Human pluripotent stem cells cradled by a mouse ovarian environment—a sort of living “dish”—differentiated into states that brought the cells close to becoming mature oocytes. The cells, which were evaluated over the course of a months-long experiment, progressed epigenetically toward reproductive competence, suggesting that induced pluripotent stem cells, given the right culture conditions, can become human egg cells.
The encouraging findings emerged from a study conducted by scientists based at Kyoto University. These scientists, led by Mitinori Saitou, M.D., Ph.D., professor, anatomy and cell biology, Kyoto University, detailed their work in a paper (“Generation of human oogonia from induced pluripotent stem cells in vitro”) that appeared recently in the journal Science. According to this paper, the scientists not only derived primordial germ cell-like cells (hPGCLCs) from human pluripotent stem cells (hPSCs), they found a way to help the hPGCLCs differentiate, show oocyte-specific characteristics, and pass through growth stages not previously reported.
“Here, we show that hPGCLCs differentiate progressively into oogonia-like cells during a long-term in vitro culture (~four months) in xenogeneic reconstituted ovaries with mouse embryonic ovarian somatic cells,” the authors of the Science article wrote. “The hPGCLC-derived oogonia display hallmarks of epigenetic reprogramming, that is, genome-wide DNA demethylation, imprint erasure, and extinguishment of aberrant DNA methylation in hPSCs, and acquire an immediate precursory state for meiotic recombination.”
This work advances reproductive biology toward a long-term goal: recreating human eggs and sperm in culture. Previous work in this field has demonstrated the ability to create mouse egg precursor cells, or oocytes, in vitro, from mouse embryonic stem cells; these cells, while not full reproductive cells, can go on to contribute to egg cell development when placed in female mice, work has shown. Separately, other scientists have been able to derive early-stage human cells akin to reproductive cells from induced hPSCs, though whether these can be transformed to mature germ (or reproductive) cells in culture remains unknown.
To push precursor cells closer to reproductive competence, the Kyoto University scientists placed early-stage germ cells into an environment of early-stage mouse ovarian cells. This mouse ovarian environment, the scientists hoped, would pass chemical signals to the induced hPSCs, precipitating epigenetic changes. The researchers then evaluated the latter cells’ progress for several months, analyzing expression of units called transcripts through RNA sequencing.
In addition to the epigenetic changes noted above, the scientists reported that in the cells they evaluated, the inactive X chromosome showed a progressive demethylation and reactivation, albeit partially. This finding, in combination with the other developmental changes observed by the scientists, led the Kyoto team to conclude the following: “These findings establish the germline competence of hPSCs and provide a critical step toward human in vitro gametogenesis.”