two things: Either cause a direct increase in surface area by affecting lipid concentrations in the RBC membrane, or by decreasing hemoglobin concentration within the RBCs. Thus, any disease that can cause these two can cause codocytes to be formed. A list of some of these diseases includes:
Thalassemia
Associated with a decrease in functional hemoglobin.
Iron Deficiency Anemia
Associated with a decrease in total hemoglobin.
LCAT Deficiency
LCAT, or lechitin-cholesterol acyltransferase is an enzyme that coverts free cholesterol into cholesteryl ester. In the absence of LCAT, the cholesterol:phospholipid ratio increases, causing cholesterol buildup in the RBC and increasing the size of the membrane of the RBC. Eventually, the surface area rises to abnormally high levels.
Obstructive Liver Disease
Obstructive liver disease is associated with an LCAT deficiency.
Hemoglobin C (and sometimes Hemoglobin S) Disease - Hemoglobinopathies
In hemoglobiopathies such as Hemoglobin C and Hemoglobin S (sickle cell anemia) disease, one of the chains in hemoglobin is genetically malformed (contrast with Thalassemias where a chain is underproduced or missing). As a result of this, typically the hemoglobin is reduced and non-functional, and it causes red blood cells to have a shortened lifespan.
Thus, codocytes are very typical of all hemoglobinopathies.
Splenectomy
Associated with removing opsonized of damaged cells, and thus target cells remain in the bloodstream and are not removed.
Echinocytes or Burr Cells
Echinocytes. Another type of red blood cell disorder. As usual, let’s allow our Greek friends to help us out. Echino- arises from the greek word echinos, which means hedgehog, or sea urchin. Let’s look at a hedgehog and see why echinocytes are named after them.
Look at those spikes, quite regular and quite short. Echinocytes are actually quite similar.
Echninocytes are burr-like erythrocyte with short, blunt, evenly spaced projections. It is a red blood cell with an abnormal membrane, that, like acanthocytes, has thorny projections. The main difference between the acanthocyte and the echinocyte however, is the shape of the thorny projections. In acanthocytes, the projections are irregularly spaced, and vary in width, length and number. In echinocytes, the projections are all short and evenly spaced. Furthermore, under the Wright Stain, echinocytes even appear to have a central area of pallor. Acanthocytes show little to no central pallor.
Both acanthocytes and echinocytes occur due to membrane abnormalities of the RBC. Recall that acanthocytes arise from either increased cholesterol buildup within the membranes, or general lipid malabsorption that causes increased membrane tension, which is what leads to the irregular projections on its surface.
Echinocytes on the other hand can be produced in vitro by incubation at high pH or in the presence of high calcium concentrations, exposure to glass surfaces, reduced albumin concentrations, and after prolonged storage, and are usually reversible creations. They can be formed during application of EDTA, drying or staining.
They may also occur in hyperlipidemias caused by liver disease, as with acanthocytes. However, the cholesterol does not become incorporated into the lipid membrane as it does with acanthocytes. Instead, it is speculated that cell surface receptors on the red blood cells bind with HDL cholesterol which induces the shape change in echinocytes.
Echinocytes are seen above. Notice how their thorny projections are short and regular, and have a central area of pallor. Look at how they compare to acanthocytes below:
The acanthocyte thorns are much sharper, longer, random and show no central area of pallor, as opposed to the echinocyte (Burr Cell) that has short, round, very regular thorny projections and show a central area of pallor.
Diseases/Conditions Associated with Echinocytes
Uremia
Pyruvate Kinase Deficiency
Microangiopathic Hemolytic Anemia
Neonates (especially premature)
As artifacts of EDTA, drying or staining.
Spherocytes
We’ve really been helped out here. With a name like “spherocytes” there’s no question what the shape of these cells will be. Spherical cells. Yeah. These cells are sphere-shaped rather than the typical biconcave disc shape expected of a normal red blood cell, and that is their most important feature.
Spherocytes are simple. They appear as spheres under a blood film with no central area of pallor.
You’ll notice that macrocytes also have no central area of pallor. To differentiate one from another, you’ll notice that the spherocytes are actually smaller than the normal red blood cells, and thus cannot be macrocytes. In contrast, microcytes show a central area of pallor, and thus you should be able to tell the difference between the three. The spherocytes are smaller due to partial loss of their membrane, without any damage to their intracellular content. The mechanism will be seen below.
Spherocytes are often always indicative of either hereditary spherocytosis or immune-mediated hemolytic anemia.
Hereditary spherocytosis is a genetic disorder, a molecular defect in one or more of the proteins of the red blood cell cytoskeleton, including, spectrin, ankyrin, Band 3, or Protein 4.2. Because the cell skeleton has a defect, the blood cell contracts to its most surface-tension efficient and least flexible configuration, a sphere.
Allternatively, during immune-mediated hemolytic anemia, there is partial phagocytosis of normal red blood cells by phagocytosis due to the presence of antigens on the surface of the red blood cell. The phagocyte destroys the membrane where the antigen was present, destroying some of the surface area of the cell. Thus, the cell must now shape itself into a sphere, decreasing its surface area:volume ratio, since the volume of the cell remains the same. This is known as immune-mediated hemolytic anemia, since the immune system in the form of phagocytes destroys (hemolyses) a portion of the red blood cells, producing spherocytes. Two simple mechanisms.
Though the spherocytes have a smaller surface area through which oxygen and carbon dioxide can be exchanged, they in themselves perform adequately to maintain healthy oxygen supplies. However, they have a high osmotic fragility--when placed into water, they are more likely to burst than normal red blood cells.
Diseases Associated with Spherocytes:
Hereditary Spherocytosis
Immune-mediated Hemolytic Anemia
Haemolytic jaundice of the newborn due to ABO antibodies
Transfused Cells
Severe Burns
Stomatocytes (Mouth Cells)
Stomatocytes are yet another membrane abnormality that occurs within red blood cells. In this condition, several cells, instead of having a central area of pallor, possess a central “slit” of pallor. For this reason, they look like a mouth, or in particular, “kissing lips.” Even easier to remember is that they look like coffee beans. Take a look at them below:
See? Coffee beans, kissing lips, you name it.
So why do these things arise? Basically, stomatocytes result from an increase in the volume of the red blood cell, and consequently a decrease in the surface area: volume ratio, due to some permeability defect in the membrane. (Contrast this with spherocytes and codocytes, which occur as a result of an increase in the surface area to volume ratio.)
The reason for the production of the mouth shaped slit is actually unknown at this point, but you can at least remember that stomatocytes are mouth shaped by picturing a mouth drinking water, similar to how water and salts move into stomatocytes due to their membrane defect.
Diseases/Conditions Associated with Stomatocytes
Hereditary Stomatocytosis
Hereditary stomatocytosis describes a number of inherited autosomal dominant human conditions which affect the red blood cell, in which the membrane or outer coating of the cell 'leaks' sodium and potassium ions.
Osmosis leads to the red blood cell having a constant tendency to swell and burst. This tendency is countered by manipulating the flow of sodium and potassium ions. A 'pump' forces sodium out of the cell and potassium in, and this action is balanced by a process called 'the passive leak'.
In the hereditary stomatocytoses, the passive leak is increased and the cell becomes swamped with salt and water. The cell lyses and a haemolytic anaemia results.
Alcoholism
Liver Disease
Rh Null Phenotype for Blood
Individuals who possess the Rh null phenotype have osmotically fragile red cells, which take the form of stomatocytes.
The osmotically fragile red cells are overfilled with water for their size even at water levels that would be normal for a normal erthyrocyte.
Thus, they appear as stomatocytes in Rh null phenotypes.
As an Artifact
If only 10% of less of the cells seen are stomatocytes, then they are most likely artifacts.
Degmacytes (Bite Cells)
A degmacyte (aka "bite cell") is an abnormally shaped red blood cell with one or more semicircular portions removed from the cell margin. These "bites" result from the removal of denatured hemoglobin by macrophages in the spleen. Glucose-6-phosphate dehydrogenase deficiency (G6PD), in which uncontrolled oxidative stress causes hemoglobin to denature and form Heinz bodies, is a common disorder that leads to the formation of bite cells.
The Heinz Bodies are seen as antigenic and are quickly phagocytosed. Because the Heinz Bodies are derivatives of hemoglobin, they are located inside the cell, and thus phagocytosis takes a significant “bite” out of the cell. This is the difference between the bite cell and the spherocyte, where only the membrane is destroyed.
This is one we all know. Sickle cell anemia is something we’re all familiar with. And it is again, a name that gives away what the cell is shaped like. You guessed it - the blade of a sickle, or a crescent. It is an autosomal recessive genetic disorder. The gene defect is a known mutation of a single nucleotide (GAG to GTG) of the β-globin gene, which results in glutamic acid being substituted by valine at position 6. This new hemoglobin is known as Hemoglobin S (HbS).
This is normally a benign mutation, causing no apparent effects on the secondary, tertiary, or quaternary structures of haemoglobin in conditions of normal oxygen concentration. What it does allow for, under conditions of low oxygen concentration, is the polymerization of the HbS itself. The deoxy form of haemoglobin exposes a hydrophobic patch on the protein between the E and F helices due to the valine. The hydrophobic side chain of the valine residue at position 6 of the beta chain in haemoglobin is able to associate with the hydrophobic patch, causing haemoglobin S molecules to aggregate and form fibrous precipitates.
In this case, as a result of the hydrophobic chains interacting, the red blood cell essentially folds in on itself, producing its unique shape.
Poikilocytes of Trauma
Dacrocytes (Teardrop Cells)
Teardrop cells is such a cool name. And they are really shaped like teardrops. Thank you to whoever named these morphological abnormalities in cells. Dacrocytes can also be said to be pear shaped. They are usually characteristic of myelofibrosis, and seen with marrow disorders or marrow infiltrations, really because of improper production of blood cells from the bone marrow. In post-splenectomy patients, the number of dacrocytes drastically increases, since the spleen cannot remove the improperly formed cells.
Diseases Associated with Dacrocytes:
Myelofibrosis with myeloid metaplasia
Myelofibrosis, also known as osteomyelofibrosis, is a rare bone marrow cancer.
Beta Thalassemia Major
Myelophthisic anemias
Myelophthisic anemia (or myelophthisis) is a severe kind of anemia found in some people with diseases that affect the bone marrow. Myelophthisis refers to the displacement of hemopoietic bone-marrow tissue either by fibrosis, tumors or granulomas.
Extramedullary Haematopoiesis
Schistocytes
A schistocyte is a fragmented portion of a red blood cell. It is literally a broken piece off a red blood cell. Obviously then, they are irregular shaped, jagged and have pointed extremities, that is, usually 2 pointy ends. There is no central pallor since they are simply fragments of red blood cells.
Schistocyte formation occurs as a result of mechanical destruction (fragmentation hemolysis) of a normal red blood cell. This occurs when there is damage to the blood vessel and a clot begins to form. The formation of the fibrin strands in the vessels occurs as part of the clot formation process. The red blood cells get trapped in the fibrin strands and the sheer force of the blood flow causes the red blood cell to break. The resulting fragmented cell is called the schistocyte.
They are thus very common in hemolytic anemias, obviously since RBCs are hemolysed and broken down in this condition.
Disseminated intravascular coagulation is an activation of the coagulation cascade which is usually a result of an increased exposure to tissue factor.
The activation of the cascade leads to thrombi formation which causes an accumulation of excess fibrin formation in the intravascular circulation. The excess fibrin strands cause mechanical damage to the red blood cells resulting in schistocyte formation and also thrombocytopenia and consumption of clotting factors.
Schistocyte values between .5% and 1% are usually suggestive of DIC.
Thrombotic thrombocytopenic purpura
Thrombotic thrombocytopenic purpura or TTP is caused by primary platelet activation. Thrombotic thrombocytopenic purpura leads to increased amounts of von Willebrand factor which then attach to activated platelets and mediate further platelet aggregation. Platelets end up being removed and the resulting fibrin strand formation remains. These fibrin strands along with the stress from the blood flow cause fragmentation of the red blood cells, leading to schistocyte formation.
In TTP, a schistocyte count between 3-10% is common, but >1% is suggestive of the disease.
Hemolytic uremic syndrome
Haemolytic-uremic syndrome or HUS is haemolytic anaemia, acute kidney failure (uraemia), and thrombocytopenia.
HUS is caused by E. coli bloody diarrhea and specific strains of shiga toxin. The bacteria in HUS cause damage to the endothelium which results in platelet activation and formation of microthrombi.
Red cells get trapped in the fibrin strands of the microthrombi and become sheared by the force of blood flow leading to schistocyte formation.
