Principal piece

The principal piece is the long middle region of the sperm flagellum. Outside the axoneme, the central microtubule cytoskeleton of the sperm tail and force generator for the flagellar beat, are the outer dense fibers (ODFs), which are believed to stabilize the axoneme and protect the sperm tail against shear forces. In the principal piece, two of the ODFs are replaced by the longitudinal columns of the fibrous sheath. The fibrous sheath protects and supports the axoneme, but also has a role in providing energy for flagellar motility through glycolysis.