A comprehensive review about sperm-oocyte interactions and key activation factors of fertilization in mammals.

Mammalian fertilization involves the migration of spermatozoa through the female reproductive system. Early embryonic development is a consequence of several steps and signaling pathways being activated, as well as biochemical and morphological modifications of spermatozoa that enable them to penetrate the membrane of mature oocytes. There are some crucial steps known to clearly explain the process of fertilization, starting with hyperactivation of spermatozoa, mutual recognition, and binding of gametes mediated by receptors located on the surface membranes of both gametes. The final step is followed by oocyte activation, which is primarily triggered via sperm-derived factors, inducing a sharp increase in intracellular calcium levels, eventually leading to polyspermy block. This review integrates current knowledge of the molecular and physiological events governing fertilization, emphasizing how ion regulation and signaling pathways converge to enable sperm function and oocyte activation. Special attention is given to sperm-derived factors such as phospholipase C zeta (PLCζ) and post-acrosomal sheath WW domain-binding protein (PAWP), which play essential roles in triggering calcium release and supporting early embryonic development.