Nature and function of endocytosis in Sertoli cells of the rat

The endocytic activity taking place at the apex of Sertoli cells was analyzed by using various tracers to demonstrate fluid-phase and adsorptive endocytosis. Native ferritin and protein-gold complexes, used to demonstrate fluid-phase endocytosis, were internalized by Sertoli cells at all stages of the cycle of the seminiferous epithelium. At 5 min after injection, the tracers were found in the large spherical or C-shaped vesicles seen in the apical processes; at 15 and 30 min, the tracers accumulated in light multivesicular bodies, and at 60 min in dense multivesicular bodies and lysosomes. At later time intervals, an increasing number of lysosomes contained the tracers. Following injection of cationic ferritin or concanavalin A-ferritin, these tracers, used to demonstrate adsorptive endocytosis, were found to be found to the plasma membrane of Sertoli-cell apical processes but were not collected by the large cytoplasmic apical vesicles, multivesicular bodies, or lysosomes. As an exception, a few special multivesicular bodies seen in the large apical processes which encapsulate the heads of late spermatids at stage VII of the cycle contained these tracers. During stage VIII of the cycle, the residual bodies which detach from the mature spermatids are phagocytosed by Sertoli cells. Such residual bodies do not contain their own hydrolytic lysosomal enzymes. However, it was observed that the lysosomes which form as a result of the fluid-phase endocytic activity of the Sertoli cells fuse with the phagocytosed residual bodies and by means of their hydrolytic enzymes contribute to the rapid disintegration of these bodies. Thus, during stage VIII of the cycle, the Sertoli cells integrate two distinct processes, i.e., fluid-phase endocytosis and phagocytosis.