Investigation of the properties of the cell surfaces of two malignant ocular melanomas with Luft's ruthenium red staining technique demonstrated that the surfaces of the tumor cells in both specimens were coated with a fairly even layer (14 to 20 nm) of the dye, suggesting the presence of an acid mucopolysaccharide substance. This same ruthenium-red positive material lined the inner surface of the membranes surrounding vesicles of pinocytotic origin. Intact cells showed no nonspecific staining or precipitated stain. The ruthenium red coat was absent from or discontinuous on the surface of damaged necrobiotic cells and cytoplasmic processes. Considerable quantities of ruthenium red-positive material accumulated in loose granular or conglutinated, highly electron-dense masses in the intercellular spaces surrounding necrobiotic cells. Necrobiotic cells and some processes extending far beyond the emitting cells displayed much stronger electron density than intact cells. This phenomenon suggests that the limiting membranes of necrobiotic forms become permeable to ruthenium red and that binding of the stain to the cell components results in increased electron density.
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