<P> During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the female egg in order to fertilize it . Fusing to the egg usually causes little problem, whereas penetrating through the egg's hard shell or extracellular matrix can present more of a problem to the sperm . Therefore, sperm cells go through a process known as the acrosome reaction which is the reaction that occurs in the acrosome of the sperm as it approaches the egg . The acrosome is a cap - like structure over the anterior half of the sperm's head . </P> <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>

When does the acrosome reaction occur in sperm