INTERACTION OF ACROSOME-REACTED MACAQUE SPERM WITH THE MACAQUE ZONA-PELLUCIDA

In this study we investigated the ultrastructure of macaque sperm indu ced to acrosome-react with calcium ionophore A23187, and the interacti on between these acrosome-reacted sperm and the macaque zona pellucida . Transmission electron microscopy revealed that the majority of ionop hore-treated sperm retained the vesiculated acrosomal cap or ''shroud. '' Untreated, acrosome-reacted sperm on the zona had a similar ultrast ructural appearance. In sperm-zona binding experiments, a mean of 4.5 ionophore-treated sperm were bound per zona after 1 min of coincubatio n compared with 41 sperm per zona in the solvent control. Vigorous pip etting was used to remove the acrosomal shrouds from approximately 50% of acrosome-reacted sperm before incubation with oocytes. Significant ly more of these mechanically treated sperm were bound to the zona aft er a 4-min coincubation compared with acrosome-reacted sperm that were not pipetted. The number of mechanically treated sperm bound to the z ona was the same whether the sperm and oocytes were coincubated in cal cium-free medium or in control medium. The percentage of mechanically treated sperm that were acrosome-reacted on the zona also was not diff erent in the two media. We conclude that macaque sperm that undergo th e acrosome reaction on the zona surface are bound by the acrosomal shr oud before zona penetration. When sperm acrosomereact before interacti on with the oocyte, their zona binding capacity is significantly reduc ed. Removal of the acrosomal shroud and exposure of the inner acrosoma l membrane increases the affinity of sperm for the zona. This sequence occurs naturally during the transition from primary binding to second ary binding on the zona surface.