Hormonal Control of Cell-Cycle Checkpoints in Mammalian Oocytes

To coordinate ovum maturation with the initiation of embryonic development at fertilization, special modifications in female meiosis have occurred during the evolution of vertebrates. Fertilization in most vertebrates occurs between gametes that exist in distinctly different cell-cycle states. Sperm are postmeiotic, and oocytes are generally arrested at metaphase of meiosis 2 at the time of ovulation. While the completion of meiosis in oocytes is triggered by fertilization, several important checkpoints are imposed on meiotic cell-cycle progression throughout oogenesis to ensure that the events of meiotic M-phase are coordinated with follicular development and ovulation.