ABOVE: An ovarian organoid made of egg-like cells (blue) generated from stem cells in culture with mouse ovarian somatic cells.

A core set of eight proteins can transform stem cells from mice into cells that look a lot like immature egg cells called oocytes. The egglike cells could not undergo meiosis to cut the total chromosomes in half as they should, but they could be fertilized by sperm and then divide until they hit the eight-cell stage of embryonic development, researchers report today (December 16) in Nature.

“This demonstrates that you can go directly from stem cells to oocytes. I think that is exciting,” Petra Hajkova, a developmental epigeneticist at Imperial College London who was not involved in the study, tells The Scientist. The work, she notes, will help researchers explore the basic biology of oocyte development. In the future, says study coauthor Nobuhiko...

Oocytes are a rare cell type in the body and not much is known about them, Hamazaki says, which is what led him and his colleagues to probe how they develop. Past studies had documented gene expression changes essential for the transition from very early stage reproductive cells called primordial germ cells to oocytes. Hamazaki and colleagues built on that work with an additional gene expression analysis to identify 27 candidate transcription factors that played a role in that transition. To test the function of each transcription factor, the team used pluripotent embryonic stem cells (ESCs), knocking out or inactivating the genes encoding the transcription factors one by one. “Making knocked out ESCs is relatively easier than making knocked out mice, although still making 27 knockout ESCs was laboring,” Hamazaki says.

This demonstrates that you can go directly from stem cells to oocytes. I think that is exciting.

—Petra Hajkova, Imperial College London

The experiments revealed eight transcription factors that were essential to oocyte development: NOBOX, FIGLA, TBPL2, SOHLH1, STAT3, DYNLL1, SUB1, and LHX8. The team then cultured another set of embryonic stem cells and overexpressed the genes that produce those transcription factors, which pushed those cells into an oocyte-like state. 

“It’s believed that oocytes develop from germ cells, but we could make oocytes from non-germ cells,” he explains. “At first, I was so surprised that I could not believe my results, so I repeated the experiment again and again and when I got the same results, I was finally convinced.” 

In additional experiments, the egglike cells did not go on to divide into daughter cells with half a set of chromosomes, which would make them viable for reproduction. But when the team introduced wildtype mouse sperm into the culture, the oocytes did divide into a mass of eight cells before becoming unviable, possibly because the cells had too many chromosomes, the researchers write.

Richard Schultz, a cell biologist at the University of California, Davis, who was not involved in the study, says the work to identify the core set of transcription factors that can drive embryonic stem cells into a state where they look like oocytes is impressive. But the egglike cells don’t undergo meiosis, so they are not functional. “It’s a big step, but only ninety-five percent there. We haven’t gotten one-hundred percent there” to understanding the factors essential for maturation of germline egg cells to oocytes and then to viable eggs with half their chromosomes.

Despite not working out the pathway to meiosis, the work “enabled us to produce a large number of oocytes. We believe that this technology can accelerate basic biological research on oocytes, which are still one of the most mysterious cell types,” Hamazaki says. He explains that the work could improve animal cloning because of the vast number of oocytes produced by the team’s technique. The plasma of the induced oocytes does not show any abnormalities, so it could also be valuable for use in reproductive technologies for women with mitochondrial disease. Children inherit the disease from their mothers, but the use of an enucleated stem cell–derived oocyte, which might have healthy mitochondria, to serve as a carrier of a cell nucleus from an affected mother’s oocyte could be a workaround for this issue, he explains. 

“That’s kind of the long vision,” Hajkova says, “but it shows how these oocytes could be very useful.”

N. Hamazaki et al., “Reconstitution of the oocyte transcriptional network with transcription factors,” Nature, doi:10.1038/s41586-020-3027-9, 2020.

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