Micrura bella (Stimpson, 1857)

Yurchenko, Olga V. & Chernyshev, Alexey V., 2022, Sperm morphology and some aspects of acrosomal complex development in four species of Heteronemertea (Pilidiophora, Nemertea), Zoologischer Anzeiger 299, pp. 38-48 : 40-42

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 ).

Figures

Fig. 1. Fine organization of spermatozoon in Micrura bella (A–C) SEM (D–J) TEM (A) General view of spermatozoon (B) Sperm head with visible acrosomal region (a), slightly curveted nucleus (n) and midpiece (mp) with mitochondria rounded or slightly elongated along anterior/posterior axis. Flagellum (f) is posteriorly oriented (C) Longitudinal section along sperm head that consists of acrosomal complex (ac), nucleus, midpiece containing mitochondria, and centriolar complex (cc) (D) Longitudinal section along acrosomal complex containing large electron-dense vesicle in apical position (dv), thin layer of electron-dense material surrounding the electron-dense vesicle (arrows), small vesicles with moderate electron density (mv) located beneath plasma membrane and lower than the region of large vesicle; flocculent subacrosomal material of moderate electron density (asterisk) fills the central region immediately under the electron-dense vesicle (E) Higher magnification of large acrosomal vesicle in longitudinal projection. Its basal part forms invagination (F) Transverse section across acrosomal complex at the level of small vesicles. The vesicles are located peripherally forming a ring, often open. Central part is filled by subacrosomal material (asterisk) (G) Transverse section across acrosomal complex at the level of electron-dense vesicle. The vesicle is surrounded by electron-dense ring (arrow) (H) Longitudinal section along midpiece with centriolar complex represented by proximal (pc) and distal centrioles (dc) and mitochondria. The basal part of the nucleus forms posterior nuclear fossa (pnf) and invaginations hosting mitochondria (arrow) at sites of their contact with the nucleus (I) Transverse section across midpiece containing five mitochondria (J) Pericentriolar complex of distal centriole formed by satellite fibers (sf) (K) Transverse section through flagellum. Scale bars: A =5 μm, B, C =2 μm, D-G =0.2 μm, H-J = 0.5 μm, K = 0.1 μm.