Factors affecting the acrosome reaction in human spermatozoa

Large pieces of human cumulus oophorus were exposed for 20-30 min to washed spermatozoa or to spermatozoa recovered after a swim-up procedure, and then fixed for electron microscopy. Spermatozoa of both populations penetrated deeply into the cumulus within that time, and none of 48 observed clearly had undergone an acrosome reaction (AR). As measured by fluorescence microscopy, an AR rate of 12% in spermatozoa obtained at 4 h following a swim-up increased to about 25% in samples incubated in culture dishes for approximately 20 h. However, this latter AR rate was no different in the presence or absence of a cumulus/oocyte complex, and was only moderately greater in 50% follicular fluid. Nor was it affected to any degree by the absence of calcium or by a low (26 degrees C) temperature, both of which are regulators of the physiological AR in other species. By contrast, a clear dose-related enhancement of the AR by the calcium ionophore A23187 was almost completely Ca2(+)-dependent. We conclude that the human cumulus oophorus does not rapidly induce an AR in spermatozoa capacitated in vitro and, unlike the situation in some other mammals, that washed human spermatozoa do not first require a period of capacitation in order to penetrate it. The results also point to the likelihood that ARs monitored in free-swimming human spermatozoa are for the most part spurious or artefactual, and they show that in-vitro AR rates in such populations do not parallel their fertilizing ability.