Development of melanosomes and cytochemical observation of tyrosinase activity in the inner ear.

The ultrastructure and tyrosinase activity of melanocytes in the inner ear of pigmented guinea pigs were observed. Melanocytes/melanocyte-like cells were seen in the stria vascularis, the vestibular dark cell area and the endolymphatic sac. In the stria vascularis, melanosomes in several stages of maturation were seen in the cytoplasm of the intermediate cells and melanin-laden endosomes existed in the basal cells. Only the intermediate cells contained tyrosinase-positive cytoorganelles; Golgi sacs and neighboring small vesicles. Numerous melanocytes containing many melanosomes were observed under the epithelium of the vestibular dark cell area, and they showed tyrosinase activity. Melanocytes were seen in the endolymphatic sac, and they also showed tyrosinase activity. However, the epithelial cells of the endolymphatic sac, which had vacuoles containing melanin, did not show tyrosinase activity. Based on these findings, it can be said that (1) most of the intermediate cells of the stria vascularis must be melanocytes, (2) melanogenesis is vigorous in melanocytes of the inner ear, and (3) melanin in the epithelial cells of the endolymphatic sac is transferred in from melanocytes and is never synthesized in the epithelial cells.