Figure 1.

Structure of a mature human sperm. Sperm flagellum is structurally divided into three parts: mid-piece, principal piece and end-piece; the cross-section of each part is shown. Annulus is a ring structure demarcating the mid-piece and the principal piece of the sperm tail. The axoneme is a highly evolutionarily conserved structure present in the whole flagellum. The schematic structure of the axoneme from the transverse view illustrates the approximate localizations of each component. The mid-piece consists of a helical MS surrounding the axoneme, which is replaced by the FS in the principal piece; the terminal piece is devoid of any peri-axonemal structures. The red box at top right is a longitudinal view illustrating the outer doublet A-tubule of IADs, OADs and regulatory structures within a single 96 nm axonemal repeat. DNAH1 may form an arc-like structure directly connected to the RS3.5 The black box at top left is the schematic structure of the connecting piece. The suggested locations of DNAH1, CFAP43, CFAP44, AK7, AKAP3, AKAP4, CEP135 are also marked in the drawing with red fonts and arrows. RSs: radial spokes; CPMA: central pair microtubule apparatus; IC/LC: intermediate chain/light chain; N-DRC: nexin-dynein regulatory complex; MIA: modifier of inner arms complex; CSC: calmodulin- and spoke associated complex; DP: distal protrusion; MS: mitochondrial sheath; FS: fibrous sheath; IADs: inner arm dyneins; OADs: outer arm dyneins.