Molecular aspects of sperm-egg fusion.

Fertilization, as one of the few well-studied physiological cell fusion systems, provides a glimpse of hierarchies of control that may exist in other membrane fusions as well. Sperm become fusogenic only after undergoing exocytosis from an apical vesicle; this acrosome reaction, induced by an egg surface component, confers upon the sperm the capacity to bind to and fuse with an egg. The acrosome reaction requires Ca2+ and Na+ and is mediated by a complex series of ionic alterations in sperm, including plasma membrane potential depolarization, Ca2+ influx, and increased intracellular pH. These changes take their toll of the sperm, which dies soon thereafter if it does not fertilize an egg. Sperm-egg fusion itself is rapid, with a negligible requirement for extracellular Ca2+, and is inhibited by depolarization of the egg plasma membrane potential. Gamete membrane fusion is followed by dramatic changes in egg physiology, including those that inhibit subsequent sperm-egg fusions. These blocks to polyspermy include a partial decrease in egg surface receptivity caused by egg membrane depolarization immediately after gamete fusion, followed by a complete inhibition of sperm entry due to a massive exocytosis from vesicles beneath the egg plasma membrane, with a concomitant change in the egg plasma membrane and cell coat. The sperm that has successfully fused with an egg contributes not only its genome, but cytoplasmic components as well. Some cytoplasmic constituents from the sperm, including several proteins, persist without degradation throughout early development; several remain localized in a single region. Whether these cytoplasmic components transferred from the sperm play a role in subsequent morphogenesis of the embryo is not clear.