Three-Dimensional Structure of Apical Vesicles of Tuft Cells in the Main Excretory Duct of the Rat Submandibular Gland

Abstract

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.

The intestinal tuft cell nanostructure in 3D

“…It has been suggested that these molecular complex may form a tubulovesicular network made of structural proteins such as citokeratin 18, fimbrin, and villin 26 . However, attempts to render the volumetric ultrastructure of the network have lacked sufficient Z resolution to prove the existence of a network 11 . These tubular network was also evident in our SBEM and ATUM images (Fig.…”

“…Some of the cells also have “lateral microvilli” but the purpose is unclear 10 . At the base of the microvilli tuft, there is a prominent cluster of vesicles and caveoli, which has been suggested to form a network but the evidence is inconclusive 11 . Since tuft cells were first reported, about a third of the published articles have focused on their ultrastructure and provided evidence of at least five peculiar features.…”

“…Since tuft cells were first reported, about a third of the published articles have focused on their ultrastructure and provided evidence of at least five peculiar features. These are the following:Associated with the microvilli, there are some undefined spherical bodies, about 2 nm in diameter, known as glycocalceal bodies 12 .Based on their close proximity to nerve fibers, tuft cells are thought to be innervated, nonetheless evidence of direct physical contact is absent 13 .Tuft cells have a small basal cytoplasmic process, similar to neuropods in enteroendocrine cells, albeit devoid of secretory vesicles 14 .Lateral cytoplasmic projections have been observed extending from the tuft cell body but their trajectory is unknown 10, 11 .At the base of the cell’s microvilli, there is evidence of a cluster of vesicles and caveola. This molecular complex is referred to as a tubulovesicular network but the true continuity of the network has yet to be demonstrated 11 .…”

“…Lateral cytoplasmic projections have been observed extending from the tuft cell body but their trajectory is unknown 10, 11 .…”

“…This molecular complex is referred to as a tubulovesicular network but the true continuity of the network has yet to be demonstrated 11 .…”