Colon Epithelium. II. In Vivo Studies of Colon Carcinogenesis. Light Microscopic, Histochemical, and Ultrastructural Studies of Histogenesis of Azoxymethane-Induced Colon Carcinomas in Fischer 344 Rats2345

Abstract

The sequential changes during histogenesis of colon carcinoma were studied with the use of 60 male inbred F344 rats given weekly im injections of 8 mg azoxymethane (ADM)/kg and serially killed at regular intervals. The malignant potentiality of induced carcinomas was confirmed by observation of invasion, metastases, and transplantation in inbred animals. The changes observed were mostly in the descending colon. Seemingly, the mucus initially increased but later decreased. The earliest changes observed by light microscopy were dilatation and distortion of colon crypts, hypercellularity of the crypts, and increased production of normally secreted sulfomucin. Later, the increased mucus secreted was sialomucin. Eventually, mucus production lessened, and the cells became more basophilic with high nucleocytoplasmic ratios. Ultrastructural examination revealed increased amounts of free cytoplasmic polysomes, diminished amounts of mucus, large nuclei with indentation of the nuclear membrane, and large, prominent nucleoli. The hypercellular crypts also showed an increased number of intestinal endocrine cells. Intestinal endocrine cells with dense core secretory granules were also frequent in the frankly carcinomatous lesions. Foci of atypical epithelium consisting of cells with basophilic cytoplasm and large nuclei were seen in grossly normal, flat mucosal areas in rats that received only a few doses of the carcinogen and that had not yet developed grossly visible tumors and also in rats bearing grossly obvious tumors in the colon. Several such foci were clearly invading the underlying submucosa and the muscle layers. Single cells in several such crypts were ultrastructurally seen to break the basal lamina and invade. Fully developed carcinomas were both polypoid and flat. No benign polyp-cancer sequence could be demonstrated in this model. Thus in this model predominance of sialomucin was associated with premalignant changes in colon epithelium, cells in the crypts either singularly or in multiple underwent carcinomatous changes de novo in the flat mucosa, and multiple foci of microcarcinomas developed during ADM carcinogenesis. Therefore, counting only the gross tumors may not reflect the true incidence of neoplasia.