Distribution and features of melanocytes during inner ear development in pigmented and albino rats

In this developmental study, the distribution and features of melanocytes in the inner ear of pigmented and albino rats was investigated with the use of an antibody, which specifically reacts with a melanocyte differentiation antigen present in the membranes of (pre)melanosomes. Melanocyte precursors could be traced from 13 days post conception onwards and the course was followed to their targets in the inner ear. Melanocytes which settle in the modiolus appeared to reach their target along another pathway than strial and vestibular melanocytes. No difference was observed in the melanocyte distribution between pigmented and albino rats. The integration of melanocytes into the stria vascularis was associated with an increased rate of melanosome production in both strains, but in the albinos far fewer melanosomes were produced. After the stria had reached maturity, melanosome production was arrested and melanosomes were subject to lysosomal digestion. In the stria of the pigmented rats, cells with aggregations of disintegrating melanosomes appeared and persisted into adulthood. In the adult, the majority of the intermediate cells contained only a few scattered melanosomes, while melanosomes could only rarely be detected in the albinos. These observations indicate that there is a close relationship between melanosome production and the process of interdigitation of melanocytes with the marginal cells. It seems unlikely that melanosomes or melanin make any important contribution to the function of the adult stria vascularis. Outside the stria, the features of melanocytes in both strains were similar to skin melanocytes.