Meiotic timelines for humans

The fate of germ cells is dictated by the somatic environment. In both the developing ovary and the testis, germ cells undergo mitotic proliferation prenatally, but the time of entry into meiosis and the duration of meiosis are strikingly different between the sexes. In the fetal ovary, a brief period of mitotic proliferation is followed by the entry of all cells into meiotic prophase. Several germ cells undergo apoptosis during this time, substantially reducing the pool of developing oocytes. Before birth, all surviving oocytes enter a period of extended meiotic arrest and, by the time of birth, all quiescent oocytes have become surrounded by somatic cells, forming primordial follicles. In a sexually mature woman, individual primordial follicles are stimulated to initiate growth throughout the reproductive lifespan. Typically, one fully grown oocyte is ovulated each month, and several growing oocytes become atretic. This process continues until the cohort of oocytes is depleted and the woman enters menopause. In contrast, in the fetal testis, a brief period of mitotic proliferation is followed by an extended period of mitotic arrest. After birth, the male germ cells, or spermatogonia, resume mitotic proliferation and, with sexual maturity, cells are stimulated to undergo meiotic division. Sperm production is maintained throughout the lifetime of the male. Throughout the meiotic divisions, individual spermatocytes remain connected by cytoplasmic bridges. These connections are lost during the postmeiotic process of spermiogenesis, which involves tight packing of the chromatin, growth of the sperm tail, and the sloughing of virtually all the cytoplasm into the residual bodies (depicted as empty cells).