Appropriate timing of the acrosome reaction is a major requirement for the fertilizing spermatozoon.

This study was undertaken to determine the site of the acrosome reaction of spermatozoa penetrating into freshly inseminated human oocytes. The inseminated oocytes were treated with an anti-acrosin monoclonal antibody and the bound antibody was visualized at the ultrastructural level with the use of a second peroxidase-conjugated antibody. Quantitative analysis of serial thin sections cut throughout the specimens showed that the number of spermatozoa within the zona pellucida (potentially fertilizing ones) corresponded to the number of acrosin deposits associated with acrosomal ghosts on the zona pellucida surface. As it is known that a large acrosin bundle is liberated from a spermatozoon at a well-defined point of the acrosome reaction, these findings indicate that the acrosome reaction of the fertilizing spermatozoon must be exactly synchronized with its penetration through the egg vestments, apparently by the action of specific acrosome reaction-promoting substances in the oocyte/cumulus complex. These results represent a theoretical basis for evaluation of direct and indirect laboratory tests for human sperm acrosome reaction. 'Good' sperm samples should display elevated levels of acrosome-reacted spermatozoa after the administration of an appropriate stimulus and low levels of spontaneous acrosome reactions.