Fig. 3
From: The annulus: composition, role and importance in sperm flagellum biogenesis and male fertility
Schematic representation of sperm flagellum biogenesis during spermiogenesis. During spermiogenesis (see steps 1 to 5), the sperm cell undergoes a set of morphological modifications allowing the transformation of a round spermatid to an elongated spermatid, and ultimately, a spermatozoon. The fusion of the Golgi vesicles allows the formation of the acrosome, and the compaction of the DNA by protamines leads to the nucleus condensation. At the same time, the flagellum initiates its growth with the assembly of the axoneme from the centriole, and positioning of the peri-axonemal structures (a). During flagellar growth the annulus slides along the axoneme, accompanied by the chromatoid body (b–d). Upstream, the mitochondria are arranged in a mitochondrial sheath. Then the annulus stops at its final position between the midpiece and the principal piece (e). The excess of cytoplasm and organelles are eliminated into the residual body (e). Ac: Acrosome, N: Nucleus, An: Annulus, Ax: Axoneme, C: Centriole, CB: Chromatoid body, G: Golgi, Mt: Manchette, M: Mitochondria, SSB: Spindle Shaped Body, FS: Fibrous Sheath, SC: Segmented Column, ODF: Outer Dense Fiber, BP: Basal Plate, RB: Residual Body. Adapted from Holstein and Roosen-Runge 1981. Created with BioRender.com