Sperm ultrastructure in Hemidonax pictus (Hemidonacidae, Bivalvia, Mollusca): comparison with other heterodonts, especially Cardiidae, Donacidae and Crassatelloidea Author Healy, John M. Author Mikkelsen, Paula M. Author Bieler, Rüdiger text Zoological Journal of the Linnean Society 2008 2008-06-30 153 2 325 347 https://academic.oup.com/zoolinnean/article-lookup/doi/10.1111/j.1096-3642.2008.00385.x journal article 10.1111/j.1096-3642.2008.00385.x 0024-4082 5447636 HEMIDONAX PICTUS ( FIG. 1 ) Acrosomal complex The acrosomal vesicle is conical, measuring 0.35 ± 0.02 Mm ( N = 4) in length and with a maximum diameter of approximately 0.4 ± 0.05 Mm ( N = 4) ( Fig. 1A–D ). Contents of the vesicle are differentiated into a highly electron-dense, faintly reticulate, basal ring sheathed by markedly less dense (and granular) material that also fills the anterior region of the vesicle ( Fig. 1A, C, D ). Associated with the vesicle apex is a broad, thin, electron-lucent layer, the extent of which is somewhat variable ( Fig. 1A, C , arrows). A narrow basal invagination extends for most of the length of the acrosomal vesicle and is filled by a granular deposit of subacrosomal material ( Fig. 1A– D ). The plasma membrane forms the outermost sheath of the acrosomal complex, as it does in other regions of the spermatozoon ( Fig. 1A, F ) (this applies in all the species of bivalves examined herein). Nucleus The nucleus is short (length 1.8 ± 0.1 Mm, N = 4) and barrel-shaped (tapers apically anteriorly), with a maximum diameter (near base) of 1.0 ± 0.1 Mm ( N = 4) ( Fig. 1B ). Nuclear contents are highly electron-dense, with scattered, irregularly shaped, electron-lucent lacunae. Broad, shallow depressions, present basally, contact the anterior surfaces of each midpiece mitochondrion ( Fig. 1B, E ). These depressions surround a smaller central recess filled with granular material loosely associated with the proximal centriole ( Fig. 1E ). Midpiece The midpiece is positioned at the base of the nucleus and consists of four spherical mitochondria surrounding a pair of orthogonally arranged centrioles ( Fig. 1B, E, F ). The entire midpiece region has a length of 0.75 ± 0.05 Mm ( N = 3) and a maximum diameter (of mitochondrial cluster) of approximately 1.44 ± 0.1 Mm ( N = 3). The centrioles lie in contact with each other and exhibit triplet microtubular structure which is largely obscured by a dense matrix ( Fig. 1E–G ). The distal centriole is connected by a series of nine satellite fibres each terminating in a Y-shaped fork attached to the plasma membrane ( Fig. 1E, G ). Flagellum The flagellum consists of a 9 + 2 microtubular configuration axoneme (nine microtubular doublets · Figure 1. Ultrastructure (TEM) of spermatozoa of Hemidonax pictus . A, longitudinal section (LS) showing acrosomal complex (acrosomal vesicle + subacrosomal material) at apex of nucleus. Arrow indicates apical electron-lucent layer. B, LS showing size proportions of acrosomal complex, nucleus and midpiece mitochondria. C, LS of acrosomal complex. Arrow indicates apical electron-lucent layer. D, transverse section (TS) through mid-length region of acrosomal complex. Note granular subacrosomal material in narrow invagination of acrosomal vesicle, and also two components of vesicle contents (basal ring and less-dense anterior region). E, LS of midpiece at base of nucleus. Paired, connected centrioles are continuous with flagellar axoneme and closely surrounded by mitochondria. F, TS of midpiece at level of distal centriole showing four mitochondria. G, TS of posterior extremity of distal centriole (triplet microtubules), showing attachment, via Y-links, to plasma membrane. H, TS of flagellae showing intact axoneme (1) and successive (narrowed, degenerate) levels of terminal region (2, 3). Abbreviations: a, acrosomal complex; av, acrosomal vesicle; br, basal ring (of acrosomal vesicle contents); dc, distal centriole; f, flagellum (featuring axoneme); m, mitochondrion of midpiece; n, nucleus; nl, nuclear lacuna; pc, proximal centriole; pm, plasma membrane; sf, satellite fibres; sm, subacrosomal material. surrounding a central pair of singlet microtubules) sheathed by the plasma membrane. The terminal region is characterized by a decreasing diameter and degeneration of the axoneme into singlet microtubules obscured by a dense matrix ( Fig. 1H ). From light microscopic observations, flagellar length is 51 ± 2.0 Mm ( N = 10). CARDIIDAE VASTICARDIUM VERTEBRATUM ( FIG. 2 ) Acrosomal complex The acrosomal vesicle is dome-shaped, 0.28 ± 0.02 Mm ( N = 4) in length and with a maximum diameter of 0.46 ± 0.02 Mm ( N = 4). The anterior surface of the vesicle may be flat, slightly concave or, most commonly, slightly convex ( Fig. 2A, B ). Contents of the acrosomal vesicle are divisible into a very prominent, and markedly electron-dense basal ring, and less dense material anteriorly ( Fig. 2B, C ). The basal ring is separated from the vesicle membrane by an electron-lucent layer, and within the ring itself two components can be distinguished: a very electrondense, internally reticulate outer layer and a less dense inner layer, both approximately 0.05–0.06 Mm in thickness ( Fig. 2B, C ). A broad invagination extends almost the full length of the acrosomal vesicle, and is filled with granular subacrosomal material ( Fig. 2B ). Such material also occupies the thin gap between the base of the acrosomal vesicle and the smooth apex of the nucleus. Nucleus The nucleus is rod shaped, curved in its anterior half and 4.4 ± 0.1 Mm long ( N = 5) ( Fig. 2A ). Maximum diameter is approximately 0.7 ± 0.1 Mm ( N = 5) just anterior to the midpiece. Nuclear contents are highly electron dense with the exception of scattered, irregularly shaped lacunae, especially in the basal region ( Fig. 2A, D, E ). Midpiece The midpiece exhibits a pair of centrioles (arranged orthogonally) surrounded by four spherical, cristate mitochondria and scattered dense granules (putative glycogen deposits) ( Fig. 2A, D–F ). The centrioles are composed of triplet microtubules slightly obscured by dense material ( Fig. 2D–F ). The proximal centriole is attached to a thin layer of material lining a very shallow indentation of the nuclear base. Proximal and distal centrioles are attached to each other by dense material, the distal one also being attached to the plasma membrane by a series of nine satellite fibres ( Fig. 2D, E ). The entire midpiece region has a length of 0.58 ± 0.05 Mm ( N = 4) and a maximum diameter (of mitochondrial cluster) of 1.27 ± 0.05 Mm ( N = 5). Flagellum The flagellum consists of a 9 + 2 microtubular configuration axoneme sheathed by the plasma membrane ( Fig. 2D–F ). From light microscopic observations, flagellar length is 48 ± 2.0 Mm ( N = 10).