Ultrastructure of melanocytes in the dark cell area of human vestibular organs: functional implications of gap junctions, isolated cilia, and annulate lamellae.

BACKGROUND

It is known that melanocytes exist in almost all parts of the inner ear, such as the cochlear duct, stria vascularis, Reissner's membrane, modiolus, vestibular organs in the region surrounding the cristae and maculae, semicircular canals, and pars rugosa of the endolymphatic sac. But there have been few studies using human materials, because of the difficulty of obtaining materials. We attempted to investigate the detailed ultrastructure of melanocytes in the vestibular organs of human inner ear.

METHODS

Eight surgical specimens obtained from patients with vestibular schwannoma were studied by light microscopy and electron microscopy.

RESULTS

Melanocytes were found in the subepithelial layer of the dark cell area. Melanocytes had round or spindle-shaped nuclei and clear cytoplasm with brown pigment granules. Besides melanocytes, there were melanophages, fibroblasts, and small blood vessels. Through electron microscopy we found melanocytes with round-shaped melanosomes in various stages of pigmentation, well-developed Golgi apparatus and endoplasmic reticulum in the cytoplasm, and many cytoplasmic processes. Gap junctions were occasionally found between the cytoplasmic processes. And there were pinocytotic vesicles just under the limiting membrane of melanocytes, and intermediate filaments were abundant in the cytoplasm. Isolated cilia of melanocytes, annulate lamellae, and fusiform banded structures in the connective tissue area around melanocytes were found.

CONCLUSIONS

Melanocytes in human vestibular organs actively synthesize melanosomes. Frequent findings of isolated cilia and fusiform banded structures and the incidental existence of annulate lamellae may be an indicator of this metabolically activated state of melanocytes. Moreover, monitoring environmental changes by isolated cilia, melanocytes in the human inner ear could act not only as one cell but also as a group to achieve their physiological functions by means of information transmission through gap junctions.