Site of the mammalian sperm physiological acrosome reaction.

I n mammals and the many other animals in which fertilization involves the establishment of a penetration path through the egg coat, at the apex of the sperm head lies the acrosome— a membrane-limited organelle that houses a complex of enzymes. For fertilization to occur, spermatozoa must first undergo the acrosome reaction (AR), which involves point fusions between the outer acrosomal and overlying cell membrane. This reaction releases the acrosome’s soluble lysins, which facilitate the creation of a path in the egg coat during penetration (although a variety of evidence now calls into question whether egg-coat lysis is the mechanism used by eutherian mammals). In PNAS, using in vitro fertilization, Jin et al. (1) report their tracking of the site and timing of the AR in spermatozoa from transgenic mice whose acrosome and sperm tail midpiece were tagged with different fluorescent markers. This approach, which used video imaging, allowed them to record that, compared with sperm on a naked zona surface, the AR was generally initiatedmore readily in the interstices of the cell mass of the cumulus oophorus that surrounds the newly ovulated mouse egg. Moreover, not only did the AR generally occur to a greater degree and often more rapidly in sperm penetrating the cumulus than on a naked zona surface, but this was also true for sperm interacting with naked eggs in the presence of cumulus-invested oocytes. Most important, Jin et al. (1) establish by retroactive examination of recorded images that some spermatozoa reacting within the cumulus were then able to go on to fertilize the egg. Given that the AR has been examined in different ways over the years and in a variety of mammals, including man, why then is this current study so important in the context of present efforts to understand the mechanisms of fertilization in eutherian mammals?