<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> <P> In mammals the acrosome reaction releases hyaluronidase and acrosin; their role in fertilization is not yet clear . The acrosomal reaction does not begin until the sperm comes into contact with the oocyte's zona pellucida . Upon coming into contact with the zona pellucida, the acrosomal enzymes begin to dissolve and the actin filament comes into contact with the zona pellucida . Once the two meet, a calcium influx occurs, causing a signaling cascade . The cortical granules inside the oocyte then fuse to the outer membrane and a transient fast block reaction occurs . </P> <P> It also alters a patch of pre-existing sperm plasma membrane so that it can fuse with the egg plasma membrane . </P>

What does the acrosome do in a sperm cell