Cells in the duct system of the rat submandibular gland.

The duct system of the rat submandibular gland (granular convoluted tubule [GCT], striated duct, excretory duct, main excretory duct [MED] and salivary bladder) was studied using scanning and transmission electron microscopy, with special reference to the dark and tuft cells. The MED was subjected to histochemistry for horseradish peroxidase (HRP) uptake. The epithelium of the duct system was composed of a heterogeneous cell population. On scanning electron-microscopic observation, principal, dark, and tuft cells were easily distinguished by their microvilli. Principal cells had small microvilli and microridges on their surface. The principal cells of the striated duct had secretory granules in the apical cytoplasm, but those of the excretory duct, the MED, and the salivary bladder had electron-lucent vesicles. Except for the GCT and salivary bladder, the basal infoldings of the epithelial cells were well developed. The dark cells were characterized by regular microvilli, and many tubules and vesicles confined to the apical cytoplasm. They existed in the striated duct, the excretory duct, the MED, and salivary bladder. The so-called pillar cells in the GCT appeared to be identical to the dark cells. Tuft cells had on their surface long, blunt microvilli with prominent rootlets. Many vesicles were observed in the apical cytoplasm. It was concluded that dark and tuft cells were common in the striated duct, the excretory duct, the MED and salivary bladder. Dark cells were observed also in the GCT. Dark and principal cells of the MED showed HRP uptake but tuft cells did not.