Melanocytes and iris color: Light microscopic findings

Objective: To systematically evaluate morphologic differences in iris stroma that contribute to clinically perceptible differences in iris color, using immunohistochemical identification of stromal melanocytes and fluorescence microscopy. Methods: Paraffin-embedded sections from 51 human irides were stained with S100a and fluorescein isothiocyanate. Cells were counted and scored as melanocytes or other. Melanocyte number, proportion, and density were determined for light-colored (blue), medium-colored (hazel) and dark-colored (brown) irides and compared. Results: No statistically significant difference was observed for mean total cellularity or mean melanocyte number among the three color groups. Mean total stromal cell count was 1177±259 (mean±SEM), and mean melanocyte number was 778±196 per 5-μm section. In human irides, 65.9% of the iris stroma is composed of melanocytes. Melanocyte density (number of cells per square millimeter) is not related to iris color. Conclusion: The number of melanocytes, the proportion of melanocytes, and iris stromal cellularity are not major contributors to iris color.