The color of the human eye: A review of morphologic correlates and of some conditions that affect iridial pigmentation

Iris color can be affected by a variety of ocular disorders. It is suspected that iris color may not remain constant throughout life. These observations have drawn attention to the morphologic correlates of iris color and its regulation. Differences in the iris color of normal eyes are the result of variable amounts of melanin pigment granules within a constant number of melanocytes in the superficial stroma of the iris. These melanocytes seem to reach their genetically determined amount of melanin in early childhood, and their melanin content usually remains constant in adulthood. Diseases such as Horner's syndrome and Fuchs' heterochromic iridocyclitis affect iris color, resulting in a decrease of iris pigmentation. Evidence suggests that melanin content of some melanocytes is subject to adrenergic regulation even past childhood. Application of the prostaglandin analogue latanoprost, on the other hand, leads to an increase in iris pigmentation in some patients. Studies with cultured dermal and uveal melanocytes, as well as with weal melanoma cells, however, show no increase in cell proliferation when treated with latanoprost in vitro. The mechanisms by which latanoprost affects regulation of iris pigmentation requires further investigation.