[Ultrastructure study of the preneoplastic colon (carcinogenesis with 1,2-dimethylhydrazine)]

100 Wistar rats, belonging to a series of 500 animals, were treated with increasing doses and for increasing periods of time with 1,2-dimethylhydrazine. A previous study has shown that tumors begin to appear only after the 19th week. Before this period there are abnormalities of mucin secretion (by the 2nd week), and then there are dysplasic lesions (after the 4th week). The ultrastructure study, done before the 19th week ("pretumor" phase) confirms some previous observations (increase in the zone of mitosis, secretory changes, mutation producing effects). These features are reminiscent of the histochemical characteristics of the human foetal colon. They show an epitheliomesenchymatous disorganisation unrelated to inflammation. There are abnormally early changes in the vasculature, there is loss of the (epithelial) coat, there is dissociation of the basal membrane and abnormal connections between it and the epithelial cells, collagen and fibroblasts; all this represents a disorganisation of the regeneration system of the colon mucosa. This loss of equilibrium between the parenchyma and the mesenchyma may represent a factor favoring excessive and disorganised growth of the mucosa.