Sperm phospholipases and acrosomal exocytosis.

At the time of fertilization, the sperm cell undergoes regulated exocytosis in response to the oocyte-associated agonists progesterone and zona pellucida. An early response generated by agonist-receptor interaction in spermatozoa is the activation of mechanisms leading to Ca2+ influx, this ion being essential for the activation of phospholipases and for the fusion of the plasma membrane with the outer acrosomal membrane. Both a phosphoinositide-specific, and a phosphatidylcholine-specific phospholipase C are involved in the generation of a variety of diacylglycerol molecular species. Phospholipase D, on the other hand, does not seem to play a significant role in the generation of diacylglycerol. Hydrolysis of phospholipids by phospholipase A2 generates free fatty acids and lysophospholipids; these are important either as substrates for the generation of other metabolites (e.g., eicosanoids) or having a direct, essential action in the final stages of membrane fusion. There is still much work to be done in the future in order to characterize phospholipase isozymes and their regulation during acrosomal exocytosis in spermatozoa.