Ultrastructure of the semicystic spermatogenesis in the South American freshwater characid Hemigrammus marginatus (Teleostei, Characiformes)

Semicystic, a rare type of spermatogenesis, was detected in the characid Hemigrammus marginatus and characterized by cysts hatching during the spermatid phase and maturation of the spermatozoa being completed at the lumen of the anastomosed seminiferous tubules. Primary spermatogonia, or type A, are distributed along the entire length of the seminiferous tubules, in an unrestricted spermatogonial pattern. H. marginatus spermiogenesis is included in type I, mainly characterized by presence of nucleus rotation. During this process, a vesicle resembling the acrosomal vesicle is visualized at the anterior region close to the nucleus of the early spermatids, however this structure did not remain in the spermatozoa. In H. marginatus, the spermatozoon is uniflagellated, primitive, type I aquasperm, with a rounded head, a short midpiece and a long flagellum with the axoneme in a 9 + 2 microtubules arrangement and no lateral fins. Residual spermatozoa are reabsorbed by Sertoli cells. Unusual biflagellate spermatozoa with three long cytoplasmatic projections originating in the midpiece are rarely observed and have not been registered in other characiforms. Ultrastructural characteristics of the spermatogenesis and spermatozoa observed in the present work provide important subsidies to systematic and phylogeny studies of Characidae fishes included in Incertae sedis groups, such as H. marginatus.