Colon epithelium. III. In vitro studies of colon carcinogenesis in Fischer 344 rats. N-methyl-N'-nitro-N-nitrosoguanidine-induced changes in colon epithelium in explant culture.

Colon explants from the inbred F344 rat descending colon pretreated in vivo with azoxymethane and maintained in explant culture were exposed to the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). One week after the MNNG treatment, the colon crypts showed marked crowding, hypercellularity, and stratification of cells. Nine weeks after the treatment, the explants showed epithelial papillary projections on the surface epithelium and within the crypts, in addition to hypercellularity and stratification. The control untreated explants maintained a single layer of epithelium during the entire culture period. Ultrastructurally, the treated cells showed an unusual concentration of free polysomes and thin and thick filaments, multiple and bizarre nucleoli, nuclear indentations and pseudoinclusions, and intracellular lumina. Sulfomucin was the predominant component in the control untreated explants as well as in the normal descending colons of rats and humans. One week after treatment the crypts of the carcinogen-treated explants showed an increase in sialomucin, and by 9 weeks after treatment, they showed mostly sialomucin. These features, compared and correlated with those of the parallel in vivo animal model as well as with human material, lend additional support to de novo histogenesis of colon carcinoma.