Interaction of acrosome-reacted macaque sperm with the macaque zona pellucida

In this study we investigated the ultrastructure of macaque sperm induced to acrosome-react with calcium ionophore A23187, and the interaction between these acrosome-reacted sperm and the macaque zona pellucida. Transmission electron microscopy revealed that the majority of ionophore-treated sperm retained the vesiculated acrosomal cap or "shroud." Untreated, acrosome-reacted sperm on the zona had a similar ultrastructural appearance. In sperm-zona binding experiments, a mean of 4.5 ionophore-treated sperm were bound per zona after 1 min of coincubation compared with 41 sperm per zona in the solvent control. Vigorous pipetting was used to remove the acrosomal shrouds from approximately 50% of acrosome-reacted sperm before incubation with oocytes. Significantly more of these mechanically treated sperm were bound to the zona after a 4-min coincubation compared with acrosome-reacted sperm that were not pipetted. The number of mechanically treated sperm bound to the zona was the same whether the sperm and oocytes were coincubated in calcium-free medium or in control medium. The percentage of mechanically treated sperm that were acrosome-reacted on the zona also was not different in the two media. We conclude that macaque sperm that undergo the acrosome reaction on the zona surface are bound by the acrosomal shroud before zona penetration. When sperm acrosome-react before interaction with the oocyte, their zona binding capacity is significantly reduced. Removal of the acrosomal shroud and exposure of the inner acrosomal membrane increases the affinity of sperm for the zona. This sequence occurs naturally during the transition from primary binding to secondary binding on the zona surface.