Structure of the sperm flagellum. (a) Mammalian sperm flagellum is structurally divided into three areas: midpiece, principal piece and end piece. (b) Schematic cross section through a representative segment of the principal piece showing the plasma membrane (PM) surrounding seven outer dense fibers (ODF, there are nine ODF in the midpiece). The fibrous sheath (FS) is composed of two longitudinal columns (LC) which are connected by transverse ribs (TR). Within the ODF, are the components of the axoneme. (c) The axoneme is enlarged and the offset shows: the nine outer microtubule doublets of the axoneme (MTs) with associated inner dynein arms (IDA), outer dynein arms (ODA), radial spokes (RS), nexin-dynein regulator complex (N-DRC), nexin links and the central pair of microtubules (CP).