Spontaneous Colon Tumors in Rats

Twenty-eight cases of spontaneous colon tumors were found during autopsy of approximately 3,000 male W rats in our laboratories between 1968 and 1974. Macroscopically, these tumors were multiple and localized in the proximal part of the colon. Many nodules protruded on the serosal surface; polypoid tumors protruding into the lumen were rare. Microscopically, hyperplastic and adenomatous polyps were observed in the mucous membrane of the colon. There was adenomatous downward growth into the submucosal layer, and tubular-type adenocarcinomas invaded all the coats of the colon wall. Structural atypism was frequent and slight cell atypism was also present. Invasive tumors sometimes had an inflammatory reaction, and bacterial growth was observed within the neoplastic glands in thionine-stained sections. Escherichia coli and a gram-negative, facultative anaerobic bacillus, which grew only with E. coli in vitro, were isolated. A cultured epithelial cell line was established from one colon tumor. After a primary tumor was transplanted intraperitoneally into young adult rats, a mass grew in the peritoneal cavity for 4 months. The tumor incidence seemed higher at different times in the animals examined.