Gamete Surface Molecules that Mediate Mammalian Fertilization

Abstract

Fertilization is a highly programmed process by which two radically different ceils, sperm and egg, unite to form a zygote, a cell with somatic chromosome numbers. The process is the net result of a complex sequence of molecular events that allow sperm to recognize and irreversibly bind to the egg's extracellular coat, the zona pellucida (ZP), undergo the acrosome reaction (AR), and fiise with the egg plasma membrane. The male gamete undergoes continuous morphological and biochemical modifications during sperm development in the testis, maturation in the epididymis, and capacitation in the female genital tract. Only the capacitated sperm are able to bind to the ZP, and undergo the signal transduction cascade that results in the exocytosis of acrosomal contents, i.e., induction of the AR. Accumulated evidence has helped consolidate the view that the carbohydrate recognizing receptor molecules (glycohydrolases, giycosyltransferases, and/or lectin-like molecules), present on the surface of capacitated spermatozoa, recognize and bind to the terminal sugar residue(s) of bioactive glycans (ligands) on the ZP, The carbohydrate-mediated adhesion event causes sperm to undergo the AR. The action of hydrolytic and proteolytic enzymes released during the AR, along with the enhanced thrust generated by the hyperactivated beat pattern of the bound spermatozoon, are important events that regulate sperm penetration through egg vestments. In this article, we have discussed extensive progress that has been made to enhance our understanding of molecules and molecular events that regulate fertilization.