Characterization of Melanins in Human Irides and Cultured Uveal Melanocytes From Eyes of Different Colors

Abstract

The presence of eumelanin and pheomelanin in irides from eyes of various colors was determined and quantified by a highly specific microanalytical procedure based on chemical degradation. Significant differences in the type of melanin were observed in the stroma and iris pigment epithelium (IPE) fractions obtained by micro-dissection of the iris specimens. Melanin from the IPE is essentially eumelanin, while the pigment in IPE-scraped iris (consisting mainly of stroma plus anterior IPE) proved to be both eumelanic and pheomelanic. A pheomelanic-type pigmentation was associated with green irides, while green-blue mixed-color irides were mostly eumelanic; by contrast, green-brown mixed-color and brown irides could not be placed into either of the two categories and probably feature a mixed pigment content. Blue irides invariably exhibited very low pigment content. Analysis of cultured iridial melanocytes in the growing stage showed a significant shift to pheomelanic pigmentation when compared with those in IPE-scraped tissues, providing evidence that growth of iridial melanocyte induce a marked change of melanin metabolism. After senescence, cultured melanocytes exhibited a marked increase in pigment content, most of the variation was associated with the eumelanin content. These results represent the first direct evidence for the presence of eumelanin and pheomelanin in human irides, and suggest that differences in stromal pigmentation are due not only to the quantity, but also the nature of the melanin pigment.