Severe burns
Keratocytes
Keratocyte formation may be associated with trauma, especially cellular damage from contact with fibrin strands within the microvasculature. Other processes that can contribute to microvascular injury include endotoxemia and antigen-antibody reactions. These forms of microangiopathy subsequently may lead to platelet aggregation, fibrin formation, and, ultimately, intravascular coagulation (DIC). As normal erythrocytes encounter a mesh of fibrin strands, they can become entrapped. Sometimes these cells are impaled on fibrin strands. Blood flow then pushes the cell against the strand of fibrin and the cell may bisect. Alternatively, the opposing sides of the cell may adhere to one another around the fibrin strand. When blood flow frees the cell, the opposing sides rejoin forming a pseudovacuole. Cells with pseudovacuoles are called "blister" cells or pre-keratocytes. When the vacuole ruptures (usually within minutes), the remaining cell resembles a helmet with straps or a horned cell that is designated a keratocyte. Prekeratocytes also can form via fusion of injured cell membranes (e.g., administration of doxorubicin6 and some iron deficiency anemias). These new cells are more fragile than the original parent cell and may rupture, forming a keratocyte.
A blister cell is seen below:
And here is a blister cell bursting:
...Leading to the formation of the final Keratocyte:
Semilunar Bodies
Semilunar bodies are types of poikilocytes caused by cell trauma. They are hypochromic, crescent shaped cells, also called ghost corpuscles/cells or phantom cells, with loss of all hemoglobin.This erythrocyte cell is an “empty” red cell membrane (the external structure or “covering”) that is left complete or intact after hemolysis, but the cell is dead. In some cases of AIHA/IMHA, semilunar bodies/ghost erythrocytes may be seen with intravascular hemolysis.
Semilunar bodies are types of poikilocytes caused by cell trauma. They are decolourised, crescent shaped cells, also called ghost corpuscles/cells or phantom cells. This erythrocyte cell is an “empty” red cell membrane (the external structure or “covering”) that is left complete or intact after hemolysis, but the cell is dead. In some cases of AIHA/IMHA, semilunar bodies/ghost erythrocytes may be seen with intravascular hemolysis.
Unique Cases of Poikilocytosis
These unique cases of poikilocytosis involve two important hemoglobinopathies: Hemoglobin SC disease and Hemoglobin C Crystals.
Hemoglobin SC Disease
Hemoglobin S-C disease is a hemoglobinopathy that causes symptoms similar to those of sickle cell disease, but milder. In this condition, only one allele of the autosomal sickle cell trait is present in patients. Thus, while the cells are not exactly “sickle cells” they still produce some abnormality in cell morphology.
This is how these cells look:
Thus, they can be described as “mitten-shaped” cells with 1-2 finger like projections.
Hemoglobin C Crystals
Hemoglobin C (abbreviated as Hb C or HbC) is an abnormal hemoglobin in which substitution of a glutamic acid residue with a lysine residue at the 6th position of the β-globin chain has occurred. This mutated form reduces the normal plasticity of host erythrocytes causing a hemoglobinopathy.
Target cells, microspherocytes and HbC crystals are found in a blood smear from a homozygous patient.
HbC crystals are the significant cell seen in this condition, and they look like this:
Yep. They look like pills. So whenever you see something like this in the blood film, along side target cells and microspherocytes, suspect HbC.
Elliptocytes/Ovalocytes
Red cells varying in shape from elongated to oval, and rich in hemoglobin, are called elliptocytes/ovalocytes. They can be seen in hereditary disorders, such as hereditary elliptocytosis, or in acquired disorders, such as iron defiency anemia, infectious anemias, thalassemia, and in newborn babies.
Elliptocytes: cigar-shaped
Ovalocytes: egg-shaped
That’s all guys! I’ve covered morphological abnormalities in this section. In the next topic, I will talk about inclusion bodies and improper arrangements of red blood cells. But for now, I hope this was useful!
Bacterial Taxonomy 2 - Classification of Bacteria Based on Metabolic Characteristics
We look at plants and animals, and agree that we all need oxygen to survive and metabolize. So what about bacteria? Bacteria must metabolize in some way, right?
Bacteria can be divided into 2 major groups, based on their metabolic properties. The two most important metabolic properties used to classify bacteria into groups include:
1) How the organism deals with oxygen.
2) What the organism used as a carbon and energy source.
There are other properties used for classification, such as the different metabolic end products that bacteria produce, but these are less important.
How Bacteria Deal With Oxygen
Oxygen, is in fact, a very dangerous chemical. We know that it is highly reactive, and easily reacts with electrons to be oxidized into dangerous radicals such as hydrogen peroxide (H2O2), superoxide radicals, (O2-), and hydroxyl radicals (•HO).
These chemicals, being very dangerous and reactive, must quickly be removed from the body. It is clear that as humans, we adore oxygen - we need it to live after all! So we obviously have ways to protect against these radicals. This protection comes in the form of a 3 enzyme defense system (man, that sounds cool).
Catalase: Catalase breaks down hydrogen peroxide (H2O2) in the following reaction:
Peroxidase: Peroxidase also breaks down Hydrogen Peroxide (H2O2). It’s in the name, after all! Peroxidase requires an electron donor molecule, and then transfers the electron over from the electron donor to the H2O2 to form water.
Superoxide Dismutase: Superoxide Dismutase (SOD) breaks down the Superoxide radical (O2-). .... like a superhero fighting a supervillain? No? Okay.
After producing H2O2 in the SOD reaction, peroxidase and catalase can take over and reduce the superoxide into something harmless.
I’ll just leave this here....
So we’ve established that humans clearly love oxygen. Thus, we possess all 3 enzymes necessary to keep oxygen in check and keeping us alive.
Bacteria, on the other hand, are arranged on a spectrum, based on how well they are able to react to oxygen radicals when they do appear. On one end of the spectrum, are bacteria that have all 3 enzymes and will die in the absence of oxygen. On the other end, are organisms that have none of the required enzymes, and die pretty much immediately even in the presence of oxygen.
This spectrum is as follows:
Obligate Aerobes
Obligate aerobes have all the faculties to carry out oxidative phosphorylation to obtain energy with oxygen quite perfectly. They use glycolysis, the Krebs Cycle and the electron transport chain, just as we do, to obtain the energy they need for their metabolism.
Obviously then, all obligate aerobes all 3 enzymes - Catalase, Peroxidase and Superoxide Dismutase.
Noteworthy is the fact that they contain no faculty to carry out anaerobic respiration, and thus, they will definitely die in the absence of oxygen.
Bacteria that are obligate aerobes include:
Nocardia
Bacillus cereus
Neisseria
Pseudomonas
Bordetella
Legionella
Brucella
Mycobacterium
Leptospira Interrogans
Branhamella catarrhalis
Burkholderia cepacia
Francisella tularensis
Spirillum minus
Coxiella burnetti
Facultative Anaerobes
Facultative anaerobes are the closest analogy to humans. They are able to carry out aerobic respiration quite perfectly, possessing both superoxide disputes and catalase (not peroxidase). However, their most noteworthy feature is that they are also able to carry out anaerobic respiration.
Thus, they are mainly aerobic, but they have the faculty to carry out anaerobic respiration. This is why they are called facultative anaerobes. When the need arises, they have the faculty to carry out fermentation to obtain energy in the absence of oxygen. This is very similar to the anaerobic respiration carried out by human muscle cells during strenuous activity like sprinting.
Facultative Anaerobes include:
Listeria
Actinomyces
Bacillus anthracis
Corynebacterium
Staphylococcus
Most other gram negative rods
To know the facultative anaerobes, you must know a list of your ABC’S.
List - Listeria
A - Actinomyces
B - Bacillus anthracis
C - Corynebacterium
‘S - Staphylococcus
Microaerophilic Bacteria
Microaerophilic bacteria are aerobic bacteria that require only a very small amount of oxygen to survive, and are poisoned by excessively high oxygen tension.
This is because they only have 1 defense enzyme, our superhero, Superoxide Dismutase (but no catalase or peroxidase).
The microaerophilic bacteria include:
Enterococcus
some Streptococci (although some species of streptococci are facultative anaerobes)
Helicobacter pylori
Spirochetes
Treponema
Borrelia
Leptospira (except Leptospira interrogans)
Campylobacter
Obligate Anaerobes
These guys.. they really don’t like oxygen. You can imagine since they’re on the extreme end of the spectrum. They have no electron transport chain, and have no enzymes to prevent against oxidative stress. Thus, if they are exposed to oxygen, they due.
Obligate Anaerobes include:
Clostridium
Bacteroides
Fusobacterium
Streptobacillus moniliformis
Porphyromonas
Prevotella
Veillonella
Peptostreptococcus
There is also a division of obligate anaerobes, known as aerotolerant anaerobes. These bacteria require no oxygen as they respire anaerobically, but unlike obligate anaerobes, they CAN survive in the presence of oxygen.
NOTE: Rickettsia and Chlamydia are both energy parasites, and steal their host’s ATP rather than utilizing aerobic or anaerobic respiration. These organisms are known as obligate intracellular organisms, and live in host cells.
The most important of these bacteria are listed in the table below.
(courtesy Clinical Microbiology Made Ridiculously Simple, 6th ed.)
How Bacteria Deal with their Carbon and Energy Source
Some organisms use light as an energy source (phototrophs), and some use chemical compounds as an energy source (chemotrophs).
Of the chemotrophs, organisms that use inorganic sources such as ammonium and sulphide are autotrophs. Those that use organic carbon sources are called heterotrophs.
However, most bacteria, and in fact, all medically important bacteria are chemoheterotrophs, because they use both chemical and organic compounds such as glucose for energy.
That’s all guys! Hope you got all of this! It may be a lot of bacteria to remember, but the most important ones you should keep in mind are summarized neatly in the table just before I speak about classification based on Carbon and Energy Source.
Bacterial Taxonomy 1 - Classification Based on Morphology and the Gram Stain.
Taxonomy, is literally the science of classification. Look at the picture above, and imagine that all those little divisions, like “firmicutes” are different phyla under the kingdom of bacteria. Then those phyla are further subdivided into different classes, then orders, then families, then genera, and then finally species! Take a look at how this works for one particular bacteria, called streptococcus mutans.
Wow, there’s a lot to classify, probably why it’s taxonomy: it’s such a taxing job.
Ahem, right, so. As you’ve probably noticed, Streptococcus mutans is named using its Genera and its Species name. Similarly, all organisms have a scientific name comprising of two parts: The genus, followed by the species. It is very important to classify organisms in this way because:
It establishes criteria for identifying organisms.
Allows arrangement of related organisms into groups.
Provides important information on how organisms evolved.
Bacteria are classified, usually, according to their morphological, metabolic and biochemical differences, although genetic and immunologic factors are also now being considered.
One of the earliest, and most fundamental methods of classifying bacteria depended on the use of the Gram Stain.
Gram Stain
Unlike large organisms like humans, parrots and dra-, erm, Komodo Dragons, which are easy to spot and have a distinct appearance to the eye, bacteria are colourless and invisible to light microscopy. Thus, gram staining had to be developed to give bacteria a colour, and visualize them. Since bacteria would either respond to the stain, or not, all bacteria were subsequently classified into gram-positive and gram-negative bacteria.
There are 4 steps to the Gram Stain Procedure.
Pour crystal violet stain (a blue dye) and wait for 60 seconds.
Wash off with water and flood with iodine solution. Wait for 60 seconds.
Wash off with water and then “decolourize” with 95% alcohol solution.
Counter-stain with safranin (a red dye). Wait 30 seconds and then wash off with water.
Basically, when viewed under the microscope, cells that absorb the crystal violet dye and hold on to it become blue: These are gram-positive. Alternatively, if the crystal violet is washed off by the 95% alcohol, the cells absorb the safranin and appear red. These are gram-negative.
Gram Positive = Blue
Imagine yourself sitting by the beach, opposite crystal blue waters, or kayaking across deep waters, or even river tubing across the bluest of rivers.. won’t you say yes to that? So, Blue = Positive. Note that Gram-Positive bacteria may also appear purple if the red safranin is not effectively washed off. This is because blue + red is purple.
Gram Negative = Red
Now you’re sitting in sweltering red heat, sweat pouring down your body, the sun red in the sky. You don’t want that, do you? Or for comic book fans, you can picture Superman’s face when he sees a Red Sun in the sky. NOPE, thinks Superman. So Red = Negative.
This difference occurs due to a difference in morphology of the bacteria.
Gram Positive vs Gram Negative Bacteria
Unlike eukaryotic animal cells which contain only one cell membrane composed of phospholipid separating the nucleus from the ECF, both gram-positive and gram negative bacteria contain more than 1 layer: the layer outside the bacterial cytoplasmic membrane is the peptidoglycan layer.
The peptidoglycan layer is composed of repeating disaccharide units that contain 4 amino acid groups attached to an O-C(CH3) unit.
These amino acids then form side linkages with other disaccharide units in neighboring chains. This allows the formation of cross linkages. The enzyme that allows the formation of these amino acid-amino acid peptide bonds across chains is transpeptidase, located in the inner cytoplasmic membrane. As a side note, this is the enzyme targeted by penicillin, vancomycin and cephalosporins.
