Molecular Mechanism of Oocyte Activation in Mammals: Past, Present, and Future Directions.

During mammalian fertilization, repetitive intracellular Ca 2+ increases known as Ca 2+ oscillations occur. These oscillations are considered crucial for successful fertilization and subsequent embryonic development. Numerous researchers have endeavored to elucidate the factors responsible for inducing Ca 2+ oscillations across various mammalian species. Notably, sperm-specific phospholipase C zeta (PLCζ) emerged as a prominent candidate capable of initiating Ca 2+ oscillations, particularly in mammals. Genetic mutation of PLCζ in humans results in the absence of Ca 2+ oscillations in mouse oocytes. Recent studies further underscored PLCζ's significance, revealing that sperm from PLCζ-deficient ( Plcz1 -/- ) mice fail to induce Ca 2+ oscillations upon intracytoplasmic sperm injection (ICSI). Despite these findings, observations from in vitro fertilization (IVF) experiments using Plcz1 -/- sperm revealed some residual intracellular Ca 2+ increases and successful oocyte activation, hinting at potential alternative mechanisms. In this review, we introduced the current hypothesis surrounding oocyte activation in mammals, informed by contemporary literature, and probed into the enigmatic mechanisms underlying mammalian fertilization-induced oocyte activation.