Relationship between calcium binding sites and membrane fusion during the acrosome reaction induced by ionophore in ram spermatozoa

Ram spermatozoa treated with the ionophore A23187, to induce the acrosome reaction, were fixed in pyroantimonate-osmium in order to demonstrate calcium at an ultrastructural level. Prior to vesiculation, granules of precipitate occurred at points of contact between plasma and outer acrosomal membranes, suggesting a discrete role for calcium during membrane fusion. The earliest stages of vesiculation always occurred in the region just anterior to the equatorial segment and it was this same site where a marked concentration of precipitate, indicating calcium binding sites, could be demonstrated.