The major difference between the gram-positive and gram-negative is the thickness of the peptidoglycan layer. The gram-positive cell wall is very thick due to a very high degree of cross-linking and high transpeptidase activity. In contrast, the gram-negative cell wall is very thin due with a simple and not complex cross-linking pattern. To visualize this easily, imagine two lines, representing the cell walls, with a space in between to specify the thickness of the cell wall. Now try squashing a +, as in Gram positive, in between the two lines. The two lines must have a high space in between to support the + shape. In contrast, a - sign is easily squashed by the two lines. This way, you can remember that:
“+” is thick peptidoglycan layer is gram positive.
“-” is thin peptidoglycan layer is gram-negative.
There is also a difference in the structure of the outer layer between gram-positive and gram-negative bacteria.
Gram-positive bacteria possess a cell wall composed of a thick peptidoglycan layer, as described previously, alongside teichoic acid, polysaccharides and other proteins. Teichoic acid is composed of polymers of either ribitol phosphate or glycerol phosphate.very important as an antigenic marker, making it important in serologic identification of gram-positive species. It should be noted that teichoic acid that remains bound to the peptide linkages keeps the name, however, teichoic acid bound to lipids from the inner cell membrane, is then called lipoteichoic acid. Furthermore, the inner surface of the cell wall touches the cytoplasmic membrane. The cytoplasmic membrane, in turn, contains proteins that span the lipid bilayer. Thus, the outer cell wall and inner cytoplasmic membrane comprise the cell envelope of the gram-positive bacteria.
In contrast, Gram-Negative bacteria possess a cell envelope made of three layers. It still possesses a peptidoglycan cell wall and an inner cytoplasmic membrane; however, a Gram-negative bacteria also possesses an outermost cell membrane.
The inner cell membrane, as in Gram-positive bacteria, still contains a phospholipid bilayer with embedded proteins. However, unlike in gram-positive bacteria, where the inner surface of the cell wall touches the cytoplasmic membrane, there exists a definite periplasmic space between the thin peptidoglycan layer and the inner cytoplasmic membrane. This space is filled with a gel-like matrix, the periplasm, that contains proteins and enzymes. In some bacteria, the periplasm contains Beta- lactamases that break down penicillin, making them more resistant to penicillin like drugs.
The thin peptidoglycan layer also does not contain any teichoic acid, instead containing a small, helical lipoprotein known as murein lipoprotein. This lipoprotein originates from the peptidoglycan layer and extends outwards to attach to the outer cell membrane.
This outer cell membrane is a very typical cell membrane, with a phospholipid bilayer structure and hydrophobic tails in the centre. It is unique, however, due to the presence of LPS or lipopolysaccharide on the outermost portion of the bilayer.
The LPS, or lipopolysaccharide is an endotoxin. It is responsible for many of the features of disease during infection by these organisms, such as fever and shock. It is different from an exotoxin since it is a vital part of the structure of the bacteria, and thus cannot be removed from the body as an exotoxin would be. An LPS is composed of 3 covalently linked components:
Outer carbohydrate chains of 1-50 oligosaccharide units that extend into the surrounding media. These differ from one organism to another and are antigenic determinants. This is known as the O-specific side chain, or O-Antigen. Since it is outermost, and is an antigenic determinant, O-antigen should come to mean “Outer-Antigen.”
The middle is composed of a water soluble core polysaccharide.
Innermost, lies lipid A, a disaccharide with multiple fatty acid tails reaching into the membrane. Lipid A is the toxic component of the LPS, and is the endotoxin of the gram-negative bacteria. The endotoxin of the gram-positive bacteria is in fact, the less powerful teichoic acid. In any case, when bacterial cells are lysed by our own immune system during infection, fragments of membrane containing lipid A are released into circulation, causing fever, diarrhea and possible fatal endotoxic shock (septic shock).
(Pictures Courtesy Clinical Microbiology Made Ridiculously Simple 6th ed. - M, Gladwin)
Also embedded in the outer membrane of the gram-negative bacteria, lies proteins known as porins. These porins allow passage of nutrients into the bacteria, and are only necessary in gram-negative bacteria.
This difference between gram-positive and gram-negative bacteria yields different medical consequences and treatment paths.
To remember ALL the differences between the Gram-positive and Gram-negative bacteria, take a look at this pretty nifty way of remembering the differences!
A LONG PPT, as in a boring, long powerpoint, is at least of some help now, right?
Gram-positive bacteria: The thickly meshed peptidoglycan layer, while providing a diffusion barrier, does not block diffusion of low molecular weight compounds. This means that substances that damage the cytoplasmic membrane can pass through, such as antibiotics, detergents and dyes. This is the reason that the 95% alcohol does not wash off the crystal violet dye, and the gram-positive bacteria appear blue.
Gram-negative bacteria: The negative outer LPS-containing membrane blocks the passage of substances that can damage the cytoplasmic membrane and fragile peptidoglycan layer. Thus, antibiotics, penicillin and lysozyme are unable to pass through.
Taxonomy Based on Bacterial Morphology
Bacteria can have 4 major shapes:
1) Cocci: spherical
2) Bacilli: rods. Especially short bacilli are called coccobacilli. (Imagine compressing a rod length wise - it gets more and more spherical, hence adding the “cocco” in the name.
3) Spiral Forms (Spirilla): Comma-shaped, S-shaped or spiral shaped.
4) Pleomorphic: Lacking a distinct shape.
These creatures all organize together to form more complex arrangements and patterns.
For example, 2 cocci pairing together forms diplococci.
A cluster of cocci form staphylococci, like a staff picture of your teachers plotting to destroy you with hard exams.
Alternatively, they can be arranged in long strips, in which case they are streptococci (think striptococci).
If the cocci are arranged in a cuboidal pattern, it implies that the bacteria belong to the genus of sarcina.
FInally, 4 cocci arranged together form a tetrad.
Notice that the bacilli follow a similar concept as the cocci. Noteworthy however, is the absence of the staph variant of bacilli. Thus, there is only streptobacilli, there is no staphylobacilli. Instead, clusters of bacilli fall under palisades.
Taxonomy Based On Staining
The only bacteria you need to remember are the gram-positive ones. This is because there are only 12 gram-positive bacteria; all the rest are gram-negative (with a few exceptions, which will be mentioned).
Gram Positive Bacteria
Of the 12 gram-positive bacteria, 6 are cocci and 6 are bacilli.
The gram-positive cocci are:
Streptococcus
Staphylococcus
Enterococcus
Enterococci were once considered streptococci, but were put into a separate category because they can appear either as diplococci or short chains.
Micrococcus
Appears in tetrads.
Peptostreptococcus
Occur in short chains, as diplococci, or even individually.
Peptococcus
(So yes, ALL streptococci and staphylococci are gram-positive!)
The 6 Gram-positive bacilli are:
Bacillus
This may sound confusing. But it’s really just poor naming. You see, a bacillus is a rod-shaped bacteria, as discussed earlier. However, Bacillus, capitalized and italicized, refers to a specific genus of bacteria that is rod shaped (sigh).
Noteworthy because it produces spores, which are spheres that protect a dormant bacterium from the harsh environment).
Clostridium
Also produces spores.
Corynebacterium
Appears in palisades
Listeria
Appears in palisades
May also be classified as a coccobaccilus.
Erysipelothrix
Appears in palisades
Propionobacterium
Appears in palisades
There are also filamentous bacilli that are gram-negative and show extensive branching, and these are the Actinomyces, Nocardia, Arachnia and Streptomyces.
Gram-Negative Bacteria
There are only 3 gram-negative cocci. Since all the streptococci and the staphylococci are gram-positive, it follows that the gram-negative bacteria are all diplococci. They are:
Neisseria
Moraxella
Veillonella
Similarly, there is only 1 group of spiral shaped bacteria, or spirilla. This group is:
Spirochetes.
The spirochetes include several bacteria, and of these, the disease causing bacteria are:
Treponema pallidum - causes syphillis
Leptospira species - causes leptospirosis
Borrelia burgdorferi, B. garinii, B. afzellii - cause Lyme Disease
Borrelia recurrentis - Relapsing fever (think recurrentis as in recurring, hence relapsing)
Brachyspira pilosicoli and Brachyspira aalborgi - causes intestinal spirochaetosis
ALL OTHERS gram-negative bacteria are either bacilli, coccobacilli or pleomorphic.
The pleomorphic bacteria include rickettsiae and chlamydiae, that are very small bacteria and cannot be seen under the light microscope effectively.
Bacteria that Stain Under Special Circumstances
As mentioned previously, some bacteria do not neatly fall into the categories of gram-positive or gram-negative. These are:
Mycobacteria: Mycobacteria are only weakly gram-positive. This is because there is a very high amount of lipid in the cell wall, and thus, the dye does not easily penetrate. Instead, you have to use a dye known as the acid-fast stain. To remember this, simply think of the mycobacterium as an obese bacteria, trying to fast to get thinner.
So mycobacteria = high lipid content in cell wall ---> acid fast stain required.
(Courtesy midlandstech)
Spirochetes: Don’t be mistaken. Spirochetes are still gram-negative. The reason they are in this list however, is because even though they will stain with a gram stain, they are too small to be seen under a light microscope. Thus, a special type of microscope, the darkfield microscope, must be used for these bacteria.
Spirochetes are also unique morphologically. While, like other gram-negative bacteria, they contain an inner cytoplasmic membrane, thin peptidoglycan cell wall, and outer cytoplasmic membrane fitted with LPS, they also contain an additional fourth outer membrane, with very few embedded proteins. This is believed to help spirochetes avoid antigenic identification, making spirochetes a “stealth organism.” A ninja, really. To remember this, think of how cheaters need to be stealthy to get away with what they’re doing. Well, so do spirochetes. Furthermore, spirochetes possess an axial flagella, that emerges from the spirochete cell wall, but runs sideways along the spirochete under the outer membrane sheath. Thus, the flagella is referred to as a periplasmic flagella. Rotation of this periplasmic flagella generates thrust, that propels the spirochete.
Mycoplasma: Mycoplasma actually have no cell wall at all! They only have a cell membrane, and thus they are neither gram-positive, nor gram-negative.
Rickettsiae and Chlamydiae: These pleomorphic bacteria are extremely small, and intracellular, and cannot be seen under the light microscope despite being gram-negative.
Legionella Pneumophila: Has poor uptake of red safranin stain, and thus, a more intense application of safranin is required.
(Courtesy Review of Medical Microbiology and Immunology, 12th ed)
(Courtesy Clinical Microbiology Made Ridiculously Simple, 6th ed.)
That’s all guys! Hope this was useful for you all. You should now have an idea for names of all the bacterial genera, and be able to classify them as gram negative or gram positive. Additionally, you should be able to identify the differences between a gram-positive and gram-negative bacteria.
Here are some other resources for you guys!
VIDEOS:
Classification of Bacteria
QUESTIONS:
1) A patient comes into your office complaining of small, painless sores and ulcers around her genital area. She claims she is sexually active. What bacteria is most likely to cause her condition?
A. Treponema pallidum
B. Rickettsiae
C. Borrelia recurrentis
D. Staphlyococcus Aureus
2) Which of the following is not representative of gram-positive bacteria:
Pathologic Cell Injury and Cell Death II - Necrosis
Cell Death
As cellular damage continues, the damage eventually becomes irreversible, after which there is no escape from death for the poor cell. When this occurs, the cell will eventually die, by one of two mechanisms:
Necrosis - Pathologic Cell Death
Apoptosis - Physiological (usually) Cell Death
Note that some people also consider autophagy as a method of cell death, but recent studies in fact link this to cell survival.
Necrosis
Necrosis is considered an unregulated and “accidental” form of cell death, caused from exogenous causes associated with damage to cell membranes and loss of ion homeostasis.
Basically, when this damage to membranes is severe, lysosomal enzymes (which we discussed under “Waste Disposal In Cells”), manage to escape the lysosomal membranes and destroy the cell from the inside, causing morphological changes known as necrosis. Furthermore, cell organelles are able to escape via the damaged membrane and into the extracellular space, where they illicit a host reaction, the inflammation associated with necrosis. The lysosomes of leukocytes called in as a result of the inflammatory reaction then also aid in the breakdown of organelles within the cell that have been exuded from it.
Speaking a little more in depth now:
Cell necrosis is heralded by the loss of the function of the cell permeability barrier, or cell membrane in the case of the cell, meaning that the cell cannot control substances leaving or entering the cell. This holds especially true for ions, most important of which are Na+ and Ca2+, for which the concentration gradients are very steep.
Usually, there are 4 factors required for maintaining the selective ion permeability of the cell:
Considerable amounts of ATP
Structural Integrity of the Phospholipid Bilayer/Cell Membrane
Intact Ion Channel Proteins
Normal association of the Membrane with cytoskeletal elements.
If any of these factors are damaged or interrupted, then the cell isn’t able to maintain normal ionic concentrations within and outside the cell. This is the point of no return for the cell - when it is no longer able to regulate internal ionic concentration.
There are a total of 7 cervical vertebrae, and with the hyoid bone, they form the skeleton of the neck extending from the base of the skull superiorly, inferiorly till the 1st thoracic vertebra. The spinal nerves emerge above the corresponding vertebrae - for example, C3 spinal nerve comes off above the C3 vertebra.
