Acrosome

The acrosome is a membrane-bound compartment found in the head of spermatozoa. It is derived partially from vesicles originating at the Golgi apparatus and partially from vesicles originating at the plasma membrane of sperm progenitor cells. The acrosome functions as a storage compartment for various hydrolytic enzymes and other factors that enables the binding to and degradation of the protective layers surrounding the oocyte. These layers consist of cumulus cells as well as an extracellular matrix known as the zona pellucida. Before reaching the oocyte, sperm cells go through a remodeling process, called the acrosome reaction, leading to the opening of channels and fusion between the anterior acrosome membrane and the plasma membrane, thus releasing the stored enzymes that degrade the egg vestments and allow penetration of the oocyte.

The acrosome can be found in the anterior part of the sperm head, as a flattened sac between the nucleus and the proximal plasma membrane. The inner acrosomal membrane lies just outside the nuclear envelope and the outer acrosomal membrane is opposed to the inner face of the plasma membrane. At the border between the iner and outer acrosomal membrane is the equatorial segment.

Read more about the acrosome as a substructure of sperm.