Ultrastructural evidence of cell communication between epithelial dark cells and melanocytes in vestibular organs of the human inner ear

Background: The possibility of interaction between epithelial dark cells and melanocytes in the mammalian inner ear has been pointed out because of their morphological and biochemical characteristics, although very few studies have dealt directly with communication between these two types of cells. We investigated the dark cell area of human vestibular organs in order to clarify the ultrastructual evidence for cell interaction between epithelial dark cells and melanocytes. Methods: All of the material was obtained from vestibular schwannoma operations. Paraffin sections were stained with hematoxylin and eosin (H&E) and by the Fontana-Masson technique. Other paraffin sections were also stained immunohistochemically for S-100 protein. Glutaraldehyde fixed specimens were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results: Light microscopy revealed melanin pigment granules in the cytoplasm of epithelial dark cells. Melanocytes in the subepithelial layer stained positively for S-100 protein. The presence of intraepithelial melanocytes was confirmed by the presence of cell profiles with a large number of melanin pigment granules and S-100 protein in the cytoplasm. SEM showed that the dark cells had a pentagonal surface with microvilli on the apical surface edge. They had complicated structures at the basal portion of their cytoplasm. Melanocytes extending cytoplasmic processes to adjacent areas were observed under the dark cells. TEM showed that the dark cells were tightly linked by junctional complexes in the upper lateral portion of their cytoplasmic membrane and interdigitated by lateral infoldings. Compound melanosomes (phagosomes or secondary lysosomes) found in the cytoplasm of the dark cells contained poorly pigmented melanosomes with a periodic internal structure. Gap junctions were clearly showed between adjacent melanocytes in the subepithelial layer. Conclusions: The characteristic substructures of dark cells and melanocytes suggested the presence of intimate cell interaction between these two types of cells in the vestibular organs of the human inner ear, although it is not clear at this stage whether such cell interaction is specific only for patients with vestibular schwannoma. Dark cells and melanocytes form a cell community that serves to maintain homeostasis in vestibular organs through communication in which cell information obtained by both dark cells and melanocytes serves to facilitate the system. © 1995 Wiley-Liss, Inc.