The typical cervical vertebrae are C3, C4, C5 and C6.
The atypical cervical vertebrae are C1 - Atlas, C2 - Axis and C7 - vertebra prominens.
It should be noted that all vertebrae except C1 and C2 consist of two broad portions:
Body: Large, central mass of bone.
Vertebral Arch: Visible in every diagram, consists of a pedicle which connects the body to the articular processes, and a lamina that connects the articular processes to the spinous process. Don’t worry, this will be explained below.
Features of a Typical Cervical Vertebra:
Small, Wide Body - Looking very carefully at the shape of the body, you can notice that some edges of the body are raised. Typically, the superior surface of the typical cervical vertebrae are raised or elevated greatly at the posterior rim, marginally at the lateral rim, and depressed at the anterior rim. This phenomenon results in the superior surface being called an uncus. [Diagram 3] What does this resemble? Think of a chair. Conveniently, the inferior surface of the cervical vertebrae have a “bevelled” surface, or a surface that complements or fits into the superior surface of the vertebra below it. This allows a special type of joint called a uncovertebral joint, that really just resembles chairs stacking one on top another. Just look at diagram 5 above, you’ll see what I mean.
Large, triangular vertebral foramen - Nerves supplying the arm come from the cervical enlargement of the spinal cord. Since the spinal cord is so large here, the vertebral foramen must also be large to accommodate it.
Tranverse Process with Foramen Transversarium - The foramen transversarium, labeled as a large hole or passageway through the transverse process of the vertebra, is the most characteristic feature of the cervical vertebrae. Its function is to allow the passage of the vertebral artery, accompanying vertebral veins and branch of inferior cervical ganglia, specifically though the foramina transversaria of C6 to C1. Hence, the name vertebral artery is given to this very important artery, as it runs within the vertebra itself. It should be noted however, that the C7 foramen transversarium is very small, and may be absent occasionally, and serves only to carry small accessory veins.
Transverse Process with Anterior and Posterior Tubercle - Labelled in Diagrams 1, 2 8, 9 and 10 above, the anterior and posterior tubercle arise due to the embryology of the cervical vertebra. The transverse process in fact, is derived from 2 parts embryologically: a posterior transverse element (part of the neural arch), and an anterior costal element, labelled in diagram 2 above. The costal element is unique, as within the thoracic vertebrae, it projects anteriorly to form the ribs. In the cervical vertebrae, you can also imagine the costal element in an attempt to protrude anteriorly to form ribs, but is not able to complete its attempt. Hence there is an anteriorly projecting anterior tubercle, and a posteriorly projecting posterior tubercle, both which serve for attachment of muscles. The portion connecting the anterior tubercle and posterior tubercle to the pedicle of the vertebra is the anterior and posterior root. Both anterior and posterior tubercle are also connected to each other by a costotransverse bar, labelled in the diagram above. The costotransverse bar forms a groove for the passage of the anterior rami of the cervical spinal nerve as it exits the vertebra. You can therefore think of the transverse process as 2 roots extending out from the lamina, connected to each other at the tips by the costotransverse bar, forming an open circle through which the vertebral artery and accompanying veins pass. It should be noted that the anterior tubercle of C6 is especially large and is called the carotid tubercle because the common carotid artery can be palpated against this tubercle, and it separates the common carotid from the vertebral artery.
Spinous Processes That are Short and Bifid - Visible in all the diagrams as a large, projecting portion of the vertebrae, the spinous processes are formed by the fusion of the two laminae along the midline. Typically, the spinous process extends inferiorly to protect the cervical vertebra below it. However, in the vertebrae C3-C5, the spinous process is very short. It should be noted however, that the spinous processes become larger from C3 - C7, and the spinous process of C6 is actually classified as a long spinous process (it is thus an exception to the generalization that the spinous process is short for the typical cervical vertebrae. The spinous process of C7 is even longer, and is usually the longest in the body - hence called the vertebra prominens, since it is so prominent. The spinous process is also classified as bifid, meaning it has two inferiorly projecting endings. All the typical cervical vertebrae (C3-C6) are bifid. C7 is again, an exception, and is in fact, not bifid.
Articular Processes - All vertebrae possess a superior and an inferior articular processes, that articulate to form a type of plane synovial joint called a zygapophyseal (facet) joint, that keeps the vertebrae together. It is very important to understand that the type of movement allowed by the vertebra depends on the shape of these articular processes, and the resulting zygapophyseal joint. For example, the most allowed movement in the cervical vertebrae is flexion and extension. Thus, the articular processes must face in a sagittal plane (i.e. must face anteriorly and posteriorly with respect to each other). Refer to the diagram of the facet joint above. As you can see, it will be impossible to carry out a different type of motion, such as rotation, as the direction the facets face do not allow that motion. In cervical vertebrae, the superior articular process faces posterosuperiorly (backwards and slightly upwards) to articulate with the anteroinferiorly facing inferior articular process of the different vertebra above it. Both superior and inferior articular process of the same vertebra are fused to the junction between the pedicle and lamina along an articular pillar, called the pars interarticularis, visible in diagram 8 above.
So what makes C7 so different?
C7 is atypical due to the following features:
Small/absent foramen transversarium, that carries accessory veins rather than the vertebral artery.
Very long spinous process that is not bifid.
Anterior tubercle and root of C7 is especially small, and provides attachment for the suprapleural membrane.
The cervical vertebrae are also used as important landmarks along the neck:
At the level of C3: mandible and hyoid bone
At the level of C4: common carotid artery bifurcates, superior border of thyroid cartilage
At the level of C4-C5: Thyroid cartilage
At the level of C6-C7: Cricoid cartilage
If you guys are confused by the zygapophyseal joints, check this youtube video:
https://www.youtube.com/watch?v=PzYo60FkojI
And here’s a nice animation for you guys to experiment with on cervical vertebrae as well!
Indeed brilliant😊Congrats man on that neuro prize in banquet! So who got the resp, endo, mbj prizes?
It was actually a little error on their part. I won the award for Endocrine, Reproduction, Muscles bones and joints, and Respiration. Somebody else won the neuro prize :)
Pathologic Cell Injury and Cell Death III - Mechanism of Irreversible Cell Injury
Here we are, looking at irreversible cell injury now. We discussed reversible cell injury and 1 type of cell death, namely necrosis, earlier in the posts:
Mechanism of Reversible Cell Injury
Necrosis
Now, we will discuss the different ways by which cells can actually become injured, and the mechanism by which the cells become injured.
So we can start by asking: When does reversible cell injury become irreversible cell injury?
To answer this question, there are 3 key factors that must be understood:
1) The cellular response to an injurious stimulus depends on the severity, duration and nature of the injury. To explain this, if a small amount of a toxin is injected into an individual, the cells may be injured and undergo reversible injury, but since the toxin levels are low, the cell is able to recover once the toxin is cleared away. If the levels of toxins persist at a very high level, the cells will undergo very critical injuries, and may eventually die, resulting in irreversible cell injury and cell death.
2) The consequences of a cellular injury depends on the state, type, and adaptability of the cell. This means that the state of the cell before the injury is a critical factor in the development of an injury. It therefore refers to the vulnerability of the cell. For example, wouldn’t a cell that is already under stress be more likely to be easily pushed to irreversible injury? If a cell is already under nutritional deficiencies and ischemia, for example, then it is in an atrophied state, and will much more easily be pushed to cellular injury if the increased stress of another condition is added. Furthermore, the type of cell is also important in determining the consequence of a cellular injury. Imagine both striated muscles in the leg and cardiac muscles of the heart have undergone cell injury via ischemia, while the patient can simply relax the leg and allow the muscles of the leg to rest and recover from cellular injury, you cannot relax the cardiac muscles since they keep you alive. Furthermore, there are some toxins that when applied to two groups of cells, can produce no effect in one, and fatal effects in the other. An example of this is with addition of CCl4, carbon tetrachloride. The fact that some cells may be immune is likely due to the presence of enzymes that metabolize the toxin, in this case CCl4, not present in all cells due to polymorphisms (different forms of enzyme, that may not be functional) in other cells.
3) Cell Injury results from different biochemical mechanisms acting on several essential components.
Here is an example of how different mechanisms of cellular injury trigger several different downstream pathways of cellular injury. These will all be explored in great detail below, have no fear. The important thing to remember is which cellular components are most easily damaged. During cellular injury, usually the mitochondria, cell membrane, machinery of protein synthesis and packaging, and DNA. However, remember that a single cause of cell injury may intact, trigger several of these biochemical downstream mechanisms.
Now, to answer the question, there are two features that characterize irreversibility from reversible injury.
Firstly, irreversible mitochondrial damage that results in ATP depletion and lack of oxidative phosphorylation despite removal of the injurious stimulus.
Secondly, profound disturbances in membrane function, especially those that affect internal ionic concentrations, and lysosomal enzyme activity, are almost always points of no return for the cell.
Cells are little, fragile things; they can be injured very easily by a variety of causes of cell injury, such as:
Hypoxia/Ischemia
Excess Heat or Cold
Radiation
Chemicals. drugs and toxins
Infectious Agents
Immunologic/Autoimmune
Inflammation
Nutrition
Genetics
These causes listed above damage the cell by one or more of the following mechanisms of cell injury:
Depletion of ATP
Mitochondrial Damage
Influx of Ca2+ and loss of Calcium Homeostasis
Accumulation of Oxygen Derived Free Radicals (Oxidative Stress)
Defects in Membrane Permeability
Damage to DNA and Proteins
See if you can match which injury uses which mechanism! If you can’t just yet, then let’s dive into this interesting topic and see what we find out!
Depletion of ATP
You know what ATP is. It is so important, if you type “ATP” into your Google search, it’s the very fir-, wait what? It’s second right after professional tennis? ...Well, alright, ATP is almost as important as tennis, which I guess is extremely important. I mean, look what you get when you image search ATP:
“Ultimate” is right, ATP is the energy currency of our body. When our body wants a process done, it pays in ATP. For example, if it wants to activate a transcription factor, it may phosphorylate an enzyme such as protein kinase A using ATP, which then acts on the transcription factor to activate it. It is also essential for several other processes such as protein synthesis,lipogenesis and membrane transport to name a few. Thus, it is essentially the cellular cash, except the consequence of having no cellular cash or ATP, is cell death.
Quickly reviewing the physiology of ATP, we must remember that ATP is synthesized by 2 major methods:
Oxidative Phosphorylation of ADP, done within the mitochondria by the reduction of oxygen using the electron transport chain.
Glycolytic Pathway, which generates ATP without Oxygen, using Glucose derived from various methods, either body fluids or hydrolysis of glycogen.
ATP depletion and reduced ATP synthesis are associated with hypoxic and chemical (toxic) injury.
Thus, the causes of ATP depletion would either be a reduced supply of oxygen and nutrients, mitochondrial damage and the actions of toxins (e.g. cyanide).
So when does ATP loss become life-threatening?
Generally, once ATP falls to a value of 5-10% of its original value, the ATP loss becomes extremely life-threatening. This is because several cellular systems are affected:
1) The very first effect is cellular swelling, discussed under Mechanism of Reversible Cellular Injury, linked above in the introduction. This occurs due to the inability of functioning of the Na+/K+-ATPase pump. As a result of this, Na+ accumulates within the cell and K+ moves out of the cell. This means the cell is no longer able to maintain internal ionic concentrations, and the point at which it is unable to do so is when the transition from reversible cell injury to irreversible cell injury is set in stone. Since water also follows Na+, it moves into the cell causing cellular swelling in the form of blebs and increased cell volume; and accumulates especially in the endoplasmic reticulum, causing ER swelling and vacuolar degeneration.
2) Alteration in cellular energy metabolism. Think about this: if a process that disrupts either ATP synthesis or causes ATP depletion such as hypoxia, will result in a decrease of oxygen, and a consequent decrease in ATP production. If ATP synthesis decreases, but ATP must still be used, it makes sense to think that AMP concentrations are gradually increasing. Usually, ATP inhibits an enzyme known as phosphofructokinase-1, that is important in converting fructose-6-phosphate into fructose-1,6-bisphosphate, an extremely important step in glycolysis. Thus, we see that high ATP inhibits glycolysis. AMP however, reverses the inhibitory effect of ATP on phosphofructokinase-1, and thus promotes the formation of fructose-1,6-bisphosphate and promotes glycolysis. This means that there is a shift in energy source from oxidative phosphorylation to anaerobic glycolysis. This process promotes production of glucose for glycolysis by the breakdown of glycogen. Hence the body’s glycogen stores are rapidly decreased.
Now recall that the end product of anaerobic respiration in animals is lactic acid. As this type of respiration is being promoted, glycogen stores and eventually fat stores and muscle proteins are broken down, all producing gradually higher amounts of lactic acid and inorganic phosphates. If allowed to build up, this acid eventually drops the intracellular pH to levels where cellular enzymes are not able to function with great efficiency.
