Acrosome reaction
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The acrosome reaction is the reaction which occurs in the acrosome of the sperm as it approaches the zona pellucida.
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.
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Variations among species
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.
Lower animal species
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 starfish and sea urchins, a major portion of the exposed acrosomal contents is a protein that temporarily holds the sperm on the egg surface.
Mammals
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.
