MELANOCYTES AND IRIS COLOR - ELECTRON-MICROSCOPIC FINDINGS

Objective: To quantitatively associate iris color with melanocyte pigm ent content. Methods: Autopsy eyes were classified as uniform-blue, un iform-hazel, or uniform-brown or showing a darker peripupillary ring. Using electron microscopic images and computerized image analysis, are a, number, and size of mature melanosomes within the perinuclear cytop lasmic area only or within perinuclear and peripheral cytoplasmic area s of the superficial stromal melanocytes combined were measured. Resul ts: Average melanosomal area per perinuclear cytoplasmic area (AMAC) a nd average number of melanosomes per perinuclear area (AMNC) significa ntly differed across iris color groups (overall P<.001). This result r eflects the large difference between blue-uniform and all other color groups. A marginally significant (nominal) trend from blue-ring throug h brown-ring was also detected (P=.06 for AMAC and P=.07 for AMNC). Th e average perinuclear cytoplasmic area was larger in the central iris zone (within 1 mm around the pupillary margin) than in the intermediat e iris zone (between 1 and 2 mm around the pupillary margin) (P=.002), but AMAC and AMNC did not significantly differ between zones. The ave rage melanosome size did not differ significantly across color groups (P=.11). Conclusion: Differences in iris colors are at least partially attributed to variable AMNC and AMAC within superficial melanocytes.