3) As ATP driven pumps continue to gradually fail, eventually, a very important Ca2+ pump fails. Given that the intracellular concentration of calcium is 10^-7 moles and the extracellular concentration is 10^-3 moles (1000 times less - imagine the concentration gradient!), calcium rushes into the cell rapidly, causing several processes that are regulated by minute amounts of calcium to be completely disrupted. At this stage, the cell is beyond all viability.
4) Eventually, as ATP depletion continues, structural disruption begins to occur. This structural disruption is most pronounced in the apparatus of protein synthesis. These disruptions include detachment of ribosomes from the rough endoplasmic reticulum and dissociation of polysomes, ultimately resulting in a decrease in protein synthesis.
5) Eventually, as ischemia and hypoxia continue, and the protein synthesis system is compromised, new proteins produced are often misfolded, and proteins that are present within the cell themselves become misfolded. This accumulation of misfolded proteins activate the ubiquitin-proteasome system, via a unfolded protein reaction. This is discussed under Waste Disposal in Cells.
6) As the unfolded protein reaction destroys the proteins within the cell from the inside, lysosomal and mitochondrial membranes are irreversibly damaged (the consequence of which is discussed just below), and lysosomal enzymes and reactive oxygen species are unleashed within the cell, destroying the cell from the inside. Eventually, the cell undergoes necrosis; most often, coagulative necrosis, as discussed already under Necrosis, linked above.
(courtesy Robins and Cotrans Pathologic Basis of Disease).
Mitochondrial Damage
Damage to mitochondria is one of the hallmarks of irreversible cell injury, and is usually a common feature of all injurious biochemical pathways that follow cell injury. We’ve seen it just above too! Mitochondrial damage occurs late in ATP depletion, and the cell dies shortly afterwards. Let us see why mitochondrial damage is so fatal to the cell.
Mitochondrial damage is so dangerous to the cell, that the cell immediately activates stress signalling and triggers autophagy of the mitochondria (called mitophagy) so as to remove the dangerous, damaged mitochondria. If the mitochondria is allowed to remain within the cell, in a damaged state, intracellular dynamics are drastically altered.
We know why mitochondria are important. They are essential in oxidative phosphorylation, involving oxygen. Thus, without them, or due to damage to them, the cell isn’t efficiently able to produce ATP and will eventually die of ATP depletion, the mechanism of which is discussed above. However, there are other major consequences and methods by which a cell whose mitochondria is directly damaged, will die.
Before we go into that, let us consider what can damage the mitochondria directly. The mitochondria is damaged by:
Increases in cytosolic Calcium, coupled by an increase in inorganic phosphate and certain fatty acids.
High inorganic phosphate and fatty acids alone cannot damage the mitochondria but coupled with high Ca2+ are extremely damaging to a cell. Note that high Calcium alone can stilldamage mitochondria.
Reactive Oxygen Species
Oxygen Deprivation, either by Hypoxia or Ischemia
Defective Turnover of Mitochondrial Proteins
Thus, they are very vulnerable as they can be easily damaged by a variety of causes.
If the mitochondria are indeed damaged, then there are 3 major consequences:
1) Mitochondrial Permeability Transition.
Mitochondrial permeability transition is a phenomenon that is defined as an increase in the permeability of the mitochondrial membranes to freely allow entry of molecules less than 1500 Daltons in molecular weight. Usually the outer mitochondrial membrane contains porins, that, although allow movement of molecules up to 5000 Daltons, very tightly regulate the movement of molecules into the mitochondria. Because all molecules below 1500 Da cannot be regulated, there is a mitochondrial permeability transition (MPT). This transition is brought about by a high conductance channel, the mitochondrial permeability transition pore (MPTP).
Notice how all solutes less than 1500 Da rush into and out of the cell in the above picture. The mitochondria slowly swells as the MPTP opens. The opening of the MPTP is trigerred primarily by an increase in intracellular calcium concentrations. This Ca2+ interacts with Ca2+ receptors on the matrix side of the MPTP, from within the mitochondria, and opens them (explained in the next section). This opening is done even faster in the presence of inorganic phosphates and certain fatty acids. Furthermore, the channel also opens when there is an abundance of reactive oxygen species, which will be explained later. The MPTP channel is closed in times of high NADH, ATP, ADP and high cations such as Mg2+, that can compete with Ca2+ for the MPTP receptors.
When this occurs, since molecules can easily move in and out of the mitochondria along their concentration gradients, the mitochondrial membrane potential is lost and H+ ions and electrons are able to freely flow out of the mitochondria. This results in a loss of the action of the electron transport chain, and thus ATP production via oxidative phosphorylation is severely compromised. As ATP depletion occurs, the cell will eventually undergo necrosis.
Using this information, it is safe to assume that we can limit cellular injury by somehow stopping the opening of this pathologic MPTP. If we examine the structure of MPTP, we can appreciate that a protein named cyclophilin D is present in the structure of MPTP, that is crucial for the proper openng of the MPTP. This protein can be targeted by an immunosuppressive drug, cyclosporine. For example, in cases of ischemia, cyclosporine can act on cyclophilin D to reduce cellular damage initiated by the mitochondria.
2) Increase in Oxidative Stress
As a result of the opening of the MPTP channels, antioxidant molecules such as glutathione, which are typically stored in the mitochondria to combat reactive oxygen species, are now removed from the mitochondria and this allows reactive oxygen species to build up within the cell. Furthermore, improper oxidative phosphorylation by the compromised electron transport chain produces oxygen free radicals and reactive oxygen species. The mechanism by which these species destroy the cell is explained lower down.
3) Induction of Apoptosis
Apoptosis is a fancy way of saying “Cell Suicide.” It is when the cell realizes it is beyond any point of return, and destroys itself. As the mitochondria is being damaged, it begins to sequester between the inner and outer membrane, a number of pro-apoptotic proteins such as cytochrome c and proteins that indirectly activate apoptosis inducing enzymes known as caspases. Thus, as permeability increases, these proteins and pro-apoptotic molecules leak into the cytosol and trigger cell death by apoptosis.
Influx of Calcium and Loss of Calcium Homeostasis
Calcium is a very tightly regulated control molecule. It is involved in the activation of a large number of molecules, and is an extremely important second messenger in the body. As mentioned already, the intracellular concentration of Calcium is extremely low, between 10^-7 and 10^-8 moles. In contrast the extracellular concentration of calcium is around 10^-3 moles, or to be more specific, 13^-3 moles.
Since several regulatory mechanisms are activated by calcium, it is extremely important to keep the concentrations of calcium within the cell at a low level. This is done by pumping calcium that enters the cell into the endoplasmic reticulum (sarcoplasmic reticulum in muscle cells) and mitochondria, to be stored as intracellular stores.
Ishcemia and toxins can initially increases intracellular Ca2+ by an increased release of Ca2+ from intracellular stores, but later due to the implications of reversible injury - namely the interruption of the Ca2+ pump, either by loss of ATP or by disruption of the electron transport chain. This means that calcium flows from the much higher concentration outside the cell to the much lower concentration inside the cell, increasing intracellular Ca2+ and triggering a number of effects:
1) High intracellular Ca2+ is sequestered and stored primarily by the endoplasmic reticulum, and to a lesser extend, the mitochondria. As intracellular Ca2+ rapidly increases, the need for storage by the mitochondria increases. Ca2+ is sequestered by the mitochondria through a mitochondrial Ca2+ uniporter (MCU), which facilitates transport of Ca2+ one way, inside the mitochondria. Usually the Ca2+ moves back into the cell and outside the mitochondria via a separate channel, by diffusing down the concentration gradient using Na+/Ca2+ and Cl-/Ca2+ exchangers, but as the intracellular Ca2+ concentration increases, Calcium does not leave the mitochondria as easily. Thus, Ca2+ concentration within the mitochondria gradually increases, until it is high enough to open the mitochondrial permeability transition pore (MPTP), and cause the cascade that results in severe mitochondrial damage, ATP depletion, reactive oxygen species formation and apoptosis, as discussed above.
2) Activation of a number of Ca2+ dependent molecules.
Remember, it is usually an increase in Ca2+ in the cell in moderate amounts that activates a vast number of proteins and molecules by triggering intracellular signaling. An example of this is the phospholipase C-trigerred Ca2+ release that promotes the activity of Protein Kinase C, a phosphorylating enzyme. Similarly, Ca2+ is involved in the stimulation or activation of phospholipases (which break down membrane phospholipids), proteases (which break down proteins), endonucleases (responsible for DNA and chromatin fragmentation) and ATPase (depletes ATP even further).
3) Induction of Apoptosis
Increase in intracellular Ca2+ directly activates caspases, which trigger apoptosis directly, and also by the increase in mitochondrial permeability (by MPTP activation), which causes the buildup of pro-apoptotic proteins such as cytochrome c, that promotes apoptosis, as explained in the above section
Accumulation of Oxygen Derived Free Radicals
Free radicals are chemicals that have a single unpaired electron in an outer orbit. Cell injury induced by free radicals, particularly oxygen derived free radicals, is a very important mechanism of cell damage in many pathologic conditions. Here is how a free radical looks, below:
Notice that in the diagram above, the “red” free radical has 7 electrons in its outer shell. This means that there are 3 pairs and 1 “lonely” electron. This one lonely electron wants another companion, and the molecule itself wants to feel completed, with no lonely electrons. Therefore, it begins to “attack” other molecules to try and steal an electron from them, to become complete. If the free radical steals an electron from the stable molecule, we say that it oxidizes the stable molecule, while the free radical itself is reduced. To remember this, remember OIL RIG - Oxidation is Loss, Reduction is Gain (of electrons). Thus, by stealing an electron, the free radical becomes reduced, while its victim is oxidized.
You can appreciate what happens here. As one free radical attacks other stable molecules, they, in turn, become free radicals. This is because when an electron is “stolen” from a stable molecule, it in turn has one unpaired electron, or a lonely electron, since its partner was stolen. It’s like one of those revenge movies where the “hero” sets out on a path of destruction. Well, in this case, it is indeed quite destructive, producing a cascade that critically injures the cells by attacking essential proteins, lipids and carbohydrates and nucleic acids.
Reactive Oxygen Species (ROS) are a type of oxygen derived free radical. They are essentially chemically reactive substances containing oxygen, such as oxygen ions and peroxide ions. These ROS are produced in moderate amounts naturally during normal oxygen metabolism, such as during mitochondrial respiration and energy generation, but they are quickly removed by cellular defense mechanisms. Thus, cells can in fact maintain a transient state of low concentrations of ROS without damaging the cell. In fact, ROS actually have functional roles within the body, in cellular signaling, modulation of gene expression, activation of MAP kinases (involved in insulin signaling) and reversible gene modulations. However, an increase in ROS production, or a decrease in ROS scavenging leads to an excess of these oxygen derived free radicals, resulting in a condition known as oxidative stress.
This is usually resisted by cellular defense mechanisms so as long as they are active. An example of a cellular defense mechanism has already been discussed - antioxidants, such as glutathione in mitochondria.
Antioxidants are, exactly as the name says, molecules that resist oxidative stress, or a buildup of oxygen derived free radicals, ROS. Antioxidants both resist the formation of free radicals and scavenge and remove free radicals when they have been formed. It does this by acting as a reducing agent and willingly entering an oxidized state. This is because it is in fact more stable in a state where it has lost an electron. Thus, it donates its electron to the free radical, converting it to a free radical and preventing the continuation of the cascade. It, itself does not become a free radical since it remains stable.
Oxidative Stress is a serious condition, implicated in a very wide number of diseases. Look at all the diseases it is involved in below!
So now that we know what free radicals are, and what diseases they are involved in, we can discuss how exactly these free radicals are formed, and the exact mechanisms by which they carry out their destructive cascade.
Generation of Free Radicals
Free radicals are generated very easily by the body, by a number of ways:
1) Reduction-Oxidation reactions that occur in normal metabolic processes.
Remember that as part of normal oxidative phosphorylation, molecular oxygen (O2) is the final electron acceptor at the end of the electron transport chain. Oxygen itself as an atom has 6 electrons in its outer shell (3 pairs of electrons), and Hydrogen only has 1 electron to donate. Usually, 2 H molecules donate their 2 electrons to one Oxygen, and 2 H molecules to the other oxygen (because O2), to produce 2 molecules of water.
This conversion is usually catalyzed by a number of oxidative enzymes throughout the cell. During this conversion, intermediate oxygen-derived free radicals are produced based on incomplete donation of electrons to the oxygen. For example if only 1 electron is donated to the oxygen, then there are 3 pairs and 1 lonely electron, and the oxygen becomes a free radical called superoxide anion (O2-) . Superoxide anions are in fact, the main method by which cells exert oxidative defenses that destroy pathogens, phagocytosed material and fragments of necrotized cells.
If 2 electrons are donated, 1 to each oxygen atom, then hydrogen peroxide (H2O2) is formed.
