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.

These results suggest that fibulin-5 up-regulated in response to tension strain may control the formation of microfibril bundles in periodontal ligament.

The elastic system fibers comprise oxytalan, elaunin and elastic fibers, which differ in their relative microfibril and elastin content. Human periodontal ligaments (PDL) contain only oxytalan fibers (pure microfibrils) among them. Since fibulin-5 regulates the organization of elastic fibers to link the fibers to cells, we hypothesized that fibulin-5 may contribute to the formation of oxytalan fibers. We used siRNA for fibulin-5 in PDL cell culture to examine the extracellular deposition of fibrillin-1 and -2, which are the major components of microfibrils. Fibulin-5 was labeled on microfibrils positive for fibrillin-1 and -2. Fibulin-5 suppression reduced the level of fibrillin-1 and -2 deposition to 60% of the control level. These results suggest that fibulin-5 may control the formation of oxytalan fibers, and play a role in the homeostasis of oxytalan fibers.

The results suggested that injection of basic fibroblast growth factor into palatal wounds improves the vascular supply to the operated mucosa and underlying bone during and after palatal wound healing, which may contribute to tissue remodeling of the palate during growth.

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