Ultrastructural study of spermatogenic stages in the protandrous sparid fish Diplodus cervinus cervinus (Lowe, 1838) from the South Eastern Mediterranean coast

Abstract

The current study was designed to study the ultrastructure of the spermatogenic stages of the protandrous hermaphrodite sparid Diplodus cervinus cervinus. Although, it is a useful tool to enhance understanding of germ cells differentiation in this economic species, none of the available references paid attention to the studied species. The testis of the studied specie is tubular in shape and the germ cells are arranged in cysts or clusters within the seminiferous lobules. Spermatogenesis occurs in several places along the length of each lobule and induced by the action of the somatic steroidogenic secretory cells which are known as Leydig cells. Such cells contained four main morphological structural characteristics a vesicular nucleus, ovoid and elongated mitochondria with tubular cristae, a number of smooth endoplasmic reticula, and a considerable amount lipid droplets in the cytoplasm. Spermatogenic cyst displays round shaped cells with large nuclei containing clumps of heterogenic dense chromatin and reduced cytoplasm known as primary spermatogonia. They undergo a series of mitotic divisions to reach the secondary spermatogonia stage; such cells irreversibly divide meiotically to form primary and secondary spermatocytes. Spermatids are seen in different stages of spermiogenesis, as indicated by the degree of chromatin condensation. They underwent a shape remodeling and a size reduction during spermiogenesis as the morphology of the spermatid nucleus gradually changed and several mitochondria and centrosomes moved to a position just behind the nucleus of the spermatid. Spermatozoan ultrastructure showed that it is anacrosomal primitive teleostean type (type I) with round head, the posterior part of the nucleus is indented a nuclear fossa which displays a bell-shaped in longitudinal section and circular in transverse section containing centeriolar complex, part of the basal body and cytoplasm. The chromatin is heterogeneously granular, high electron-dense containing tightly packed fibers. The mid piece is short containing mitochondrial ring which consists of spherical or ovoid mitochondria. The axoneme consists of nine double outer tubules and single microtubular constructions. In conclusion, the testis structure could be described as the unrestricted spermatogonial testicular type. More attention should be rewarded to the female of this species in order to give a clear view about the process of oogenesis. Key words: Spermatogenesis, Sparid, protandrous, hermaphrodite, anacrosomal, primitive teleostean spermatozoa.