Factors governing egg production have been uncovered in zebrafish ovaries. Using a lab-grown long-term model system of egg (oocyte) development, the mTOR-Stat3-Stathmin molecular pathway is found to be key for oocyte polarisation (how a cell organises within and that is vital for developmental processes), establishing a state of 'polarisation competence' â the oocyteâs ability to develop the potential to polarise
Read the published research article here
Image from work by Amal Shawahny and colleagues
Department of Developmental Biology and Cancer Research, The Hebrew University of Jerusalem Faculty of Medicine, Ein-Kerem Campus, Jerusalem, Israel
Video originally published with a Creative Commons Attribution â NonCommercial â NoDerivs (CC BY-NC-ND 4.0)
Published in Current Biology, November 2025
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Creation of mammal with two biological fathers could pave way for new fertility treatments in humans
"Scientists have created mice with two biological fathers by generating eggs from male cells, a development that opens up radical new possibilities for reproduction.
The advance could ultimately pave the way for treatments for severe forms of infertility, as well as raising the tantalising prospect of same-sex couples being able to have a biological child together in the future.
âThis is the first case of making robust mammal oocytes [a.k.a. egg cells] from male cells,â said Katsuhiko Hayashi, who led the work at Kyushu University in Japan and is internationally renowned as a pioneer in the field of lab-grown eggs and sperm.
Hayashi, who presented the development at the Third International Summit on Human Genome Editing at the Francis Crick Institute in London on Wednesday, predicts that it will be technically possible to create a viable human egg from a male skin cell within a decade. Others suggested this timeline was optimistic given that scientists are yet to create viable lab-grown human eggs from female cells.
Previously scientists have created mice that technically had two biological fathers through a chain of elaborate steps, including genetic engineering. However, this is the first time viable eggs have been cultivated from male cells and marks a significant advance. Hayashiâs team is now attempting to replicate this achievement with human cells, although there would be significant hurdles for the use of lab-grown eggs for clinical purposes, including establishing their safety.
âPurely in terms of technology, it will be possible [in humans] even in 10 years,â he said, adding that he personally would be in favour of the technology being used clinically to allow two men to have a baby if it were shown to be safe.
âI donât know whether theyâll be available for reproduction,â he said. âThat is not a question just for the scientific programme, but also for [society].â
The technique could also be applied to treat severe forms of infertility, including women with Turnerâs syndrome, in whom one copy of the X chromosome is missing or partly missing, and Hayashi said this application was the primary motivation for the research.
Others suggested that it could prove challenging to translate the technique to human cells. Human cells require much longer periods of cultivation to produce a mature egg, which can increase the risk of cells acquiring unwanted genetic changes.
Prof George Daley, the dean of Harvard Medical School, described the work as âfascinatingâ, but added that other research had indicated that creating lab-grown gametes from human cells was more challenging than for mouse cells. âWe still donât understand enough of the unique biology of human gametogenesis to reproduce Hayashiâs provocative work in mice,â he said.
Study Methods
The study, which has been submitted for publication in a leading journal, relied on a sequence of intricate steps to transform a skin cell, carrying the male XY chromosome combination, into an egg, with the female XX version.
Male skin cells were reprogrammed into a stem cell-like state to create so-called induced pluripotent stem (iPS) cells. The Y-chromosome of these cells was then deleted and replaced by an X chromosome âborrowedâ from another cell to produce iPS cells with two identical X chromosomes.
âThe trick of this, the biggest trick, is the duplication of the X chromosome,â said Hayashi. âWe really tried to establish a system to duplicate the X chromosome.â
Finally, the cells were cultivated in an ovary organoid, a culture system designed to replicate the conditions inside a mouse ovary. When the eggs were fertilised with normal sperm, the scientists obtained about 600 embryos, which were implanted into surrogate mice, resulting in the birth of seven mouse pups. The efficiency of about 1% was lower [although not THAT much lower] than the efficiency achieved with normal female-derived eggs, where about 5% of embryos went on to produce a live birth.
The baby mice appeared healthy, had a normal lifespan, and went on to have offspring as adults. âThey look OK, they look to be growing normally, they become fathers,â said Hayashi.
