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Cytochemical Studies on the Mouse Mammary Tumor Virus

Viral Biology Branch, National Cancer Institute, Bethesda, Maryland 20014

The chemical nature of the ultrastructure of the mouse mammary tumor virus has been investigated in situ. Small pieces of spontaneous mammary tumors from C3H mice were fixed in phosphate-buffered glutaraldehyde, infiltrated, and embedded in glycol methacrylate, a water-miscible plastic introduced in 1963 by Leduc and her collaborators as a medium suitable for cytochemical studies at the ultrastructural level. Electron microscopic examination of the tissue revealed the presence of numerous cytoplasmic A particles and extracellular B particles. Considerable evidence has accumulated demonstrating that the latter is the active, mature mammary tumor virus. The cytoplasmic A particle is considered to be formed in the cytoplasm as a precursor of the B particle; however, there is little evidence from immunologic, biochemical, and biophysical parameters to support this hypothesis. Ultrathin sections of the tumor containing both particles were subjected to digestion with RNase, DNase, pepsin, papain, and trypsin and combinations of these enzymes. Consecutive sections were incubated in the appropriate control solutions. The results reported (a) provide evidence for the presence of RNA in the cytoplasmic A particle, (b) indicate that the stereochemical nature of the A particle is synonymous with the nucleoid of the B particle, and (c) present further support of Bernhard's hypothesis that the cytoplasmic A particle is the intracellular precursor of the mammary tumor virus.

¹ NIH, USPHS, HEW.

Received 5/ 8/67. Accepted 7/11/67.