Ultrastructures of mechanoreceptors in the oral mucosa

Abstract

The present review describes the fine structures of lamellated mechanoreceptive corpuscles, Merkel cell-neurite complexes and free nerve endings in the oral mucosae of mammals, with special attention to axon terminals and lamellar cells. The mechanoreceptive nerve endings of the oral mucosa were studied using histochemistry, immunohistochemistry and transmission electron microscopy techniques. The organized mechanoreceptive corpuscles are present in the mucosae of gingiva, cheek, tongue and soft and hard palate. They are elongated or globular in shape, being located in the connective tissue papillae. The capsule is composed of several layers of cytoplasmic extensions of perineural cells. Numerous bundles of collagen fibers are noted at the periphery of the corpuscle. The lamellated corpuscles are surrounded by several layers of superimposed flattened capsular cell processes. The interlamellar spaces are 0.2-0.4 micron in width and filled with thin fibrillar collagen fibers embedded in the amorphous substance. The lamellar cells contain rich microtubules and are characterized by the presence of caveolae on the surface plasma membrane. The terminal axon contains an abundance of mitochondria and small clear vesicles (20-50 nm in diameter). There are neurofilaments in the center of the axon terminal. Intermediate-type junctions are seen between the adjacent lamellar cells and between the axon and adjacent lamellae. The free nerve endings are found in the subepithelial regions, very close to the basal laminae of mucosal epithelium. They are surrounded by a thin cytoplasm of Schwann cells. Sometimes Schwann cell basal larinae become multilayered. Merkel cells are present within the basal layer of mucosal epithelium and contain characteristic electron-dense granules that are located almost exclusively at the side of cytoplasm in contact with axon terminals. Intermediate-type junctions are noted between axon terminals and Merkel cells.

Innervation of the Human Incisive Papilla: Comparison with Other Oral Regions

“…SCN7A-containing nerve fibers could not be detected at the vicinity of these cells. Previous and present studies demonstrated that Merkel cells and their myelinated axons contain PGP 9.5 in the human oral mucosa and labial skin [Hilliges et al, 1996;Watanabe, 2004]. It is also known that some Merkel cells are isolated and devoid of sensory innervation [Ramieri et al, 1992].…”

“…Also, unmyelinated axons of free nerve endings are surrounded with unspecialized Schwann cells [Watanabe, 2004]. Immunohistochemistry for SCN7A may be able to detect Schwann cells of corpuscle endings and free nerve endings in the human oral mucosa.…”

Distribution and ultrastructure of Merkel cell of the fishing bat (Myotis ricketti)

“…Transmission electron microscopy studies of Merkel cells have been conducted in previous studies [5,31] . As for the reason of intermediate filamentous aggregation, Kuhajda et al [32] and Kagoura et al [33] …”

Pathophysiology of merkel cell

“…[17] Intermediate-type junctions have been observed between terminal axons and MCs (Watanabe, 2004). [18] It should be noted that a number of MCs are not associated with the nerve axon. Finally, MCs are attached to neighboring keratinocytes by a few small desmosomes and contain melanosomes similar to keratinocytes.…”