HUMAN OCULAR MELANOCYTES AND RETINAL-PIGMENT EPITHELIAL-CELLS DIFFER IN THEIR MELANOGENIC PROPERTIES IN-VIVO AND IN-VITRO

Purpose. The vertebrate eye contains both melanocytes and retinal pigm ent epithelial (RPE) cells. Little is known of the pigmentary behaviou r of these embryologically dissimilar cells. The aim of this study was to examine aspects of the pigmentary properties of both cell types in vitro and ex vivo to learn more of the function of these cells. Metho ds. Sections of normal adult human eye were stained for tyrosinase rel ated protein 1(TRP1), and cultures of RPE cells and choroidal melanocy tes were examined immunocytochemically for TRP1 and 2 and enzymaticall y for tyrosinase activity (by assaying dopa oxidase activity). Results . Over half of the choroidal melanocytes expressed TRP1 ex vivo; in co ntrast, a very small percentage of RPE cells were TRP1 positive. In vi tro, passage 1 to 3 ocular melanocytes expressed TRP1 and TRP2 and had tyrosinase activity, which was influenced by the choice of substrate on which the cells were grown. Tyrosinase activity was highest when ce lls were grown on fibronectin and plastic, intermediate on laminin and lowest on vitreous extracellular matrix (ECM) containing pigment to w hich they attached and spread out poorly. In contrast, passage 3 RPE c ells (which were unpigmented) showed little evidence of tyrosinase act ivity in short-term culture, irrespective of the substrate on which th ey were grown, and failed to express TRP1 and TRP2. When cells were gr own on plastic for greater than 3 weeks in culture. a very low percent age of cells (<0.1%) became TRP1 positive and this percentage was incr eased threefold if cells were cultured on laminin in the presence of b FGF. A few cells were also seen to contain pigment but cultures failed to show any tyrosinase activity. In contrast, RPE cells (but not mela nocytes) showed a marked ability to take up pigment granules in vitro. Conclusions. The data suggest that normal human ocular melanocytes re tain the capacity to produce pigment throughout adult life, and this c an be demonstrated both ex vivo and in vitro. In contrast, we were una ble to confirm that the majority of RPE cells play any significant rol e in active pigment production in the adult.