Calyx

The calyx is the postacrosomal part of the perinuclear theca, thus localized between the nuclear envelope and the plasma membrane. Proteins in this region help shape and organize the sperm head, but also have roles in sperm–egg interactions and egg activation during fertilization.

The calyx can be seen as a section in the posterior part of the sperm head.

Read more about the calyx as a substructure of sperm.