Three-dimensional structure of apical vesicles of tuft cells in the main excretory duct of the rat submandibular gland.

Tuft cells are present in the mucosal epithelium of a number of hollow organs including the main excretory duct (MED). Despite their distinctive features such as the long, thick, blunt microvilli with prominent rootlets and the large number of vesicles in the apical cytoplasm, the hypolemmal terminal-tuft cell relationship and the true form of the various vesicles and tubules are still controversial. The present study investigated the above mentioned features of tuft cells in the MED of rat submandibular gland by computer three-dimensional (3-D) reconstruction with focus on their function. Computer 3-D reconstruction revealed that nerve endings are present at both sides of the basal portion of the lateral cytoplasmic branch of tufts cells and that the apical tubulovesicular system of these cells consists of two separate components: the complex and coherent vesicles and the small network of tubules. We suggest that such a system may be involved in the rapid changes of surface area observed in tuft cells. Furthermore, our findings demonstrate that the images seen in thin sections and formerly regarded as evidence for the presence of variations in the shape of the tubules and of the vesicles are in reality the product of the different angles at which the tubulovesicular system was sectioned. Finally, a few vesicles and tubules that were not part either of the complex or of the network, also were found.