Micrura bella

3.1. Sperm morphology in M. bella

The sperm head is elongated, with a maximum length of ca. 9 μm and a width of ca. 1 μm in the widest region of nucleus, and slightly asymmetric, mainly due to a slight curvature of the nucleus (Fig. 1A, B). In longitudinal sections, the small acrosomal complex, the nucleus, mitochondria, and centrioles are well discernible (Fig. 1C). The dimensional characters of sperm organelles are summarized in Table 2.

In the acrosomal complex, two types of acrosomal vesicles can be recognized (Fig. 1D): (1) single large, cap-shaped, electron-dense vesicle of ca. 0.5 μm in diameter, with an invagination in its basal part (Fig. 1E), located apically and centrally (Fig. 1D), and (2) several small, rounded vesicles of ca. 0.1 μm in diameter with moderate electron density, located below the large acrosomal vesicle and peripherally (Fig. 1D, F); the central region, the space between the small vesicles is filled with subacrosomal flocculent material (Fig. 1D, F). A thin layer of electron-dense material is visible between the large acrosomal vesicle and plasma membrane (Fig. 1D). In transverse sections, this layer appears as a ring around the large acrosomal vesicle (Fig. 1G).

The slightly concave nucleus contains well-packed, electron-dense chromatin (Fig. 1C, H). The anterior part of the nucleus is slightly convex (Fig. 1C), whilst posteriorly it has a nuclear fossa, partly hosting the proximal centriole and several depressions at sites where the nucleus contacts the mitochondria (Fig. 1H, I).

The midpiece consists of five rounded mitochondria (Fig. 1I) slightly elongated along the anterior/posterior axis (Fig. 1B, H), with the greater diameter of ca. 0.7 μm. The mitochondria surround a pair of centrioles located perpendicularly to each other (Fig. 1H). The distal centriole has a pericentriolar complex formed by 9 satellite fibers directed towards the plasma membrane (Fig. 1J).

The microtubullar organization of the axoneme of flagellum is 9 × 2 + 2 (Fig. 1K).