Diffuse iris nevus in oculodermal melanocytosis: a light and electron microscopic study.

We studied by light and electron microscopy the diffuse iris nevus of an eye in a 16-year-old patient with oculodermal melanocytosis and choroidal malignant melanoma. The nevus cells in the anterior border layer of the iris appeared to be poorly differentiated, showing moderate infolding of the nuclear membrane with heterochromatin clumping, watery cytoplasm, abundant mitochondria, fine filaments, rough endoplasmic reticulum, and numerous pinocytotic vesicles. Small scattered immature melanosomes and occasional giant melanosomes were observed in these cells. Deep in the iris stroma, however, nevus cells were found singly or in small groups, and were associated with an increasing number of melanized melanosomes and cytoplasmic filaments and reduced numbers of other cytoplasmic organelles, such as mitochondria and free ribosomes. Differentiation of the iris nevus cells appeared to progress from the anterior border layer toward the iris stroma. This observation suggests that intrastromal nevi may be more benign than nevi with surface plaque. The ultrastructural characteristics of the diffuse nevi of oculodermal melanocytosis were compared with those of other iris nevi.