Significance of egg's zona pellucida glycoproteins in sperm-egg interaction and fertilization

Mammalian fertilization is a highly programmed process by which sperm and egg unite to form a zygote, a cell with somatic chromosome numbers. To fertilize an egg, the capacitated (acrosome-intact) spermatozoa recognize and bind to the egg's extracellular glycocalyx coat, the zona pellucida (ZP). The tight and irreversible binding of the opposite gametes in the mouse and many other species studied, including man, results in the opening of Ca2+ channels on sperm plasma membrane (PM) and influx of Ca2+. The transient rise in Ca2+ and other second messengers, such as cAMP and IP3, initiates a cascade of signaling events that elevate sperm pH and triggers the fusion of the sperm PM and underlying outer acrosomal membrane at multiple sites (induction of the acrosomal reaction). The fusion of the two membranes results in the exocytosis of acrosomal contents at the site of sperm-egg adhesion. The hydrolytic action of the acrosomal enzymes (glycohydrolases, proteinases, esterases, sulfatases etc), released at the site of sperm-egg adhesion, along with the enhanced thrust generated by the hyperactivated spermatozoon, are important factors that regulate the penetration of the ZP and the fusion of the acrosome-reacted spermatozoon with the egg. Evidence accumulated over the past two decades strongly suggests that glycan units of the ZP have a significant role in the recognition and adhesion of the opposite gametes and induction of the AR. In this review article, we intend to highlight well programmed molecular events that results in the sperm-egg adhesion and fertilization. Our intention is also to discuss the increasing controversy on the role of ZP glycan chains in sperm-egg interactions.