The activation of mammalian sperm

Our understanding of the cascade of events that result in the activation or capacitation of sperm which thereby acquire the ability to effect fertilization has developed considerably in recent years. Capacitation of mammalian sperm in vivo is a complex, dynamic process which may be mimicked in vitro by a number of different methods. A variety of components have been implicated in capacitation including the alteration of membrane cholesterol/phospholipid ratios, removal of seminal plasma-coating proteins and cell-surface components, and alterations of membrane potential. All these events appear to be implicated in an eventual change in membrane permeability to calcium ions. The subsequent rise in intracellular calcium precipitates the multiple membrane fusion event known as the acrosome reaction. The calcium-dependent enzyme phospholipase A2 has been implicated in this event.