Spermatogenic cycle in the testis of the Japanese black bear (Selenarctos thibetanus japonicus).

Spermatogenic cycle in the testis of the japanese black bear (Selenarctos thibetanus japonicus) was studied by light and transmission electron microscopy. By light microscopy, spermatids were allocated into eleven steps based on morphological changes in the nucleus and the acrosome of spermatids. Cellular associations of the seminiferous epithelium were allocated into eight stages based on the changes in the nucleus and acrosome of spermatids, appearance of meiotic figures and time of spermiation. Cross-sections of the seminiferous tubule seldom contained more than one type of stage. Spermatids at steps 1-2 had the well-developed Golgi complex. The crescent-shaped Golgi complex was accompanied by the acrosome extending over the nucleus at steps 3-5. At step 6, spermatids faced the base, and the outer membrane of the acrosome converged upon the plasma membrane of spermatids. The acrosome projected into the cytoplasm of Sertoli cells at step 9. At step 11, most of the cytoplasm was phagocytosed by Sertoli cells, and spermatids were released in the lumen to become spermatozoa.