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Submicroscopic Cytoplasmic Particles Occasionally Found in the Ehrlich Mouse Ascites Tumor^*

Divisions of Experimental Pathology and Experimental Chemotherapy, Sloan-Kettering Institute for Cancer Research, New York, N.Y.

1. Striking cytoplasmic masses of submicroscopic spheres have been observed to be present in nearly all cells of four and completely absent in 66 of the Ehrlich mouse ascites tumor specimens which have been extensively studied.

2. The individual particles consist of a dense osmiophilic core surrounded by a shell with an outer diameter of 580 ± 40 A.

3. The particles are exclusively cytoplasmic and aggregate in groups often in hexagonal close-packing without an enclosing membrane.

4. The particles occur in identical morphology in resting, mitotic, or degenerating cells and may occur extracellularly in the ascitic fluid.

5. Smooth-walled tubules with diameters similar to those of the particles are often associated with them and may be a normal cell component.

6. The identity of these particles is unknown. Various possibilities which have been considered include altered cell component, degeneration product, tumor agent, fungus, or virus, of which the last is thought to be the most likely.

^* This investigation was supported by an institutional research grant by the American Cancer Society and by a grant from the Lillia Babbitt Hyde Foundation.

Received 6/10/54.