Current knowledge in the renewal capability of germ cells in the adult ovary.

It is a central dogma in reproductive biology that oogenesis is completed before or just after birth and that the postnatal ovary is endowed by a fixed and non-renewing number of oocytes in mammals. However, this widely accepted doctrine was recently challenged by studies showing regeneration of oocytes from putative germ cells in bone marrow and peripheral blood. These results not only triggered an enormous amount of interest among reproductive biologists but also a great deal of debate. In this review we will provide an update on the molecular aspects of the formation of primordial germ cells (PGC), the precursors of adult gametocytes, beginning from their specification to their migration to prospective gonads and formation of the ovary and follicular structures. We will also discuss more recent studies that showed in vivo regeneration of germ cells in the postnatal ovary in situ, along with other pioneering works that demonstrated generation of germ cells in vitro from embryonic and somatic stem cells.