Degenerative retinopathy over six choroidal tumors was studied by light and electron microscopy. Special effort was made to evaluate pathologic changes of the retinal pigment epithelium (RPE). Two types of proliferation of the RPE were observed: (1) diffuse hyperplasia, and (2) plaques of partially depigmented spindle-shaped cells closely packed together in a matrix of fibrillar material. On evaluation of serial light microscopic sections and on electron microscopic examination, both types were found to be due to growth of the cells in a flattened papillary distribution. In plaques of proliferated RPE cells generally referred to as areas of "fibrous metaplasia, " basement membrane-like material containing a few collagen fibrils was present between the cells. The cells retained many of their epithelial characteristics. True metaplasia into fibroblasts was not observed. Proliferated RPE cells dedifferentiated partially and showed some embryonic features. The presence of glycogen clearly distinguished these cells from both normal embryonic and mature RPE. New formation of melanin could not be identified. Accumulation of lipofuscin granules presumably accounted for the hyperpigmentation of some proliferated cells.
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