If 3 electrons are donated, 2 to one oxygen atom, and 1 to another, then while one oxygen is stable, the other oxygen become a free radical bound to only 1 hydrogen atom, a hydroxyl ion radical (•OH). This •OH radical is in fact, the most reactive radical. Note that when H2O2 is produced in excess, it is converted to •OH by reaction with the superoxide anion. This is known as the Haber-Weiss Reaction.
2) Absorption of Radiant Energy
Radiant energy, or radiation, includes X-rays and UV rays. Ionizing radiation is simply radiation that carries enough energy to liberate and remove electrons from atoms and molecules, thereby ionizing them and generating free radicals. Ionizing radiation of this sort can hydrolyze water into •OH and H radicals.
3) Inflammation
During the normal process of inflammation, rapid bursts of ROS are actually produced in activated leukocytes as a mechanism of self-defense. This mechanism is a mechanism of inflammation, and will be explained when I talk about Inflammation. Some intracellular oxidases such as xanthase oxidase can produce free radicals, particularly, superoxide anion, (O2-). Furthermore, oxidases in peroxisomes particularly produce large amounts of H2O2.
4) Enzymatic Metabolism
The enzymatic metabolism of drugs and exogenous substances can produce free radicals that are not technically oxygen derived, and thus not ROS, but are still extremely dangerous. eg. CCl4, metabolized can form •CCl3).
5) Transition Metals
It is possible for transition metals such as Iron and Copper to contribute to the formation of free radicals. This is because they can very easily accept or donate electrons during chemical reactions. The most important example of this is the Fenton Reaction, that occurs between Fe2+ (Fe, hence Fenton Reaction) and H2O2, that produces •OH radicals.
There is also a photo-Fenton reaction, used in industries since Fe3+ is more easily obtainable. This reaction involves using photons of light as catalysts, but the important factor to consider is the ultimate generation of the hydroxyl radical. The Fenton reaction usually is also promoted by the effects of the superoxide anion (O2-), and thus, high amounts of superoxide anion and Fe2+ promotes the formation of free radicals.
6) Nitric Oxide (NO)
This one just spells bad news. The chemical starts out as literally having the chemical symbol, “NO”. NO, a potent vasodilator, is produced by epithelial cells, macrophages, neurons and other cell types, and can individually act as a free radical. However, the main way in which NO exerts oxidative stress is by reaction with superoxide (O2-) to form peroxynitrite anion (ONOO-).
Yep, NO produces ONOO- (Oh noooo!). You do not want NO being a free radical in your body, especially because the peroxynitrite anion radical attacks a wide variety of molecules, including DNA and proteins. Oh no!
Let us quickly talk about the most damaging of these radicals, the Hydroxyl Radical (•OH). The •OH radical is produced, in summary, by 3 mechanisms:
1) Radiolysis (via ionizing radiation) of water into •OH and H.
2) Fenton Reaction
3) Haber-Weiss Reaction.
Removal Of Free Radicals
Before we get into the mechanism of how free radicals are removed, and the body keeps safe from free radicals, remember that free radicals are very unstable, simply because they really want to find a partner for their lonely electron. Because they are so unstable, free radicals often decay spontaneously simply due to the extremely high amount of energy they carry, The decay free radicals undergo is called dismutation.
Dismutation is a process whereby both the oxidized and reduced form of a species are produced simultaneously. For example, the superoxide anion (O2-) usually decays to O2 and H2O2 in the presence of water. This is a dismutation. To explain this, first refer to the diagram below:
Essentially, one superoxide anion donates its extra electron to another superoxide anion, as depicted above. Thus, the donator superoxide is oxidised (recall OIL RIG), and the receiver superoxide is reduced, becoming a peroxide ion (O2 2-). This peroxide anion is stabilized by reacting with 2 H+ ions, forming hydrogen peroxide. This explains the dismutation reaction.
Besides the normal dismutation of free radicals, the body also has natural defenses when the number of free radicals becomes elevated and uncontrollable by simple spontaneous decay. These methods include:
1) Antioxidants
Antioxidants have been explained above, so I can now be specific regarding which antioxidants are used within the body. As mentioned above, antioxidants either resist formation of free radicals entirely, or act as free radical scavengers that remove them after they have been formed. Commonly used antioxidants within the body are:
Vitamin E (alpha-tocepherol): This is a terminal electron acceptor, and thus reacts with free radicals to accept their extra electron, thereby blocking free radical chain reactions. Since this is a fat soluble vitamin, it mainly exerts its effects in the lipid membranes, protecting them from a phenomenon that will be explained below known as lipid peroxidation.
Vitamin C (ascorbate): This reacts directly with O2, •OH and other products of lipid peroxidation to remove them. It also serves an important role in Vitamin E regeneration, thereby preventing lipid peroxidation indirectly. Because it is water soluble, it cannot exist in the membranes and exists instead, in the cytosol.
Retinoid (precursors to Vitamin A): These are also lipid soluble, and function in similar ways to Vitamin E, acting as chain breaking antioxidants.
Glutathione: This is a reducing agent, and thus reduces or donates an electron to the free radical to make it stable. In this way, it is converted to glutathione disulphide (GSSG), as it is oxidised itself.
NO: I know we said that NO is bad, but being a potent vasodilator, it has its very important uses. It is generated as a product of an enzyme known as Nitric Oxide Synthase (NOS). NO can increase proteasomal activity, and thereby decrease cellular intake of Fe2+ by acting on the transferrin receptor, one of the receptors that intakes Fe2+, thus reducing the production of •OH by the Fenton reaction.
2) Transition Metals
It has already been mentioned that transition metals can easily form free radicals. In normal circumstances however, transition metals that are taken up and stored by being bound to a multitude of storage and transport proteins. These include Transferrin (transfers iron in blood plasma), Ferritin (stores iron in tissues), Lactoferrin (binds iron in milk) and Ceruloplasmin (stores Copper, and Iron to a lesser extent.) These transport proteins greatly reduce the availability of transport proteins, thereby preventing free radical production.
3) Enzyme Cascade
There is a cascade of 3 important enzymes that are especially important in acting against oxygen derived free radicals.
1) Superoxide Dismutase: We have already discussed dismutation of O2- above. That is exactly the reaction that this enzyme, Superoxide Dismutase (SOD) catalyses. Thus, SOD converts O2- into H2O2, via the reaction illustrated below:
There are different variations of the SOD enzyme, each of which uses a different metal in their active site. Manganese associated SOD (Mn-SOD) is associated with the mitochondria. Copper and Zinc associated SOD (CuZn-SOD) is associated with the cytosol.
2) Catalase: Catalase is located within peroxisomes, an organelle within the cytosol.
Catalase is especially important for the removal of H2O2, which it does by decomposing it into water and oxygen via the reaction:
3) Glutathione Peroxidase:
We already mentioned the role of glutathione, as a reducing agent. When Glutathione (GSH) acts as a reducing agent, it, itself is oxidized to Glutathione Disulphate (GSSG). Glutathione peroxidase catalyses the general breakdown of free radicals H2O2 and •OH using glutathione. It reacts via two reactions:
Notice how GSH is being converted to GSSG in the presence of radicals. Thus, the intracellular ratio of GSH:GSSG is a very important indicator of the number of free radicals within a cell. A high ratio of GSH:GSSG (high GSH) means low oxidative stress, while low GSH:GSSG (high GSSG) means high oxidative stress.
Pathologic Effects of Free Radicals
There are 3 major ways in which free radicals cause irreversible damage to cells:
1) Lipid Peroxidation In Membranes
Lipid peroxidation is the oxidative degradation of lipids. This is most cell damaging in the membranes. In this reaction, free radicals ‘steal’ electrons from the molecules in the membranes, causing the lipid itself to now become a lipid radical. It consists of initiation, propagation and termination. Lipid peroxidation is initiated by ROS, particularly, •OH, by reacting with polyunsaturated fatty acids within the membrane. Since ROS generate lipid radicals, which are highly unstable, these lipid radicals react with oxygen to form a peroxyl-fatty acid radical. These peroxides themselves are very unstable, and reacts with other lipids to produce a free radical and lipid peroxide. Thus, it forms a free radical chain reaction, and this step is known as propagation. Finally, termination occurs when two radicals react with each other, and cancel each other out. This is a termination reaction. Molecules that react with these radicals to terminate the reaction include Vitamin E and retinoids, as discussed before.
Typically, the end products of lipid peroxidation are reactive aldehydes, such as malondialdehyde (MDA) and 4-hydroxynonenal (HNE). HNE in particular, is a marker of lipid peroxidation, and high concentrations of HNE indicate lipid peroxidation. If membranes, particularly membranes of the lysosome and outer cell membrane are damaged, then enzymes within the lysosome can completely destroy the cell, or ECF contents can destroy the cell and disrupt calcium homeostasis.
2) Oxidative Modification of Proteins
We already know that free radicals want another electron, so they can be complete. They therefore go around stealing electrons from other molecules. A molecule that has an electron “stolen” has lost an electron, and thus becomes oxidized. In proteins, this is a real problem, as there are several important proteins that allow our cells to stay alive.
Thus, free radicals can promote the oxidation of amino acid side chains, oxidation of the protein backbone and the formation of new protein-protein bonds such as disulphide bonds, by donating the high energy to bond formation.
This oxidation of proteins may alter the active site, rendering the protein dysfunctional if it is an enzyme, and misfolding, raising proteasome activity and consequent protein breakdown. It also can disrupt structural proteins, resulting in a collapse of the structure of the cell, destroying the cell completely.
3) Lesions in DNA
Free radicals are capable of causing single- and double-strand breaks in DNA, cross-linking of DNA strands, and formation of adducts. This sort of disruption in DNA can cause malignancies, and is also implicated in cellular aging.
(courtesy Robbins and Cotrans Pathologic Basis of Disease)
It is still important to note that free radicals, while mostly cause death by necrosis, can also cause cellular death by apoptosis. Taking O2- (superoxide anion) into special consideration, this free radical actually is known to activate several degradative enzymes, indicating that it may be involved in apoptosis.
Defects In Membrane Permeability
Early loss of selective membrane permeability is a common feature of necrosis, not apoptosis. Eventually, overt or wholesale damage to the membrane occurs. Also note that all cellular membranes may be damaged, including those in lysosomes, peroxisomes and the cell membrane of the cell.
Mechanism of Membrane Damage
Membranes are damaged in different ways, based on different pathologies. For example, during ischemia, membrane damage may occur due to ATP depletion and the activation of phospholipases due to disruption of Ca2+ homeostasis. The plasma membrane can also be damaged due to bacterial toxins, viral proteins, lytic complement components and a variety of physical and chemical components.
All these pathologies however, follow a similar biochemical mechanisms to disrupt the membrane:
Reactive Oxygen Species: These ROS carry out lipid peroxidation to damage the membrane, as explained above.
Decreased Phospholipid Synthesis: The production of phospholipids in cells may be disrupted due to hypoxia and ischemia, or defective mitochondrial function via damage to the mitochondria. This is because ATP depletion caused by these occurrences affects energy dependent biosynthetic pathways, including the synthesis of phospholipids. Thus, the production of membranes for a wide variety of organelles, including mitochondria themselves, is disrupted.
Increased Phospholipid Breakdown: As mentioned above, traumatic cell injury can disrupt Ca2+ homeostasis, raising intracellular Ca2+ and activating Ca2+ dependent phospholipases, that break down and degrade membranes. This sort of breakdown leads to the formation of lipid breakdown products. These lipid breakdown products include molecules such as unesterifed fatty acids, acyl carnitine, lysophospholipids, which have a detergent effect on membranes. This means that the membranes become more soluble in water, and are thus more easily disrupted. Furthermore, lipid breakdown products may also insert into the cell membrane themselves, altering membrane permeability and electrophysiologic alterations.
Cytoskeletal Abnormalities: Cytoskeletal filaments serve as anchors, connecting the plasma membrane to the cell interior.
Upon disruption of Ca2+ homeostasis and influx of Ca2+, Ca2+ dependent proteases are activated that destroy and break down cytoskeletal filaments. Furthermore, upon cellular swelling, the first sign of cell injury, the cytoskeletal filaments may burst or be torn from the membrane, causing cellular swelling to occur much more easily, and cell shape to be much more easily disrupted or ruptured.
Consequences of Membrane Damage
Answering this question is actually quite easy. Just think about some organelles whose membranes are very important, and think of what will happen without them. There are 3 that should come to your mind: The cell membrane itself, lysosomes and mitochondria.
Cell Membrane: As mentioned earlier, damage to cell membrane results in a loss of osmotic gradient, and disruption of ion channels that usually regulate internal ionic concentrations. Furthermore, damage to the cell membrane can eventually cause the loss of cell contents, as well as metabolites that usually allow ATP regeneration, resulting in energy depletion.
Lysosomes: We already know about how dangerous the lysosomal enzymes are. They are all acid hydrolyses, and contain numerous enzymes that destroy various components of the cell, such as Nucleases, proteases, lipases, phosphatases, DNases, RNases, just to name a few. Release of these enzymes can destroy the entire cell from the inside, and the cell dies by necrosis.
