Acrosome reaction

Acrosome reaction

During fertilization, the acrosome reaction is the reaction which 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.

As the sperm approaches the zona pellucida of 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.

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.


In mammals the acrosome reaction releases hyaluronidase and acrosin; their role in fertilization is not yet clear. The acosomal reaction does not begin until the sperm comes into contact with the Oocytes gelly layer. Once the sperm comes into contact with the Oocytes gelly layer the acrosomal process begins. After the Jelly layer is penetrated 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 graduals inside the Oocyte then fuse to the outer membrane and a transient fast block reaction occurs.

It also alters a patch of pre-existing sperm plasma membrane so that it can fuse with the egg plasma membrane.

ee also

* ZP3
* Cortical reaction

The acrosomal reaction takes place in the fallopian tube (site of fertilization) when the sperm penetrates the primary oocyte.First stage is the penetration of corona radiata, by releasing hyaluronidase from the acrosome. Second stage is penetrating the zona pellucida, this occurs by releasing acrosin from the acrosome. After the two stages the head and tail of the sperm go into the oocyte's cytoplasm while the membrane fuses with the oocyte's membrane, meanwhile zona reaction occurs in order to prevent another spermatocytes to enter.

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