Fine Structure and Axonal Transport Labeling of Intraepithelial Sensory Nerve Endings in Anterior Hard Palate of the Rat

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Abstract
The fine structure of intraepithelial free nerve endings, Merkel receptors, and special (possibly chemosensory) corpuscular receptors has been examined in anterior hard palate of the rat. Samples were taken from the two anterior large rugae as well as from the more rostral incisive papilla. Most nerve endings were concentrated in regions in which the epithelium and connective tissue interdigitated extensively. The nerve endings contained relatively pale cytoplasm, a variety of vesicles, and numerous mitochondria. Distinctions could be made among different endings depending on their intraepithelial location, or on the degree of specialization of the neighboring epithelial cells. Simple (free) nerve endings entered the epithelium between epithelial papillae; upon entering, their basal lamina became associated with the epithelial basal lamina in various ways, and their Schwann cell sheath also entered the epithelium for a short length. Free nerve endings were found at various sites among the regular epithelial cells in the basal layers, the stratum granulosum, and stratum spinosum, and next to the stratum corneum border; they did not form specialized junctions with the epithelial cells. Intraepithelial nerve endings at all levels could be labeled by axonal transport from the trigeminal ganglion. Some nerve endings in anterior hard palate were associated with specialized epithelial cells, either in Merkel complexes or in apparent chemosensory corpuscles; both were labeled by axonal transport from the trigeminal ganglion. The specialized epithelial cells in the corpuscular receptors formed possible efferent and afferent synapses with the nerve endings. The nerve endings in the Merkel complexes formed junctions with the specialized Merkel epithelial cells. Both the Merkel complexes and corpuscular receptors were found in their respective characteristic epithelial papillae, which were surrounded by a dense plexus of subepithelial terminal axons and nerve endings.