Germ stem cells in the mammalian adult ovary: considerations by a fan of the primordial germ cells.

At or early after birth, mammalian ovaries are filled with primordial follicles each composed by an oocyte blocked at the end of prophase I surrounded by a single layer of granulosa cells. The doctrine that female mammals are born with a finite number of oocytes fated to be exhausted with the age has been challenged by recent results claiming that new oocytes can be continuously formed in the post-natal mouse ovary. In my view, this notion, termed neo-oogenesis, is strictly linked to the process of the germline specification which presents unique features. Therefore, in the present paper, I am going to discuss two aspects of neo-oogenesis related to this process: first, evidence showing that adult mammalian ovary contains cells able to undergo germline specification and produce new oocytes; and second, the possible origin of such cells. In conclusion, I favour the possibility that a small number of primordial germ cells (PGCs)/oogonia or of PGC-derived undifferentiated cells with stem cell characteristics could remain in the post-natal ovary and under certain conditions may resume mitosis, enter meiosis and give rise to oocytes.