Abstract
The concept that oogenesis continues into reproductive life has been well established
in nonmammalian species. Recent studies of mice and women indicate that oocyte formation
is also not, as traditionally believed, restricted to the fetal or perinatal periods.
Analogous to de novo oocyte formation in flies and fish, newly formed oocytes in adult
mammalian ovaries arise from germline stem cells (GSCs) or, more specifically, oogonial
stem cells (OSCs). Studies of mice have confirmed that isolated OSCs, once delivered
back into adult ovaries, are capable of generating fully functional eggs that fertilize
to produce healthy embryos and offspring. Parallel studies of OSCs recently purified
from ovaries of reproductive-age women indicate that these cells closely resemble
their mouse ovary–derived counterparts, although the fertilization competency of oocytes
generated by human OSCs awaits clarification. Despite the ability of OSCs to produce
new oocytes during adulthood, oogenesis will still ultimately cease with age, contributing
to ovarian failure. The causal mechanisms behind these events in mammals are unknown,
but studies of flies have revealed that GSC niche dysfunction plays a critical role
in age-related oogenic failure. Such insights derived from evaluation of nonmammalian
species, in which postnatal oogenesis has been studied in depth, may aid in development
of new strategies to alleviate ovarian failure and infertility in mammals.
Keywords
germline stem cell - oogonial stem cell - oogenesis - oocyte - ovary - human