Mitochondrial Membrane: Mitochondrial membrane damage, as discussed above, activates the mitochondrial permeability transition pore (MPTP), which causes ATP depletion due to deceased ATP synthesis, and the trigger of apoptosis.
Detection of Cell Injury
There is a very important technique of detecting, en masse, if a region of the body is undergoing irreversible cell injury and necrosis. Since membranes are damaged, tissue-specific proteins can leak into the extracellular fluid. For example:
Acute heart injury can be indicated by the presence of an isoenzyme located only in the heart, CPK-MB (creatine kinase, MB variant), and contractile protein troponin.
Liver/Bile Duct Damage can be detected by the elevated presence of enzymes Alkaline Phosphatase (ALP) together with Gamma Glutamyltransferase (GGT).
Damage to hepatocytes of the liver is indicated by increased concentrations of transaminases.
Damage to Proteins and DNA
Usually, cells have mechanisms to repair DNA after damage. However, if damage to DNA is too severe, by DNA damaging drugs, ROS or radiation, then the cell triggers a suicide program, apoptosis.
Similarly, if enough proteins are damaged and misfolded, then the cell also triggers apoptosis. Apoptosis will be explained in the next topic in Pathology I cover.
Finally, that took a long while to do. Hope you guys like it! I’ll do reperfusion injury and hypoxia in the next post, followed by Apoptosis. Thank you for your continued support, and I really hope this is helping at least some of you out there.
VIDEOS:
Dr. Rabiul Haque (0:00 - 36:18)
Robbins Pathology AudioBook (30:32)
QUESTION:
1) Absorption of radiant energy can cause radiolysis of water and produce dangerous radicals. Which of the following enzymes protect and prevent the dangerous action of radicals produced by radiolysis of water?
Sorry about the lack of posts over the past couple days guys! I've been a little busy, but I will have something ready for you all tomorrow - Mechanisms of Irreversible Cell Injury, where I'd be explaining things like Free Radicals, consequences of ATP depletion and mitochondrial damage, and disruption of Calcium homeostasis. Looking forward to seeing your responses. And I really appreciate all the positive feedback I've gotten, it really motivates me to help out even more of you guys. Thank you for your hopefully continued support!
Did you ever get so hungry, that you ate yourself? (No would be the right answer). But sadly, your body does indeed eat itself, just at a cellular level. This phenomenon is autophagy, which literally translates to “self-eating.” [Auto- self; phagy- eating]
Autophagy is a process in which a cell eats its own components. It is the basic catabolic mechanism that involves cell degradation of unnecessary or dysfunctional cellular components through the action of lysosomes. It therefore involves the delivery of cytoplasmic materials to the lysosome for degradation.
We did discuss this already under, “Waste Disposal in Cells” as being one of the mechanisms of action of lysosomes, and a very important one at that.
You must still be wondering though - Why would my cell eat components of itself? What even?
Well, truth of the matter is, every component within a cell has its own needs. The more organelles within the cell, the greater its metabolism since more energy would be needed to sustain the cell. So when there is a deficiency of nutrients, or an inability to sustain this high metabolism by the body, the cell begins to destroy components within itself so as to decrease the metabolism to levels the body can afford to supply. We say this is decreasing nutrient demand to match nutrient supply.
Classes of Autophagy:
Needless to say, there are several different organelles that can be consumed via autophagy, and this leads to different types of autophagy, very easily explained:
I bet this was easier than you thought it would be.
Yes, if autophagy involves a mitochondrion, it is called mitophagy, a peroxisome - pexophagy, protein aggregates - aggrephagy, glycogen - glycophagy, lipids - lipophagy.
The only one that will need special attention is xenophagy, which will be explained in detail later.
Types of Autophagy:
Note the difference here, between the classes of autophagy and the types of autophagy, as they refer to completely different things. While the classes of autophagy refer to the particular substance broken down in the autophagy process, the type of autophagy refers to the mechanism by which autophagy occurs.
With this in mind, it can be said that there are 3 types of autophagy:
Basics of Pharmacology I - Characteristics of Drugs
Pharmacology, oh pharmacology. Truly, quite a challenging field to master. Do I really have to know ALL these drug names?
But not to worry, mastering pharmacology comes with mastering the basics of this field.
For those of you who don’t know, pharmacology is a very broad term that refers to the study of substances that interact with living systems through chemical processes. These chemical processes can be triggered especially due to two major mechanisms:
Activating or inhibiting normal body processes.
Binding to regulatory molecules.
Furthermore, these substances that induce these chemical reactions can either have:
Therapeutic Effects, by disrupting some negative process occuring in the patient.
Toxic effects, as used by parasites on their host.
That’s right, pharmacology isn’t all the drugs we all assume it to be - it literally includes any substance that can act on the body, therapeutic or toxic.
Thus, it is important to specify what we mean exactly, by pharmacology.
The study of the therapeutic portion of pharmacology is referred to as medical pharmacology, which is the science of substances used to prevent, diagnose and treat a particular illness.
In contrast, the study of the toxic or undesirable effects of substances on individual cells or entire ecosystems is toxicology.
However, remember that although toxicology probably makes you think about poisons and parasites, any substance, even those intended to be therapeutic drugs, can be toxic in high amounts.
Just as an additional point of note, different drugs can actually have marginally (or majorly) different effects on the body based on each individual’s genetics. The study of the relationships of drugs in pharmacology to the genetic makeup of an individual is known as pharmacogenomics.
Now that we’ve covered some general terms, let’s talk about the real meat of pharmacology.
Nature of Drugs
We can define a drug as any chemical that brings about a change in biologic function through its chemical actions. Compare this to the definition of pharmacology! The “substances” referred to in the definition of pharmacology are in fact, drugs, be it therapeutic or toxic.
Any substance or drug can act either as an agonist or an antagonist. An agonist is a substance that acts as an activator, or promotes activity of a specific regulatory system or body process. An antagonist is a substance that acts as an inhibitor, having the opposite effect to the agonist.
But wait, what do these drugs act on? Drugs act on a regulator molecule, known as a receptor, which literally receives the agonist or antagonist molecule, and sends the signal to the body system it regulates, changing it to the liking of the agonist (activate) or antagonist (inhibit).
Pathologic Cell Injury and Cell Death II - Necrosis
Cell Death
As cellular damage continues, the damage eventually becomes irreversible, after which there is no escape from death for the poor cell. When this occurs, the cell will eventually die, by one of two mechanisms:
Necrosis - Pathologic Cell Death
Apoptosis - Physiological (usually) Cell Death
Note that some people also consider autophagy as a method of cell death, but recent studies in fact link this to cell survival.
Necrosis
Necrosis is considered an unregulated and “accidental” form of cell death, caused from exogenous causes associated with damage to cell membranes and loss of ion homeostasis.
Basically, when this damage to membranes is severe, lysosomal enzymes (which we discussed under “Waste Disposal In Cells”), manage to escape the lysosomal membranes and destroy the cell from the inside, causing morphological changes known as necrosis. Furthermore, cell organelles are able to escape via the damaged membrane and into the extracellular space, where they illicit a host reaction, the inflammation associated with necrosis. The lysosomes of leukocytes called in as a result of the inflammatory reaction then also aid in the breakdown of organelles within the cell that have been exuded from it.
Speaking a little more in depth now:
Cell necrosis is heralded by the loss of the function of the cell permeability barrier, or cell membrane in the case of the cell, meaning that the cell cannot control substances leaving or entering the cell. This holds especially true for ions, most important of which are Na+ and Ca2+, for which the concentration gradients are very steep.
Usually, there are 4 factors required for maintaining the selective ion permeability of the cell:
Considerable amounts of ATP
Structural Integrity of the Phospholipid Bilayer/Cell Membrane
Intact Ion Channel Proteins
Normal association of the Membrane with cytoskeletal elements.
If any of these factors are damaged or interrupted, then the cell isn’t able to maintain normal ionic concentrations within and outside the cell. This is the point of no return for the cell - when it is no longer able to regulate internal ionic concentration.
Pathologic Cell Injury and Cell Death I - Mechanism of Reversible Cell Injury
We already know that cells are able to adapt to their surroundings when there is an increased amount of stress to overcome them, just as we will in all our medical exams. If you want a reminder about this, we talked about this in “Cellular Adaptation to Stress.”
But what happens when the cell isn’t able to adapt to its stress and overcome its environment? Alas, it is inevitable that some cells simply are not able to adapt in time, or the stress is far too great to overcome. In this scenario, when cells are stressed so severely that they are unable to adapt to persevere in time, or when they are exposed to some intrinsic abnormality (e.g. genetic abnormality) or inherently damaging agent (eg. virus/bacteria).
From this stage, the cell will be damaged, and will reach a stage of reversible cell injury. If the stress persists, the cell eventually dies, resulting in cell death.
If we were to map out what happens when stress is added to a normally functioning cell, here’s a summary I tried to make for you guys:
Let’s discuss the latter two in detail to fully and completely understand what happens when a cell is stressed.
In this series, we will discuss Reversible Cell Injury, then I will discuss Necrosis, then Apoptosis, and finally, Mechanisms and Causes of Cell Injury.
Yes, Atlas. The Greek titan who held up the celestial spheres.
Wait, we’re talking about medicine, right. Oh well, guess I’ll have to talk about the significantly less cool (although the significantly more real) Atlas of our body, the C1 vertebra. And while we’re at it, let us also complete learning about all the vertebrae in the body, and finish the Axis, C2, as well.
Remember, for a review of the Cervical Vertebrae, you can check out my blog post: https://theartofmed.wordpress.com/2015/05/23/what-is-this-blog-about/
Back to the Atlas and Axis, these two vertebrae are together called the craniovertebral vertebrae, due to their very important articulations with the skull, as will be discussed lower down.
Let us begin with the Atlas for now.
Atlas
The atlas is the 1st Cervical vertebra. It directly articulates with the base of the skull, as shown in this figure below:
Here is the Atlas, colored in green.
What is the most striking thing you notice about it when you compare it to other vertebrae below?
I’ll tell you what I see, I see that the Atlas is somehow much wider than the other vertebrae. It’s as if the C1 vertebra is lifting its hands and holding up the skull, our own celestial sphere, or the globe of our head. It really is like Atlas, isn’t it? Holding out its “hands” and holding the weight of the skull up, just as Atlas Himself did.
It is obvious it is an irregular vertebra, because it’s function is definitely different from the other Cervical Vertebrae.
Structure of Atlas
The atlas is irregular because it contains no body nor spinous process, and is much wider than the other cervical vertebrae.
It can thus be described as a ring-shaped vertebra, with the ring divided into an anterior arch in the front and a posterior arch in the back, each separated from one another by laterally extending structures collectively called the lateral masses, and transverse processes.
Thus, there are 4 main features we need to consider about the Atlas:
Short Anterior Arch
Long Posterior Arch
Lateral Masses
Transverse Processes
Anterior Arch
The anterior arch forms about 1/5 of the ring. The anterior surface is convex and projects outwards, in an anterior direction. It serves as a rudimentary body of the C1 vertebra.
The most anterior part of the anterior arch forms the anterior tubercle.
This anterior tubercle, shown above in green, is extremely important for the attachment of a ligament, the anterior longitudinal ligament.
The anterior longitudinal ligament is a strong, fibrous band that runs along the anterolateral aspects of vertebrae and IV discs, extending superiorly from the pelvic (anterior surface) surface of the sacrum to the anterior tubercle of the Atlas. The anterior longitudinal ligament is thin over the vertebrae, but thick over the IV discs, and functions as the only ligament to prevent hyperextension of the vertebral column. The anterior longitudinal ligament also extends superiorly above the anterior tubercle, to connect to the anterior surface of the foramen magnum as the anterior antlanto-occipital membrane, a membrane attached to the sueprior border of the anterior arch. Furthermore, the region of the anterior longitudinal ligament that connects the atlas to the axis is the anterior atlanto-axial ligament, a membrane attached to the inferior border of the anterior arch. Both of these will be explained later.
On either side of the midline, the anterior tubercle also gives attachment to the superior oblique part of the longus colli muscle. This it the whole muscle here:
The longus colli muscle is greek for long muscle of the neck, and we can see why: The longus colli muscle spans superiorly from the anterior tubercle of the atlas, as stated before, inferiorly until the T3 vertebra.
Looking at the diagram, it appears to be somewhat fusiform shaped, i.e, wide in the middle, and pointed on either end, somewhat like this:
With this in mind, the longus colli muscle is divided into 3 parts:
1) Superior Oblique Portion: The superior oblique portion originates from the anterior tubercles of the transverse processes (remember the anterior tubercles of the normal vertebrae are located on the transverse process. The anterior tubercle of the Atlas is in the midline) of C3, C4 and C5 vertebrae. It ascends obliquely (hence superior oblique) and in a medial direction, and attaches to the anterior tubercle of the atlas.
2) Inferior Oblique Portion: The inferior oblique portion is the smallest part of the longus colli muscle. It originates from the front of the bodies of T1, T2 and sometimes T3. You can visualize this in the diagram above. It ascends obliquely and laterally inserts to the anterior tubercles of the transverse processes of C5 and C6.
