Fine structure of tuft cells of the main excretory duct epithelium in the rat submandibular gland.

Tuft cells, which are characterized by long microvilli with prominent rootlets and by vesicular and tubular profiles, are present in the mucosal epithelium of a number of hollow organs, including the main excretory duct of the rat submandibular gland. Despite their widespread occurrence, little is known of their function. Main excretory ducts of the submandibular gland were obtained from 20 male Wistar rats and prepared for electron microscopic examination. Specimens also were subjected to various histochemical procedures, including HRP (horseradish peroxidase) uptake, the demonstration of catalase in peroxisomes, glycoconjugate cytochemistry, and ruthenium red staining. The features of the tuft cells in the MED (main excretory duct) basically were similar to those described in other organs. However, for the first time it was observed that in the submicrovillus zone and intermicrovillus space, there were many membrane-bound electron-dense granules, which resemble the so-called glycocalyceal bodies found on the apical surface of normal and neoplastic intestinal-type epithelial cells. Hypolemmal nerve terminals were seen to be in contact with the basal portion of tuft cells. After injection of HRP, reaction products were observed on the luminal surface of the microvilli, but not within the vesicles. Vesicles and tubules in the supranuclear cytoplasm exhibited a positive cytochemical reaction for glycoconjugates. The surface coat of tuft cell microvilli was stained with ruthenium red, but the vesicles and tubules in the supranuclear cytoplasm remained unstained. Our observations suggest that the tuft cells may engage in secretory activity in addition to reception.