Abstract
This study reports a novel form of testicular development in nurse sharks (Ginglymostoma cirratum) and provides the first histological description of spermatogenesis in this species. Radially structured testes developed from the caudal to the cranial regions, a pattern that appears to be unique among shark species. Testes from immature males had spermatogonia and Sertoli cells, while in testes of developing and capable to reproduce males, the nuclei of Sertoli cells were evident in basal positions of spermatocyst. The spermatogonia divide to form primary spermatocytes, secondary spermatocytes divide to produce spermatids, and spermatids then move to the periphery of the spermatocyst. Subsequently, the cysts with spermatids were observed at different stages of maturation, and spermatids became spermatozoa. In the final stage of spermatogenesis, spermatozoa had their heads directed towards the basal membrane and flagella directed towards the lumen. Examinations of testes development in a broad range of shark species are now required to determine whether this novel form of testes development observed in nurse sharks is shared by other elasmobranch species.
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