Four independently derived, spontaneously transformed, murine embryo-cell strains and tumors produced by intramuscular injection of the cell strains were studied by electron microscopy. This report emphasizes the close structural similarity between such cells and normal connective tissue fibroblasts as seen in vivo. Like normal fibroblasts, the fusiform spontaneously transformed cells contained numerous fine cytoplasmic filaments (especially as aggregates in the peripheral cytoplasm) and irregularly dilated, extensively developed, rough-surfaced endoplasmic reticulum. The interstitium of the tumors had an abundance of collagen fibers and smaller, nonbanded filaments. In the cultures, though nonbanded extracellular filaments were abundant, morphologically identifiable collagen fibers were much less common than in the tumors. Viral particles were not consistently observed in either the tumors or the strains from which they were derived.
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Richard G. Cornell (1969) studied this question.