The zona pellucida stimulates the spermatozoon to undergo the acrosome reaction (a fusion of the plasma membrane and the outer acrosomal membrane and eventual loss of the vesicles formed). In the absence of large numbers of human zonae, and the failure of recombinant human ZP3 to be of benefit, calcium ionophore has been used to mimic the increased intracellular calcium normally induced by the zona. The percentage of spontaneously acrosome-reacted human epididymal sperm is high but similar for sperm from all epididymal regions but upon challenge with ionophore, the response of cauda cells is greater than that of corpus cells and caput sperm do not respond at all (Fig. 4.2E). Clearly a change in the response of the cells to calcium influx occurs upon maturation that results in their being able to acrosome react. Epididymal sterol binding and lipid binding proteins (see Table 4.2) are capable of making membranes more fluid, which is a prerequisite for acrosomal vesiculation. In this regard it is interesting to note that human epididymal dysfunction has been associated with abnormally high sperm membrane rigidity (Wiese et al., 1996).
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