Equatorial segment

The equatorial segment is a specialized ring-shaped region in the middle of the sperm head. The distinction of this region becomes evident during late spermatogenesis. The composition includes membrane proteins as well as some intracellular proteins, like annexins and heat shock proteins, some of which appear at this region after the acrosome reaction. The function of the equatorial segment is elusive, but it seems to play a role in the plasma membrane fusion between the spermatozoon and the oocyte.

The equatorial segment can be seen as a peripheral ring, located at the border of the underlying outer acrosomal membrane and inner acrosomal membrane. Some proteins that localize to the acrosome prior to the acrosomal reaction, are instead seen at the equatorial segment in acrosome-reacted sperm.