Going Further
He and colleagues are now attempting to replicate the creation of lab-grown eggs using human cells.
Prof Amander Clark, who works on lab-grown gametes at the University of California Los Angeles, said that translating the work into human cells would be a âhuge leapâ, because scientists are yet to create lab-grown human eggs from female cells.
Scientists have created the precursors of human eggs, but until now the cells have stopped developing before the point of meiosis, a critical step of cell division that is required in the development of mature eggs and sperm. âWeâre poised at this bottleneck at the moment,â she said. âThe next steps are an engineering challenge. But getting through that could be 10 years or 20 years.â
Oocytes â the female germ cells â divide as they mature. But unlike the rest of our cells, the non-sex cells or somatic cells â they don't have centrosomes (structures within the cell for polarisation of chromosomes) to accurately organise the process, instead they assemble a complex from microtubules prior to division. This study shows the importance of the enzyme MPS1 in preventing errors as oocytes divide
Read the published research article here
Image from work by Shuhei Yoshida and colleagues
Laboratory for Chromosome Segregation, RIKEN Center for Biosystems Dynamics Research (BDR), Kobe, Japan
Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)
Published in The EMBO Journal, June 2025
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Few cells are more precious than eggs, but although women are born with millions their store is vastly reduced by their mid-30s. One reason for this is an accumulation of DNA damage causing the loss of these cells before their time. Researchers investigated how the bodyâs DNA repair mechanisms act in this setting, and found that DNA damage persisted longer in eggs from older women, and that the repair machinery was organised into distinct, connected compartments (such as the green filaments made of the repair protein Rad51 in this video of a series of mouse egg precursors). They saw that with age, the organisation of this machinery changed and the protein complex that maintains DNA stability â cohesin â deteriorated. Eggs in older women are relying on an ageing team of mechanics, but if we could substitute in a more spritely repair team, perhaps the supply could last a little longer.
Written by Anthony Lewis
Adapted from movie from work by Ninadini Sharma and colleagues
Max Planck Institute for Multidisciplinary Sciences, GĂśttingen, Germany
Video originally published with a Creative Commons Attribution â NonCommercial â NoDerivs (CC BY-NC-ND 4.0)
Published in Current Biology, November 2024
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Analysing the reading of genes (transcription) and their translation into proteins as stem cells differentiate in the eggs (oocytes) of living fruit flies reveals two distinct waves of development with a transient return to a stem cell-like program of transcription and translation
Read the published research article here
Image from work by Tamsin J Samuels and Jinghua Gui, and colleagues
Department of Genetics, University of Cambridge, Cambridge UK
Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)
Published in The EMBO Journal, March 2024
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The role of lipid droplets in egg cell development and fertility revealed in a fruit fly model
Read the published research paper here
Image from work by Michelle S. Giedt and Jonathon M. Thomalla, and colleagues
Anatomy and Cell Biology, University of Iowa Carver College of Medicine, Iowa City, IA and Department of Biology, University of Rochester, Rochester, NY, USA
Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)
Published in Development, June 2023
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Fertilised eggs contain hundreds of molecular âprotein-buildingâ instructions called mRNAs. They need to be followed precisely in the right place, at the right time to create embryos. As egg cells, or oocytes, mature, they produce these mRNAs in preparation, but hereâs the problem: accumulating these mRNAs before they're needed can hinder development. Quite a conundrum, but researchers have identified a protein that can balance being ready to spring into action with successful embryo development. Nanog is known to amplify mRNA messages in the early embryo. Now researchers have found that it helps silence pre-made mRNAs in eggs and how that impacts development. By injecting a dye (red) into fertilised oocytes, they followed how cells move during normal development (pictured). In oocytes without Nanog, this movement was more sluggish so embryos donât develop properly. While the full extent of Nanogâs hidden talent needs to be uncovered, itâs clear that it is important for successful development.
Written by Sophie Arthur
Video from work by Mudan He and Shengbo Jiao, and colleagues
State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Innovation Academy for Seed Design, Chinese Academy of Sciences, Wuhan, China
Video originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)
Published in Development, December 2022
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