Many investigators maintain that spermatozoa that have initiated the acrosome reaction (AR) before reaching the surface of the egg's zona pellucida (ZP) are unable to bind and penetrate the ZP. A recent study has revealed that most fertilizing mouse spermatozoa initiate the AR before contacting the ZP. We found that acrosome-reacted spermatozoa collected from the perivitelline space of Cd9-null mice (whose egg plasma membranes are incapable of fusing with spermatozoa) were able to pass through both the cumulus and ZP of WT mouse eggs and produced live offspring. This means that the spermatozoa we used had the ability to pass through the ZP at least twice. Apparently, some spermatozoa that had undergone the AR long before contact with the ZP remained capable of crossing the ZP and fertilizing eggs. Thus, the concept that acrosome-reacted spermatozoa are unable to bind to the ZP and have lost their fertilizing capacity must be reconsidered.
