Mammalian Fertilization: Scientific Basis and Recent Progress

Successful fertilization in mouse and other studied species, including man, is a highly programmed process by which two radically different looking haploid cells, sperm and egg, unite to form a zygote, a diploid cell with somatic chromosome numbers. The process is the net result of a complex sequence of biological events that prepare ejaculated spermatozoa to recognize and irreversibly bind the egg's extra-cellular matrix, the zona pellucida (ZP). The species-specific tight binding of the opposite gametes results in the opening of Ca2+ channels on sperm plasma membrane (PM) and influx of Ca2+. The transient rise in intra-sperm Ca2+ triggers a signal transduction cascade that results in the fusion of the sperm PM and the underlying outer acrosomal membrane at multiple sites (i.e., induction of the acrosomal reaction). The hydrolytic action of the acrosomal enzymes (i.e., proteinases, glycohydrolases, sulphatases, esterases, etc.) released at the site of sperm-egg binding, along with the hyperactivated beat pattern of the bound spermatozoon are important factors that regulate the penetration of the acrosome-reacted spermatozoon through the ZP and fertilize the egg. This review focuses on the most important aspects of biological processes that prepare spermatozoa to fertilize an ovulated egg. Our intention is to discuss recent data from this and other laboratories that provide useful insights into the biology underlying sperm-egg interactions leading to fertilization.