Muscularis mucosae in desmoplastic stroma formation of early invasive rectal adenocarcinoma.

The origin of myofibroblasts or myofibroblastic cells in the desmoplastic stroma associated with carcinoma invasion has been controversial. In the early invasive area of a rectal adenocarcinoma reported here, an obvious transition between the muscularis mucosa and the bundles of eosinophilic stromal cells observed in the carcinomatous stroma was demonstrated both in morphology and in their cytoskeletal phenotype, which conceivably suggests that the smooth muscle cells of the muscularis mucosa could convert to the eosinophilic stromal cells, namely myofibroblasts. Moreover, type I procollagen was demonstrated in both protein and mRNA levels in the areas of eosinophilic stromal cells with a lesser degree of differentiated smooth muscle phenotype that showed a transition from the muscularis mucosa, implying that the myofibroblastic cells converted from smooth muscle cells of the muscularis mucosa could be responsible for type I collagen production. These findings suggest that the muscularis mucosae may not be a passive barrier through which colorectal carcinomas infiltrate into the submucosa, but may play an active role in the formation and remodeling of tumor stroma.