Testis Structure, Spermatogenesis, and Spermatozoa in Teleost Fishes

In all teleosts the testes are mainly paired organs composed of seminiferous tubules or lobules entering the testicular main ducts and the spermatic duct forming an unpaired structure opening at the genital papilla. The interstitial tissue and the germinal epithelium consist of germinal cells (spermatogonia, spermatocytes, spermatids, spermatozoa) supported by somatic (=Sertoli) cells. During spermatogenesis the germinal and somatic cells form spermatocysts, which are the functional unit providing a blood testis barrier. This cyst starts to develop when Sertoli cells processes enclose a single spermatogonium comprising an isogenic clone of secondary spermatogonia and all germinal cells at further stages of spermato- and spermiogenesis. The testes in more basal taxa are tubular with anastomosing tubules. The tubules in the testes of most externally fertilizing species show a similar structure alongside the entire testis, but in species practicing insemination, different morphological adaptations coupled with dividing testis into a spermatogenic and a aspermatogenic part occur. The spermatogenic part is a place filled with cysts containing developing spermatozoa, while the aspermatogenic part is a storage organ for spermatozoa as well as sperm bundles called spermatozeugmata or spermatophores. In more derived taxa the testes are lobular. Additionally, according to the distribution of spermatogonia, the lobular testis are classified as: a) unrestricted spermatogonial testis type, where spermatogonia are situated along the lobules (e.g. in Percidae), and b) restricted spermatogonial testis type, where spermatogonia are situated only at the distal end of the lobule just beneath the tunica albuginea as in Atherinomorpha. Independent of testis type the process of spermatogenesis finally leads to the development of species-specific spermatozoa, which in all teleosts are anacrosomal. The spermatozoal ultrastructure shows a great diversity in shape and size of the head (mainly the nucleus), location and number of mitochondria, number of flagella or their lack, mutual arrangement of centrioles and the intercentriolar apparatus as well as the presence of additional structures such as pseudoflagella, striated rootlet, accessory microtubules, lateral fins, cytoplasmic sleeve, fenestrated membrane, dense fibers etc. According to the mode of fertilization and the presence or lack of the flagellum, the spermatozoa are classified as aquasperm (aflagellate, uniflagellate and biflagellate) or introsperm with a simple or complex structure.