ニホンザル (Macaca fuscata) の硬口蓋粘膜にみられる神経終末装置の微細構造的研究

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  • Ultrastructure of Sensory Nerve Endings in the Hard Palate of the Japanese Monkey, Macaca fuscata

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The mechanoreceptors in the hard palate of the Japanese monkey were studied with paticular regard to their types and fine structures. These included Merkel cell-neurite complexes in the epithelium and lamellated sensory corpuscles in the subepithelial connective tissue. The latter was further classified into simple corpuscles and Meissner corpuscles. These mechanoreceptors were concentrated in the crests of transverse palatine rugae. The Merkel cells were clustered at the basal portion of the epithelial rete pegs in every palatine rugae. They are characterized by the presence of numerous dense-cored granules, ranging from 80 to 120 nm in diameter. They made contacts with nerve terminals by junctional specializations, which are characterized by a thickening of apposed plasma membranes. Numerous microvillous cytoplasmic projections abutting from the Merkel cell were intercalated with adjacent keratinocytes. Occasionally the cells possessing characteristic features of both Merkel cells and keratinocytes appeared in the epithelium. These cells were characterized by the presence of clumps of tonofilaments and dense-cored granules in their cytoplasm and were regarded as so-called "transitional" Merkel cells. These findings strongly suggested that Merkel cells might be a keratinocyte origin. Well-developed encapsulated sensory corpuscles were seen in the subepithelial connective tissue at the top of transverse palatine rugae. The simple corpuscle was composed of a discoid axon terminal with 3-10 layers of flattened lamellae of the lamellar cells. Interlamellar spaces were separated by the basal lamina. A large number of caveolae were observed along the plasma membranes facing the basal lamina. The Meissner corpuscle, on the other hand, was relatively small in size and characterized by a winding back and forth of the nerve terminal separated by many stacks of thin lamellae derived from the lamellar cells. Numerous caveolae also appeared along the plasma membranes of the lamellae facing the basal lamina. In favorable sections, characteristic collagen bundles showing ladder-shaped structures were observed in the interlamellar spaces and occasionally in the connective tissue surrounding the corpuscles. It was suggested that the hard palate of the monkey were highly adapted for mechanical sensation.

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