<P> As the sperm approaches the zona pellucida of the egg, which is necessary for initiating the acrosome reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the sperm's head, exposing the contents of the acrosome . The contents include surface antigens necessary for binding to the egg's cell membrane, and numerous enzymes which are responsible for breaking through the egg's tough coating and allowing fertilization to occur . </P> <P> There are considerable species variations in the morphology and consequences of the acrosome reaction . In several species the trigger for the acrosome reaction has been identified in a layer that surrounds the egg . </P> <P> In some lower animal species a protuberance (the acrosomal process) forms at the apex of the sperm head, supported by a core of actin microfilaments . The membrane at the tip of the acrosomal process fuses with the egg's plasma membrane . </P> <P> In some echinoderms, including starfish and sea urchins, a major portion of the exposed acrosomal content contains a protein that temporarily holds the sperm on the egg's surface . </P>

The acrosomal proteins break through the layer of granulosa cells