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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.

By removing all or most organelles, we have exposed the cytoplasmic side of the plasmalemma and its specializations in serous cells and in cells of striated and excretory ducts of human major salivary glands. The areas of plasmalemma located beneath the lumen and those bordering the intercellular canaliculi are covered by evenly distributed particles arranged in a continuous band and, below it, in regularly spaced clusters. A similar pattern of particles is seen on the internal aspects of the juxtaluminal plasmalemma of cells of both striated and excretory ducts. Small isolated clusters of particles are seen in other regions of serous and ductal cells as well, being particularly numerous along the basal processes of cells of striated ducts. A distribution of particles resembling that present along intercellular canaliculi of serous cells also is seen on the plasmalemma bordering the biliary canaliculi where, however, the clusters look smaller and farther apart. Large clusters of particles, matching those seen on salivary glands and on liver, are present at the base of the short processes of cells of the stratum spinosum of squamous stratified epithelia. Since the sites of location of the clusters closely correspond to the areas where transmission electron microscopy (TEM) reveals the presence of desmosomes, we believe that the clusters may be related to these cellular junctions. Of more difficult interpretation are the particles present on the juxtaluminal band corresponding both to the zonula occludens and to the zonula adhaerens.

The parotid gland of a female African mole-rat, Tachyoryctes splendens, was examined by light and electron microscopy. A pure seromucous gland of conventional histology, its striated and excretory ducts contain prominent cytoplasmic inclusions that are lightly stained with toluidine blue. At the ultrastructural level, the inclusions are seen to consist of a light to moderately dense structureless matrix in which are suspended scattered, randomly oriented filaments that either are 4-7 nm or approx 14 nm thick. A few multivesicular bodies or putative lipid droplets are sparsely distributed in the inclusions. Even though the inclusions lack an encompassing membrane, all other formed cytoplasmic structures are excluded. These inclusions are not present in the ducts of the submandibular gland of the mole-rat. The inclusions may confer special properties on the parotid gland of this fossorial animal that permit it to cope with the exigencies of a subterranean existence.

The duct cells of the mandibular glands of mice (and many other mammalian salivary glands) absorb NaCl from an isotonic, Na+-rich primary saliva, formed by the gland's secretory endpieces, utilising an amiloride-sensitive Na+ channel in the apical (luminal) domain of the plasma membranes. The present study focuses on the mechanisms whereby the apical membrane Na+ conductance is controlled so that the rate of Na+ influx from lumen to cytosol via the Na+ channels is matched to the rate of Na+ extrusion from cytosol to interstitium via the basolateral Na+-K+-ATPase (so called homocellular regulation or epithelial cross-talk). Our results show that the apical membrane Na+ conductance is not controlled by a sensor of extracellular (luminal) Na+, as has been previously believed, but by sensors of cytosolic Na+ and Cl- which down-regulate the Na+ channels when the cytosolic concentration of either ion increases. These effects of cytosolic Na+ and Cl- are mediated, respectively, by G proteins of the Gi and Go subclasses.