#oocytes

Potential to Polarise

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' – the oocyte’s 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 – NonCommercial – NoDerivs (CC BY-NC-ND 4.0)

Published in Current Biology, November 2025

"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.

“This is the first case of making robust mammal oocytes [a.k.a. egg cells] from male cells,” 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. Hayashi’s 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.

“Purely in terms of technology, it will be possible [in humans] even in 10 years,” 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.

“I don’t know whether they’ll be available for reproduction,” he said. “That is not a question just for the scientific programme, but also for [society].”

The technique could also be applied to treat severe forms of infertility, including women with Turner’s 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 “fascinating”, but added that other research had indicated that creating lab-grown gametes from human cells was more challenging than for mouse cells. “We still don’t understand enough of the unique biology of human gametogenesis to reproduce Hayashi’s provocative work in mice,” 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 “borrowed” from another cell to produce iPS cells with two identical X chromosomes.

“The trick of this, the biggest trick, is the duplication of the X chromosome,” said Hayashi. “We really tried to establish a system to duplicate the X chromosome.”

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. “They look OK, they look to be growing normally, they become fathers,” 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 “huge leap”, 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. “We’re poised at this bottleneck at the moment,” she said. “The next steps are an engineering challenge. But getting through that could be 10 years or 20 years.”

Separating an Egg

Oocytes – the female germ cells – divide as they mature. But unlike the rest of our cells, the non-sex cells or somatic cells – 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

Getting Repairs Right

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 body’s 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 – cohesin – 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, Göttingen, Germany

Video originally published with a Creative Commons Attribution – NonCommercial – NoDerivs (CC BY-NC-ND 4.0)

Published in Current Biology, November 2024

Return of Stemness

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

Eggs and Oil

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

The Eggs Factor

Fertilised eggs contain hundreds of molecular ‘protein-building’ 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 here’s 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 don’t develop properly. While the full extent of Nanog’s hidden talent needs to be uncovered, it’s 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