ABSTRACT
For more than half a century, reproductive biologists have upheld the theory that, in most mammalian species, oocytes are formed before or shortly after birth, but never in adulthood. This doctrine was established1 and germline stem cells have not been recorded in juvenile or adult ovaries, apart from a few isolated exceptions in certain species. In fact, although spermatogenesis is maintained throughout adult life, oocyte production in mammalian females is considered to cease before birth. This cornerstone of reproductive science has stood strong despite the rapid development of new technology and knowledge, remaining virtually unchallenged until the publication of two reports.2,3
It is a dogma within reproductive biology that females are born with a finite and non-renewable number of germinal cells, all of which are arrested in the first phase of meiosis and surrounded by somatic cells in structures known as follicles.1 The number of follicles decreases during postnatal life4 through mechanisms involving apoptosis. Three waves of this dramatic apoptosis have been described. The first wave, and most important, takes place in the prophase of the first meiotic division. From a maximum of 6-7 million around week 20 of gestation, the total follicle number drops to approximately 1 million in the female newborn. The second wave occurs just before puberty, and sees the ovarian reserve drop to around 400 000 primordial follicles during puberty. The third wave is gradual and cyclic, and takes place in response to the monthly deprivation of
follicle-stimulating hormone (FSH), leading to the total depletion of germ cells in the ovaries in what represents the menopause, which occurs around the fifth decade of life.4
