Fate of the germ cells in mammalian ovary: A review

Ovary has a fix number of germ cells during fetal life in mammals. The germ cells are depleted rapidly and a large number of germ cells (≥99%) are eliminated from the cohort of ovary through follicular atresia during prepubertal life. The various cell death pathways including apoptosis, autophagy, necrosis, and necroptosis are involved in follicular atresia. Hence, <1% of germ cells are culminated into oocytes that are available for meiotic maturation and ovulation during entire reproductive life. These oocytes are arrested at diplotene stage of meiotic prophase-I and remain arrested for few months to several years during entire reproductive life. Resumption from diplotene arrest in follicular oocytes starts in response to gonadotropins surge and progresses through metaphase-I to metaphase-II stage that extrudes first polar body at the time of ovulation. Surprisingly, oocytes do not wait for fertilizing spermatozoa and quickly undergo abortive spontaneous oocyte activation (SOA) in few mammalian species including humans. The abortive SOA makes oocyte unfit for fertilization and limits assisted reproductive technologies outcome. Indeed, majority of germ cells and oocytes are eliminated from the cohort of ovary and only few oocyte that are of good quality get selectively recruited to become right gamete after ovulation during entire reproductive life span in mammals.