Binding of mouse spermatozoa to the zonae pellucidae of mouse eggs in cumulus: evidence that the acrosomes remain substantially intact.

Cumulus-intact mouse eggs inseminated in vitro with capacitated mouse sperm have sperm bound to the zonae pellucidae as early as 20 min postinsemination. Maximal sperm binding was observed at 60 min, after which time the number of sperm bound per egg decreased to about 30% of maximal. This extent of binding remained constant from 120 to 180 min postinsemination. All the sperm bound to the zonae had intact acrosomes as determined by chlortetracycline fluorescence assay. The sperm binding was retarded during the first 60 min by 3-quinuclidinyl benzilate, a specific inhibitor of the acrosome reaction induced by zona glycoprotein, but this compound had no effect on sperm binding during the next 60 min. The observation that sperm, which bind to zonae pellucidae after traversing the cumulus, have intact acrosomes is consistent with the same observation of intact acrosomes made with sperm binding to cumulus-free eggs [Saling and Storey (1979). J. Cell Biol. 83:544-555], and further supports the hypothesis that the sequence of events leading to fertilization of mouse eggs is: binding of intact sperm to the zona pellucida followed by the acrosome reaction induced by zona glycoprotein.