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Fig. 1 | Basic and Clinical Andrology

Fig. 1

From: Assessment of the frequency of sperm annulus defects in a large cohort of patients presenting asthenozoospermia

Fig. 1

a Flagellum assembly during sperm terminal differentiation in mammals. Scheme from Aminata Toure. Following meiosis, the spermatids undergo a set of morphological changes that specify the flagellum and the acrosome (Ac), required for motility and interaction with the oocyte, respectively. The acrosome (Ac) is formed by the fusion of vesicles from the Golgi apparatus, and the nucleus (N) is highly condensed by the germ cell-specific histones and protamines (steps 1–3). The flagellum is formed by the assembly of the microtubules, which constitute the axoneme (Ax) and the periaxonemal structures, the dense fibers (DF) and fibrous sheath (FS). The annulus (An) is assembled in the cytoplasm, at very early stages of flagellum assembly (step 2). It then associates with another electron-dense structure, the chromatoid body (CB), which consists mostly of RNA and RNA-binding proteins (step 3). During the extension of the flagellum,, the mitochondria (M) align along the axoneme (Ax) and the excess cytoplasm is removed as residual bodies (RB). The annulus (An) then moves towards its final position at the junction of the midpiece and the principal piece (steps 4–7). Ac: acrosome; An: annulus; Ax: axoneme; CB: chromatoid body; RB: residual body; DF: dense fibers; FS: fibrous sheath; M: mitochondria; N: nucleus. b Electron micrograph of human spermatozoa. Image from Aminata Touré & Alain Schmitt. Human spermatozoa with the head on the right, and the flagellum on the left. The flagellum is divided into two main compartments: the midpiece, which comprises the mitochondrial sheath, and the principal piece, characterized by the presence of a fibrous sheath surrounding the axoneme. The annulus is distinguishable at the junction of the midpiece and principal piece, as a fine electron-dense structure apposed to the plasma membrane. Bar: 1 μm. Ac: acrosome; An: annulus; Ax: axoneme; CB: chromatoid body; RB: residual body; DF: dense fibers; FS: fibrous sheath; M: mitochondria; N: nucleus

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