Uptake of cationized ferritin by the epithelium of the main excretory duct of the rat submandibular gland.

Previous studies demonstrated that the main excretory duct (MED) of the rat submandibular gland can internalize exogenous protein in addition to reabsorbing and secreting electrolytes. However, more precise studies have not been conducted. The aim of this study was to elucidate the cell types responsible for endocytosis of an exogenous protein (ferritin) and to follow the movements of the endocytosed protein in the ductal epithelial cells. The MEDs of the right submandibular gland of male Wistar rats were exposed near the glands proper and cationized ferritin solution was injected into each MED through a fine glass cannula. The MEDs were removed at intervals after ferritin injection, fixed and examined by transmission electron microscopy. The epithelium of the MED of the rat submandibular gland was pseudostratified and consisted of light (types I and II), dark, tuft and basal cells. Uptake of ferritin by the light (types I and II) and dark cells occurred frequently. Small vesicles and multivesicular bodies containing ferritin particles were observed in the supra-nuclear and lateral nuclear cytoplasm. Endocytosis of tracers by tuft cells was rare. Some of the small vesicles and the multivesicular bodies were acid phosphatase-positive. By 60 min after treatment, ferritin-containing small vesicles and multivesicular bodies appeared in the basal cytoplasm. Ferritin particles were also observed in basal extracellular spaces. The light (types I and II), dark and tuft cells (latter rarely) participated in endocytosis of exogenous proteins in the epithelium of the MED of the rat submandibular gland. Almost all of the internalized proteins appeared to be processed by the lysosomal system, and some proteins were released into the extracellular spaces.