Oogenesis in antenatal development in man

Summary Ninety-seven female embryos and foetuses aged 6–40 weeks were quantitatively analyzed for germ cell development, mitotic activity in the ovary, and dynamics of chromosome transformations in oocytes at the stages of meiotic prophase I and follicle genesis. For the first time, chronology of oocytes dynamics at the stage of the preleptotene chromosome condensation and decondensation is described. Oocytes at the leptotene stage occur in embryos aged 10–11 weeks. Oocyte transfer at the zygotene and pachytene stages starts by 10.5–11 and 11.5–13 weeks, respectively. Their number is maximum after 26 weeks and by the 40th week decreases to just single oocytes. The first oocytes at the diplotene stage appear in foetuses aged 11.5–12 weeks. Oocyte transition to the dictyotene stage is observed in single oocytes after 16 weeks of development, but active bivalent decondensation begins after 26 weeks.The formation of a follicular layer takes place not earlier than around the oocyte at the diplotene stage. Follicle genesis occurs after 11–12 weeks. Transformation of primordial follicles into primary ones is intensified after 19–20 weeks. By the moment of birth, the majority of oocytes in the human ovary are contained in primary follicles, and only a few are contained in primordial ones. The number of secondary and tertiary antral follicles is extremely small. The dynamics of degeneration of germ cells throughout intra-uterine development is also described.