Mucinous colloid adenocarcinoma of colon in Fischer-344 rats. Light microscopy, histochemistry and ultrastructure.

Azoxymethane (AOM) induced mucinous colloid adenocarcinomas of the colon have been studied by light microscopy, histochemistry and transmission electron microscopy. This neoplasm constituted 2.6% of the induced carcinomas. Histogenetically they were formed directly from the flat mucosa without going through a benign polyp-cancer sequence. By light microscopy, the neoplasms were characterized by lakes of abundant extracellular mucin within which were cells often distended with intracellular mucin. Histochemically, this mucin was composed of a mixture of neutral and acidic mucopolysaccharide, the latter being predominantly sialomucin. Ultrastructurally, the cells exhibited a mixed differentiation. The majority of the cells were hyperdistended with mucin, resembling the goblet cells. Other cells contained abundant rough endoplasmic reticulum, free polysomes and very little intracellular mucin. There were a few endocrine cells as well as mucous cells with dense core granules. It seems that similar to the normal colon, the neoplastic colon also contains cells at various stages of differentiation. A hypothesis to explain these events in colon carcinogenesis is proposed.