Human foetal ovary shares meiotic preventing factors with the developing testis

How can pre-meiotic germ cells persist in the human foetal ovary?

Numerous oogonia escaping meiotic entry were retrieved throughout human ovarian development simultaneously with the expression of signalling pathways preventing meiosis, typically described in the rodent embryonic testis.

The transition from mitosis to meiosis is a key event in female germ cells that remains poorly documented in research on the human ovary. Previous reports described a strikingly asynchronous differentiation in the human female germ line during development, with the persistence of oogonia among oocytes and follicles during the second and third trimesters. The possible mechanisms allowing some cells to escape meiosis remain elusive.

In order to document the extent of this phenomenon, we detailed the expression profile of germ cell differentiation markers using 73 ovaries ranging from 6.4 to 35 weeks post-fertilization.

Pre-meiotic markers were detected by immunohistochemistry or qRT-PCR. The expression of the main meiosis-preventing factors identified in mice was analysed, and their functionality assessed using organ cultures.

Oogonia stained for AP2γ could be traced from the first trimester until the end of the third trimester. Female germ cell differentiation is organized both in time and space in a centripetal manner in the foetal human ovary. Unexpectedly, some features usually ascribed to rodent pre-spermatogonia could be observed in human foetal ovaries, such as NANOS2 expression and quiescence in some germ cells. The two main somatic signals known to inhibit meiosis in the mouse embryonic testis, CYP26B1 and FGF9, were detected in the human ovary and act simultaneously to repress STRA8 and meiosis in human foetal female germ cells.

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Our conclusions relied partly on in vitro experiments. Germ cells were not systematically identified with immunostaining and some may have thus escaped analysis.

We found evidence that a robust repression of meiotic entry is taking place in the human foetal ovary, possibly explaining the exceptional long-lasting presence of pre-meiotic germ cells until late gestational age. This result calls for a redefinition of the markers known as classical male markers, which may in fact characterize mammalian developing gonads irrespectively of their sex.

This research was supported by the Université Paris Diderot-Paris 7 and Université Paris-Sud, CEA, INSERM, and Agence de la Biomédecine. The authors declare no conflict of interest.