Cellular organisation of the mature testes and stages of spermiogenesis in Danio rerio (Cyprinidae; Teleostei)—Structural and ultrastructural studies

The male gonads of Danio rerio occupy a position typical of the Teleostei species. The structure of the testes corresponds to the anastomosing tubular type with unrestricted spermatogonia and represents a cystic type of spermatogenesis. The results of this study indicate that four distinct stages of cell differentiation can be identified during spermiogenesis. These stages are characterised by chromatin condensation, the development of flagellum, nuclear rotation, the formation of nuclear fossa and the elimination of excess cytoplasm. A round head and the absence of an acrosome characterise the differentiated sperm. The midpiece is short and large, and C-shaped mitochondria form a ring surrounding the initial region of the flagellum. The axoneme shows a 9 + 2 pattern. In the D. rerio spermatozoa the flagellar axis is at an angle of 110° to the nucleus diameter running through the centriole.

► The process of spermatogenesis in Danio rerio has been described.
► The testes correspond to the anastomosing tubular type with unrestricted spermatogonia.
► Four distinct stages of cell differentiation can be identified during spermiogenesis.
► Chromatin condensation, development of flagellum, nuclear rotation and rejection of excess cytoplasm.
► The flagellar axis is formed at an angle of 110° to the nucleus diameter running through the centriole.