Ultrastructural aspects of the main excretory duct of rat submandibular gland

Recent data indicate active transport of Na+ and K+ by the main excretory duct (MED) of the rat submandibular gland. In view of these data submandibular MED's of four adult rats were investigated by transmission electron microscopy. Luminal surfaces of two MED's were examined with the scanning electron microscope. The duct is surrounded by connective tissue, blood vessels and neuron perikarya. Blood supply to the ductal epithelium consists of a plexus of sinusoidal capillaries. Three principal cell types compose the epithelium: light cells, dark cells, and basal cells. The basal cells resemble those of striated ducts except for the presence of numerous hemidesmosomes along basal plasma membranes of MED basal cells. Light cells show basal infoldings, and bulbous enlargements may occur at their distal aspects. Dark cells are narrow, electron dense cells with prominent microvilli at the luminal surface. Intercellular clefts penetrate MED epithelium from the lumen to a depth of one‐third the epithelial thickness. Surfaces of the clefts are lined by dark and light cells. Intercellular tissue spaces are present at lateral boundaries of all cells and extend from the epithelial base to distal cytoplasmic levels.