Ultrastructure of malignant melanomas of the choroid.

Eleven vialignant melanomas of the uveal tract have been studied by light and electron microscopy. Two types of cells were found in the epithelioid tumors: one, a darker type, contained large amounts of free RNP particles in single granules or in clusters, many smooth membranes and vesicles, and numerous mitochondria; the lighter type had similar cytoplasmic components, but in a lesser amount, especially in the number of free RNP particles. The epithelioid cells were closely packed, in contrast with the findings of light microscopy, and the cell walls were smooth and straight. In mixed tumors it was more difficult to distinguish spindle and epithelioid cells, except by prior identification with stained 1 n sections. The three spindle cell B tumors showed a very uniform pattern in low-power field with the electron microscope, but in higher magnifications exhibited considerable variety in cytoplasmic composition, with numerous large mitochondria, many smooth-surfaced vesicles, and an amorphous component in the cytoplasm. In contrast with the epithelioid cell there is more rough-surfaced endoplasmic reticulum, and an absence of free RNP particles in the cytoplasm. All tumors contained a variable number of macrophages, characterized by large masses of cytoplasmic pigment. It was not possible to determine whether these were true reticuloendothelial cells, or tumor cells which had phagocytized pigment. The origin of the melanin granules was also not revealed by this study. Possible modes of origin of these tumors were considered. None of the cells resembled Schwann cells of normal human beings, nor did they exactly resemble normal melanocytes. In addition to pigmentation, many similiarities to the normal melanocyte could be detected in the spindle B type cells.