Transmission electron microscopy (TEM) was used to study ultrastructural changes that accompanied the tumorous transformation of the descending rat colon epithelial cells, following short treatment with a direct carcinogen, N-methyl-N’-nitro-N-nitrosoguanidine (MNNG), with subsequent prolonged treatment with secondary bile acids, lithocholic (LCA) and deoxycholic (DCA), which enhanced tumor formation. Colon epithelial cells after long treatment with bile acids alone were characterized by the presence of an irregular nuclear membrane, ring-shaped rough endoplasmic reticulum (RER), collagen-like tonofilaments and membrane-bound mucous vacuoles. Tumor cells which developed following treatment with MNNG alone were characterized by the irregular shape of the nuclear membrane and, sometimes, by polynuclei, accumulation of large amounts of mitochondria, loss of cell-cell contacts and by endocytosis of the cell membrane. After combined treatment with MNNG and LCA, many mitochondria lost their membranous envelope; in the cytoplasm many collagen-like tonofilaments ring-shaped RER and many free ribosomes were present. After treatment with MNNG and DCA, many polysomes were found in the cytoplasm. It was apparent that treatment with MNNG alone caused the development of adenocarcinoma-like tumors, while additional treatment with secondary bile acids significantly enhanced these changes, which were accompanied by the development of atypia and anaplasia of epithelial cells, with many irregularities in intracellular organization.

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