*4.1.1 Ca2+-dependent process of cortical granule exocytosis*

Cortical granule exocytosis enables polyspermy block by altering the zona pellucida with the content of the granule (proteases, peroxidases, and glycosaminoglycans) and prevent more sperms from fertilizing oocytes. The proposed model is that calcium stimulates Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) and myosin light-chain kinase (MLCK) that phosphorylate a vesicle targeting protein and myosin II to promote exocytosis [33]. It is a quick response; in mouse oocytes, exocytosis starts 15 min after exposure to capacitated sperm, and 30 min after insemination, 78% of cortical granules disappear from cytoplasm [35]. It is proposed that each calcium oscillation cycle moves cortical granule one step closer to the egg plasma membrane toward the fusion with the plasma membrane and exocytosis of the contents [33].
