Acrosome reaction in human spermatozoa.
Human spermatozoa were allowed to undergo the acrosome reaction in vitro by incubation in media with or without reagents known to accelerate the onset of the acrosome reaction. The first observable change before the acrosome reaction was a partial decondensation of the acrosomal matrix. This was followed by invaginations of the outer acrosomal membrane alone or of both the plasma and outer acrosomal membranes, which resulted in formation of many vesicles within the acrosomal cap. Subsequently, the plasma and outer acrosomal membranes fused, but the fusion was seldom seen in the acrosomal cap region. On the other hand, fusion of the two membranes was observed consistently at the anterior end of the equatorial segment of the acrosome. Soon after the membranes over the acrosomal cap disappeared, many vesicles, which were originally within the cap, were seen on or in the vicinity of the inner acrosomal membrane. These vesicles dispersed eventually, leaving the inner acrosomal membrane completely exposed. Thus the acrosome reaction in human spermatozoa seems to proceed in a way somewhat different from that in spermatozoa of most other species, although the end result of the reaction is the same.