Metaplastic polyp of the colon develops in response to inflammation.

Background: The metaplastic polyp of the colon is a non-neoplastic lesion that is generally identified as white, flat and having a smooth surface. In general, this polyp is small, is less than 5 mm in diameter but is occasionally larger than 5 mm in diameter, and forms a swelling. The aims of the present study were to clarify the factors that determine the morphology of protruding metaplastic polyps. More specifically, we investigated whether the metaplastic polyp forms as a result of an abnormality in cell proliferation or inflammation of the region. Methods: We examined 15 endoscopically resected metaplastic polyps of the colon having a longitudinal diameter of more than 5 mm. To study aspects of cell proliferation, we used proliferating-cell nuclear antigen (PCNA) staining. For examination of histological changes caused by inflammation, we used α-smooth muscle actin (αSMA) staining, in which myofibroblasts are specifically stained. Results: Metaplastic polyps showed significantly higher expression of PCNA, not only in the deep layer, but also in the intermediate and superficial layers, compared with the normal mucosa of the colon. In the protruding metaplastic polyps, anti-αSMA staining revealed bundle-like myofibroblasts in the interstitium. Conclusions: The factors responsible for the formation of non-neoplastic metaplastic polyps larger than 5 mm with a protruding morphology are: an increased number of epithelial cells due to the movement of these cells toward the ductal epithelium in the proliferating zone; and expansion of interstitial tissues due to infiltration of myofibroblasts and other inflammatory cells in response to inflammation.