Melanocytes in vestibular dark cell areas in human fetuses.

Melanocytes in human vestibular dark cell areas were observed to have round or spindle-shaped cell bodies with several processes. They were located below the basement membrane but the melanocyte processes were found in the dark cell epithelial layer closely adjacent to the dark cell processes or bodies without any separation from the basement membrane. Submicroscopic observations revealed melanin pigment in various stages of accumulation on the tyrosinase filaments in melanosomes. Three-dimensional stereoscopic observations clearly revealed the morphological characteristics of tyrosinase filaments, their helicoidal structure and some variations. Mature melanin granules developed from premelanosomes as the melanin pigments were gradually deposited onto the filaments. Through this progress of pigment accumulation, the melanosomes became darker. In the four mid-term fetuses used in the present investigation, few differences were seen in the mode of melanin deposition on the tyrosinase filaments. In view of the close spatial arrangement and contact of the melanocytes with the dark cells, it seems plausible that the melanocytes play some role in vestibular metabolism via the dark cells.