Spontaneous mammary carcinoma of C3H mice was studied by electron microscopy.
Cancer cells showed prominent nucleoli and increased nucleocytoplasmic ratio, containing a few mitochondria, vacuoles, and endoplasmic reticulum in the cytoplam. There were inclusion bodies in some of cancer cells.
Mature virus particles were seen in the lumina of the milk ducts, intercellular space and intracytoplasmic vacuoles. They were round in shape and composed of an eccentrically-located, electron-dense nucleoid containd in a triple-layered shell ranging 100 to 120 mμ in diameter. It was demonstrated that the outermost layer of the shell consists of numerons 100 Å spines. This finding is closely consistent with the negative contrast figure of the mature virus particles.
A new interpretation of type A particles was also presented.