Zona pellucida and progesterone-induced Ca2+ signaling and acrosome reaction in human spermatozoa.

The mammalian acrosome reaction (AR) is the fusion, at multiple points, of the outer acrosomal membrane with the overlying plasma membrane. This fusion causes the release of acrosomal contents and the exposure of the inner acrosomal membrane. The time taken for this process to occur is not accurately known. Estimates for AR in mouse spermatozoa vary from ;2 minutes after stimulation with solubilized zona to 130 minutes with intact zona (Lee and Storey, 1989; Rockwell and Storey, 2000). In the human, latencies of 15–60 minutes have been observed with intact zona (Morales et al, 1994). Fusion of the plasma and outer acrosomal membranes in the human appears to progress in a different way from that of the rodent and other mammals, but the final result is the same (Nagae et al, 1986; Yanagimachi, 1994). Release of acrosomal enzymes (including hyaluronidase and acrosin), combined with vigorous sperm motility, has been postulated to enable sperm penetration through the zona, allowing binding of the sperm to the oolemma, the egg plasma membrane (Talbot, 1985). It should be noted that experiments with mouse genetic knockouts have shown that zona penetration and fertilization can still occur in the absence of acrosin (Baba et al, 1994). Receptors revealed on the surface of the spermatozoon (after the vesicle and outer acrosomal membrane have been shed) are important in oolemma binding. After binding, the sperm