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. 2023 Feb 1;14(2):383. doi: 10.3390/genes14020383

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© 2023 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Schematic view of a human sperm cell and transmission electron microscope (TEM) images of a cross-section from the subregions of the sperm flagellum, which is subdivided into the connecting piece (CP), the midpiece (MP), the principal piece (PP; which can be further subdivided into proximal PP (PPP) and distal PP (DPP)), and the endpiece (EP). (a) Cross-section of the sperm MP showing the axoneme (Ax, light-blue dotted circle), the outer dense fibers (ODFs), and the mitochondrial sheath (M). (b) Cross-section of the sperm PPP showing the axoneme (Ax, light-blue dotted circle), the ODF, and the fibrous sheath (FS). (c) Cross-section of the sperm DPP showing the axoneme (Ax, light-blue dotted circle), and the FS. (d) Cross-section of the sperm EP, showing the axonemal disorganization that distinguishes this region. Legend: A, acrosome; An, annulus; C, proximal centriole; CtD, cytoplasmatic droplet; N, nucleus; NV, nuclear vacuole; RM, redundant membrane.