Spontaneous mammary carcinoma of AKR mice and Xray-induced mammary carcinoma of C58 mice, both known as low-cancer strains, were investigated by electron microscopy.
In AKR mammary carcinoma, intracytoplasmic type A and extracellular type B particles were present. In C58 mammary carcinoma, extracellular type C particles were observed. In both tumors, mature virus particles, i.e. type B and type C particles, were formed by the process of budding of cytoplasmic membrane of carcinoma cells and present in the lumina of the milk ducts. Aberrant form of type C particles, which contained a filamentous structure instead of a nucleoid was also seen in C58 mammary carcinoma.
In serial in vivo transplantation of those carcinoma cells to the same strain mice, these virus particles were always present and formed from carcinoma cells, although the number of virus particles reduced almost successively with transplantations in AKR mammary carcinoma.
In tissue culture of C58 mammary carcinoma cells, type C particles similar to those in the original tumor were observed extracellularly. On the contraty to the presence of type B particles in the original AKR mammary tumor, however, two tissue culture strains derived from in vivo transplanted AKR carcinoma cells contained type C particles. These type C particles were conceivable to be the leukemia virus that naturally harbored in AKR mice and happened to have been tranferred to the tissue culture probably through its latent infection in carcinoma cells.
