[Melanocytes, Langerhans and Merkel cells in oral epithelium]

The fine structural morphology of melanocytes, Langerhans cells and Merkel cells in the normal dorsal epithelium of the human tongue has been studied by electron microscopy. Melanocytes were regularly found in normal papillary epithelium, even in the absence of clinical signs of pigmentation. The basal epithelial melanocyte contains individual pigment organelles at different stages of maturation and rudimentary half-desmosomes were found between the melanocyte and the basal membrane. The suprabasal Langerhans cells can be identified by their specific organelles and by the lack of tonofilaments and desmosomes. The morphological features of these cells suggest functional relations to cell structures of the keratinocytes and to the epithelial keratinization process. The Merkel cell is characterized by a horizontal orientation in the basal epithelial layer, the development of desmosomal attachments to keratinocytes, the lack of tonofilaments, the typical Merkel cell granules and an associated axon terminal. The observation of Merkel cell granules in relation to the Merkel cell plasmalemma and the morphological similarity of these granules to the monoamine-storing granules in certain types of chromaffin cell system suggest a symbiotic union between the specialized Merkel cell and the associated neurite with a neuroepithelial tactile receptor function. Because of their specific ultrastructure, the Merkel cells are thought to constitute a self-maintaining cell population, independent of keratinocytes and dendritic cells, in the oral epithelium.