3) Vertical Portion: The vertical portion literally ascends vertically, originating from the anterior part of the bodies of C3/C4 - C5 and T1-T3. After ascending vertically, it inserts into the anterior portion of the bodies of the C2-C4 vertebrae.
On the other hand, the posterior surface of the anterior tubercle holds an oval shaped facet. This facet attaches a structure known as the odontoid process/dens of the axis, a long tooth-like projection that extends from the C2 vertebra.
Notice where the label is in the diagram above.
Now look at how the odontoid process looks below:
Posterior Arch
The posterior arch forms about 2/5 of the ring structure. It extends posteriorly until the posteriormost tubercle, the posterior tubercle of the atlas. The posterior tubercle serves as a rudimentary spinous process of the C1, since no proper spinous process exists.
Posterior Tubercle of Atlas:
Looking at the diagram above, you can see the posterior arch is larger, and ends in a posterior tubercle as its most posterior region, in the midline. The posterior tubercle gives attachment to a ligament, known as the ligamentum nuchae.
The ligamentum nuchae is a superior continuation of the supraspinous ligament.
The supraspinous ligament is a thick, cord like fibrous band that connects the apices of the spinous processes, from C7 to the sacrum. It is continuous with the ligamentum nuchae above C7, and it continuous with interspinous ligaments between the spinous processes. The interspinous ligaments and thin, fibrous membranes that connect adjacent spinous processes together, by running obliquely from the root of the spinous process to the apex. They are continuous with the supraspinous ligament posteriorly, and continuous with a ligament known as the ligamentum flavum anteriorly (we discuss this ligament below). The interspinous ligaments are narrow in the thoracic region, but thicker in the lumbar region. In the cervical region, they are very underdeveloped, and are considered part of the ligamentum nuchae. We will see why just now.
Both the supraspinous ligament and the interspinous ligament prevent hyperflexion of the vertebral column.
Now, it is important to remember that C7 is the vertebra prominens, with the largest spinous process of the body. Beyond this point, the supraspinous ligament is not able to change its direction so as to directly come into contact with the spinous processes of vertebrae C6 and above. Hence, instead, the supraspinous ligament gives off extensions that spread to the spinous processes of the C1- C6 vertebrae and this forms a median, fibro-elastic band known as the ligamentum nuchae.
The ligamentum nuchae extends superiorly up till the external occipital protuberance of the skull, visible in the diagrams above. The important thing about this ligament is its location. Because it is located in the midline, and it is located over the very short and deeply located spinous processes of the C1-C6 vertebrae, it:
Compartmentalizes the neck into a left and right compartment.
Allows attachment for muscles inserting into the midline above the C7 spinous processes. These muscles are the splenius capitis and upper fibres of the trapezius.
[Note that although the semispinalis captious and splines cervicis are labelled here, the semispinalis capitis is inserted deep to the ligamentum nuchae, and splenius cervicis is inserted below the ligamentum nuchae.]
On either side of the posterior tubercle is the insertion of the rectus capitis posterior minor muscle.
Furthermore, the inferior surface of the posterior tubercle attaches a very small, deep muscle of the back, the interspinalis cervicis.
Don’t worry about all the muscles you’re seeing here, when we cover muscles of the neck, we’ll go through these in great detail.
Superior Surface of Posterior Arch:
The superior surface of the posterior arch contains a groove for the vertebral artery and C1 spinal nerve (suboccipital nerve). This groove is located just adjacent to the lateral masses of the Atlas.
Directly behind this groove, the superior surface of the posterior arch provides attachment to the posterior atlanto-occipital membrane.
The posterior atlanto-occipital membrane is a broad, thin fibrous membrane that extends from the superior surface of the posterior arch, superiorly till the posterior margin of the foramen magnum.
As seen in the diagram above, there is a defect in the inferolateral part of the membrane that allows the passage of the C1 spinal nerve and vertebral artery.
Inferior Surface of the Posterior Arch:
The inferior surface of the posterior arch provides attachment for the posterior atlantoaxial membrane. It is a broad, thin ligament that attaches superiorly to the inferior surface of the posterior arch, and inferiorly to the superior surface of the lamina of the axis, the C2 vertebra.
Both the posterior atlanto-occipital membrane and posterior atlanto-axial ligament are continuations of the ligamentum flavum. The ligamentum flavum is a yellow, elastic ligament that extends between the pedicles of the vertebrae. It begins inferiorly at the 1st Sacral segment below, and extends up till the Body of the C2 Vertebra, the Axis. Above the C2 vertebra, it forms the posterior atlanto-axial membrane till the Atlas, and the posterior atlanto-occipital membrane from the Atlas to the Occipital Bone.
These ligaments extend almost vertically from one lamina to another, and they meet and blend with the ligamentum flavum on the opposite side along the midline. They are thinnest in the cervical region, but get thicker in the thoracic region, and thickest in the lumbar region. The marked elasticity of the ligamentum flavum keeps the vertebral column rigid and fixed in place, preventing abrupt flexions of the vertebral column that may damage it.
Lateral Masses
The lateral masses are the most bulky part of the Atlas, very large (like the muscles of Atlas) in order to support the weight of the globe of the head, which is our own celestial sphere.
The superior view, seen above, shows the lateral mass, labelled. The superior part of the lateral mass shows a kidney shaped, concave superior articular facet, that articulates with the occipital condyles of the base of the skull, to form the atlanto-occipital joint.
The “ ? “ above shows the occipital condyles, also kidney shaped, but convex or protruding outwards. Notice then that it shows a perfect complement to the Atlas. No wonder our head sits so well, Atlas is doing a great job.
This atlanto-occipital joint is formed between the occipital condyles and the superior articular facet of the lateral mass of the Atlas. The joint is a condyloid type synovial joint. Look at how the joint is shaped:
The joint functions like a seesaw:
Imagine each end of the seesaw is the direction in which you move your head. The little pivot, or triangle in the middle in this case is the Atlas, and it will allow the seesaw to move towards the direction more weight is applied in. Thus, if you want to say yes, and move your head forwards then backwards, it is the atlanto-occipital joint that allows this movement, by moving the occipital condyles within the superior articular facet of the lateral mass. We can thus say the atlanto-axial joints allow a ‘yes’ motion (extension and flexion of the neck) and also allows a small amount of lateral flexion (sideways tilting, as in confusion).
Medial Surface of Lateral Mass:
The lateral mass contains a small tubercle for the attachment of a ligament known as the transverse ligament of the atlas, that extends from the medial surface of one lateral mass to the medial surface of the lateral mass on the opposite side.
Notice that the transverse ligament of the atlas bounds the odontoid process posteriorly. This is known as the median atlanto-axial joint, formed between the odontoid process, the posterior surface of the anterior tubercle, and the transverse ligament of the atlas. This is a pivot type synovial joint. The black arrows in the diagram very effectively illustrate what kind of movement we can expect from the median atlanto-axial membrane.
To picture this, imagine Emma Stone playing with a hula hoop. The hula hoop swings around Emma’s body in circles as she play with it.
Superimposing that onto the median atlanto-axial joint, the odontoid process or dens is Emma Stone, and the entire Atlas is the hula hoop, able to swing all the way around the odontoid process! But thankfully it doesn’t. It is restricted by a joint described immediately below, that restricts the degree of rotation allowed by the median atlanto-axial joint. Thus, the main motion allowed by the media atlanto-axial joint is rotation, allowing us to say “no” with our heads. The transverse ligament of the atlas is very important so as to keep the odontoid process in place.
Inferior Surface of Lateral Mass:
The inferior surface of the lateral mass contains the inferior articular facet. This inferior articular facet is nearly circular, is more or less flat, and articulates with the superior articular facets of the Axis, to form the lateral atlanto-axial joint.
Notice how round the inferior articular facet looks from the inferior surface. It forms 2 lateral atlanto-axial joints, and these joints are gliding synovial joints, that function mainly to restrict the degree of motion of the median atlanto-occipital joint.
There are also a number of ligaments associated with the atlanto-axial joints that restrict the degree of movement of the odontoid process and atlanto-axial joints. These ligaments are the apical ligament, alar ligament, and cruciate ligament.
The apical ligament is a thin, string like ligament that extends from the very apex of the odontoid process and attaches just onto the clivus of the occipital bone, passing through the foramen magnum. The clivus is a region located on the inside of the skull, on the basilar part of the occipital bone. It is shown in blue below, ignoring all the lines:
It prevents the odontoid process from moving too inferiorly in relation to the Atlas.
The alar ligaments are a pair of cord like ligaments that extend from each side of the odontoid process, and attaches to the medial ends of the occipital condyles. It prevents lateral displacement or sideways displacement of the odontoid process.
The cruciate ligament is actually a collection of 3 ligaments. These are the transverse ligament of the atlas, superior longitudinal band, and inferior longitudinal band. We have discussed the transverse ligament of the atlas already. The superior and inferior longitudinal bands are simply superior and inferior extensions from the middle of the transverse ligament of the atlas. This results in a cross-shaped structure of ligaments, and hence the name, cruciate (cross like). The superior longitudinal band extends superiorly from the transverse ligament of the atlas, attaching on the clivus deeper through the foramen magnum than the apical ligament. The inferior longitudinal ligament extends inferiorly from the inferior border of the transverse ligament of the atlas towards the body of the axis, where it attaches.
[The superior longitudinal band is blocking the apical ligament from view].
The anterior surface of the lateral mass also attaches a muscle, the rectus capitis anterior muscle. This muscle is a short, flat muscle of the anterior neck situated immediately behind the longus capitis muscle. From its origin on the anterior surface of the lateral mass of the atlas, it ascends obliquely in a medial direction to insert in the inferior surface of the basilar part of the occipital bone, on the anterior margin of the foramen magnum. This muscle actually is the muscle that helps you look down! So it is a flexor of the head.
Note in the picture above, that the longus capitis muscle is cut off on the left side, and hence does not block the rectus capitis anterior anymore.
Transverse Processes
The transverse processes are the lateralmost structures of the Atlas, extending laterally from the C1 vertebra’s lateral masses. It is actually quite long, and can be felt between the angle of the mandible and mastoid process. It indeed, contains features typical and expected of the transverse processes of the regular cervical vertebrae. This means that the transverse process still contains the most characteristic feature, the foramen transversarium, that transmits the vertebral artery.
Recall that there is also a groove on the superior surface of the posterior arch just adjacent to the lateral masses. Thus, we can see how the vertebral artery travels along the C1 vertebra: It travels through the foramen transversarium and then immediately enters the groove for the vertebral artery on the superior surface of the posterior arch. From here, it travels through the defect in the posterior atlanto-occipital membrane.
Notice it travels through the foramen transversarium, then over the groove, then straight through the membrane in the picture above.
It also provides attachment to a large number of muscles.
These muscles include:
1) Rectus Capitis Lateralis: Has origin from superior surface, anteriorly, of transverse process of C1, and inserts into the jugular process of occipital bone.
2) Obliqus Capitis Superior Muscle: Has origin at superior surface, posteriorly of transverse process of C1, and inserts into the lateral half of the inferior nuchal line.
3) Obliqus capitis Inferioris Muscle: Originates from the apex of the spinous process of the C2 vertebra (Axis) and inserts into the posterior surface of the transverse process of C1, close to the tip.
4) Levator Scapulae Muscle: This muscle originates from the posterior tubercles of the transverse processes of C2-C4 and the lateral margin and lower border of the transverse process of C1, the atlas. It inserts into the upper part of the medial border of the scapula, and the superior angle, as discussed under “Scapula.”
5) Splenius Cervicis: This muscle originates from the spinous processes of T3-T6, and inserts into the posterior tubercles of C2 and sometimes C3, and the transverse process of C1.
6) Scalenus Medius: This muscle originates from the transverse process of C1-C6, and inserts into the upper surface of the 1st rib.
7) There are an additional two deep instrinsic muscles of the back, the intertransversarius posterior cervicis and intertransversarius anterior cervicis, attached to the transverse process.
Occasionally, the transverse process sometimes fuses with the occipital bone, a process known as occipitalization, and this is a serious condition that can compress the spinal cord.
Vertebral Canal
We already know the vertebral canals of cervical vertebrae are large. However,the vertebral canal of the C1 vertebra is divided into two compartments by the transverse ligament of the atlas. As visualized above, the odontoid process passes through the anterior compartment. It is the spinal cord that passes through the posterior compartment.
Thus, if the transverse ligament of the atlas is ever torn, the odontoid process may compress the spinal cord causing death.
Ossification of Atlas
The atlas is ossified from 3 centres, by endochondral ossification.
One appears in each lateral mass by the age of around 7 weeks, and extends posteriorly to form the posterior arches. By the age of 3-4, they fuse in the midline to form a posterior tubercle either by fusion, or by the use of a cartilage medium formed by a separate centre.
At birth, the anterior arch consists of only cartilage, but produces an ossification centre, which converts the anterior arch to bone.
