Endocytosis in epithelial cells lining the rete testis of the rat.

The endocytic activity of the low cuboidal cells lining the rete testis was analyzed by electron microscopy following injection of various tracers into the lumen of these anastomotic channels. At 1 and 5 minutes after injection, cationic ferritin (CF) and concanavalin A-ferritin (Con A) were seen bound to the apical plasma membrane and to the membrane of subjacent vesicles or invaginations connected to this apical membrane. At 30 and 60 minutes, these tracers were found in intracytoplasmic vesicles and in vesicles connected to the lateral or basal plasma membrane as well as in the lateral intercellular space and in the lamina lucida of basal lamina. At 30 minutes, CF and Con A also appeared in the matrix of pale multivesicular bodies while at 1 hour dense multivesicular bodies were labeled. At 2 hours and later time intervals, the tracers accumulated in dense granules identified as lysosomes. Native ferritin (NF), concanavalin A-ferritin in presence of alpha-methyl-D-mannoside, and horseradish peroxidase or albumin bound to colloidal gold were all to be incorporated by the lysosomal system of these epithelial cells, as just described for CF and Con A, but these various tracers were not bound to the apical plasma membrane or to the membrane of cytoplasmic vesicles, nor were they found in the intercellular spaces or the lamina lucida at the base of the cells. Thus, the epithelial cells of the rete testis do not appear to be only involved in the uptake of substances from the lumen and their disposal by the lysosomal system, but also appear to contribute to the transport of certain macromolecules from the lumen to the laterobasal surfaces of the cells. These cells may thus play a role in determining the composition of the rete testis fluid.