Annulus

The annulus is a cytoskeletal structure, composed of septin rings, that resides between the midpiece and principal piece of sperm flagella. It has been hypothesized to act as a selective diffusion barrier between these regions, thus contributing to spatial compartmentalization of the flagella. During spermatogenesis, the annulus migrates towards the midpiece border, at the same time as mitochondria are assembled in a helical pattern behind it, ultimately lining the the axoneme in the mid piece

The annulus is visible as a small dense region between the midpiece and principal piece of the flagellum.

Read more about the annulus as a substructure of sperm.