Ultrastructure of the epithelial cells in the aspermatogenic part of the testis in Mimagoniates barberi (Teleostei: Characidae: Glandulocaudinae) and the role of their secretions in spermatozeugmata formation.

Two types of unencapsulated sperm bundles are present in the milt of the internally inseminating characid fish, Mimagoniates barberi. Our ultrastructural study revealed that sperm bundles of both types are formed within the efferent ducts of the posterior, aspermatogenic part of the testis. The epithelial cells of the testicular tubules of the aspermatogenic portion of M. barberi testis show signs of both secretory and phagocytic activity. Rough endoplasmic reticulum, polyribosomes, Golgi complexes, secretory vesicles and vacuoles abound in the cytoplasm. Adluminal surface of the epithelial cells often exhibits extensions of various form. Two types of secretions are present in the efferent ducts. In more proximal ducts the secretion is finely granular, whereas in more distal ducts groups of coalescing spermatozoa are accompanied by a fibrous secretion. The sperm are aligned along strings of this secretion which is most probably instrumental in sperm clumping. In the most caudal portion of the testis where only fully formed sperm bundles of both types are present no structured secretion in the lumen of the efferent ducts is found. No secretion between the sperm within the bundles is visible. The results of histochemical staining indicate that the secretion in the tubules has no carbohydrate component and is proteinaceous.