Cellular and molecular biology of capacitation and acrosome reaction in spermatozoa

A comparative account is given of advances in cellular and molecular biology of capacitation and acrosome reaction in spermatozoa by comparing and contrasting their biochemical and physiological changes in response to various factors in vivo and in vitro. It can now be stated that phenomena of sperm capacitation and acrosome reaction are endogenous molecular events occurring at the membrane level which can be modulated by external environmental factors. The molecular mechanisms and the signal transduction pathways mediating the process of capacitation and acrosome reaction are only partially defined and appear to involve modification of intracellular Ca2+ and other ions, lipid transfer, and phospholipid remodeling in the sperm plasma membrane as well as changes in protein phosphorylation. Evidences for the involvement of cAMP-dependent kinase pathway in the acrosome reaction are discussed. The mediation of one or more external signals by the sperm plasma membrane appears to activate this pathway after or simultaneously with the influx of Ca2+. Concurrent with or following entry of Ca2+, adenylate cyclase is activated, leading to increased concentrations of cAMP-activation of cAMP-dependent kinase and protein phosphorylation; the identity of such proteins and their role in the acrosome reaction must be determined. The roles of biological effectors of the acrosome reaction, such as ZP3 and follicular fluid are still to be defined at the molecular level. The gaps in our knowledge about the cellular and molecular aspects of capacitation and acrosome reaction are emphasized.