Acrosome

In spermatozoa of many animals, the acrosome is an organelle that develops over the anterior half of the spermatozoon's head. It is a cap-like structure derived from the Golgi apparatus. Acrosome formation is completed during testicular maturation. In Eutherian mammals the acrosome contains digestive enzymes (including hyaluronidase and acrosin).

Acrosome reaction
As the sperm approaches the egg, the membrane surrounding the acrosome fuses with the plasma membrane of the sperm, exposing the contents of the acrosome and rendering the sperm capable of fusing with the egg. 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. 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 plasma membrane. In some lower species, including sea stars and sea urchins, a major portion of the exposed acrosomal contents is a protein that temporarily holds the sperm on the egg surface. In mammals the acrosome reaction releases hyaluronidase and acrosin; their role in fertilization is not yet clear. It also alters a patch of pre-existing sperm plasma membrane so that it can fuse with the egg plasma membrane.