Fig. 1 in Sperm morphology and some aspects of acrosomal complex development in four species of Heteronemertea (Pilidiophora, Nemertea)

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.