In spermatozoa from primitive species (such as sea urchins and fishes) and in flagellated unicellular organisms (such as Chlamydomonas), the flagellum consists of the 9 + 2 axoneme surrounded by the cell membrane. In animals with internal fertilization, the sperm axoneme is surrounded by outer dense fibers and the fibrous sheath (Fig. 5.1).
Each of the nine microtubule doublets is associated with an outer dense fiber, that has a characteristic size and shape (Fig. 5.1). These structures are composed of several cystein-, serine- and proline-rich intermediate filament-like proteins with a high degree of zinc-dependent disulfide cross-bridging (Oko and Clermont, 1990). Outer dense fibers could be involved in the maintenance of the passive elastic structure of the flagellum but their high and variable level of phosphoserine on proteins suggests some yet undetermined role in the regulation of flagellar motil-ity (Inaba, 2003; Oko and Clermont, 1990).
The fibrous sheath is composed of three longitudinal columns attached to microtubule doublets 3 and 8 that run along the principal piece of spermatozoa (Oko and Clermont, 1990) (Fig. 5.1). This structural arrangement and the stabilization of proteins by disulfide bonds, suggest that the fibrous sheath influences the flexibility, the plane of flagellar motion and the shape of the flagellar beat (Eddy et al., 2003). AKAPs represent more than 50% of the fibrous sheath mass in some species (Eddy et al., 2003) and their tyrosine phosphorylation could be associated with fibrous sheath sliding (Carrera et al., 1994;Mohri et al., 1998; Si and Okuno, 1999). The Rho-binding protein, rhophilin, is located at the outer surface of the outer dense fibers and its binding protein, ropporin, is anchored at the inner surface of the fibrous sheath (Fujita et al.,2000). Enzymes associated with the glycolytic pathway, such as glyceraldehyde 3-phosphate dehydrogenase-S (Miki et al., 2004; Westhoff and Kamp, 1997), hexokinase HK1-S, glutathione S-transferase GSTM5 and an intermediate filament-like protein FS39, are also found in the fibrous sheath (Eddy et al., 2003; Mohri et al., 1998). The scaffolding of glycolytic enzymes and the presence of constituents of signal cascades on the fibrous sheath supports the importance of this structure for sperm motility.
Mammalian sperm mitochondria are condensed and elongated and they are tightly and helically disposed end to end around the outer dense fibers forming the mitochondrial sheath at the level of the sperm middle piece. These mitochondria have specific functional characteristics adapted for sperm motility such as resistance to hypotonic conditions and ability to use lactate as an oxidative substrate (Oko and Clermont, 1990).
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