Electron Microscopy of Mouse Parotid Tumor (Polyoma) Virus

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Abstract
Crude tissue-culture fluids containing a virus which had been shown by previous workers to induce multiple tumors in mice and hamsters yielded a characteristic particle when examined with the electron microscope. Absorption and elution from red blood cells and density gradient centrifugation were utilized for further purification. The size of the particle in arrays was 40 to 45 mμ; in flattened, oblate spheroids it was 59 mμ in the major diameter and 24.2 mμ in height, which gave 44 mμ as the diameter of the sphere having the same volume as the flattened particle. Virus from 2 tissue-culture strains, one cultured from a parotid-gland tumor and the other from a mesenteric lymph-node tumor, appeared identical. Sedimentation experiments indicated that the particles were somewhat larger in solution. Particle counts from the electron micrographs were parallel to the hemagglutination (HA) titer. In one experiment the number of virus particles was 1 × 107 times the HA titer. The end point of hemagglutination corresponded to the virus dilution in which the ratio of virus particles to red blood cells was unity. The uniformity of virus size is consistent with the concept of a single virus, but morphology alone is an insufficient basis for such a conclusion.