The Hundred Trillion Stories in Your Head Santiago Ramón y Cajal, “the father of modern neuroscience,” visualized the brain by imagining “neurons as protagonists in an intense cerebral drama.” The Hundred Trillion Stories in Your Head thumbnail courtesy of theparisreview.org
Mouse blastocyst greatly enlarged with electron microscopy. Individual cells can be seen.
We have considered in previous posts the very early stages of embryonic development before implantation called the preimplantation period. The embryo is properly called a “zygote” from the point following fertilization and fusion of genetic material from Mom and Dad up until there are multiple cells. After there are multiple cells the term embryo (from Greek, “young one”) is typically used as a general name for these early stages. In these posts we discussed cell divisions that produced the multicellular embryo and referred to those cells as “blastomeres.”
But what is a cell?
We will now embark on a brief tour of this most basic unit of our body, what’s in it, what it does, and how it divides. Later we will consider why and how cells lose control and instead of making normal body structures make cancer.
The basic building block of all tissues, the stuff that we are made of, is the cell. A cell is essentially a bag of highly refined machines that work together to acquire nutrients, produce energy and perform the tasks that define the tissue. We have hundreds of trillions of cells arrayed in 200 tissues in our adult bodies.
We begin as just two cells: an egg from Mom and a sperm from Dad.Those two types of cell are as different from each other as any two cells in the body. For starters the egg is one of the largest cells measuring 100 microns in diameter. [A micron, abbreviated as µm, is one millionth of a meter (0.000001 meters) or 0.00004 inches.] That seems pretty small, but actually as cells go it is quite large and is near the limit of our ability to see with the naked eye. The sperm on the other hand is really tiny. The sperm has a mobile tail and moves around if in liquid. These main differences and a few others raise important issues about what cells are and where they came from [for some good reading on this subject check out What Is Life? by Lynn Margulis and Dorian Sagan, UC Press 1995].
But let's consider one crucial part of the cell first: The membrane.
The membrane is a bag or limiting wall that contains the cytoplasm, nucleus and mitochondria, the main organelles that make up a cell. [There are some exceptions to this though, red blood cells lose their nucleus and perform their main function of supplying oxygen to tissue without one.] The membrane is richly endowed with proteins which perform many functions, but one of the most important is transmitting signals to the inside of the cell from other parts of the body or the external environment.
A space-filling model of a membrane phospholipid showing its hydrophilic head and a hydrophobic tail.
The membrane is reminiscent of a soap bubble but much more stable. Membranes comprise phospholipid a very special fatty substance that naturally forms two tightly opposed layers called not surprisingly a bilayer. This is a chemistry which is critical to life, the inside of the bilayer avoids water it is “hydrophobic” while the outside can exchange chemicals that are soluble in water, it is “hydrophilic.”
The cell membrane is made up of a phospholipid bilayer. The hydrophobic tails of each phospholipid interact with each other while the hydrophilic heads define the surface of the cell's interior and exterior.
Protein molecules poke through the entire bilayer structure creating a continuum from the outside of the cell to the inside. These proteins can fit in one way, their structures have a hydrophobic part and a hydrophilic part that match the properties in the bilayer so that they orient themselves just right. Sometimes the proteins link together in cylinder or lifesaver shape that creates a physical channel through the membrane. This allows critical elements to be moved, transported, through the membrane. [More information of the subject can be found in the definitive text book by Lon J. Van Winkle, Biomemebrane Transport, Academic Press 1995].
Some proteins line up across the membrane and form channels that allows small molecules and other elements to pass from inside the cell to the exterior, or from the exterior to the inside of the cell.
This time we'll review in musical form:
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Coming soon: rest of the skeleton, anatomy of the knee Cell anatomy overview (complete w/ mitosis & meiosis) Bone Tissue anatomy & physiology The beginning of neurology.
