Metaplastic polyp of the colon develops in response to inflammation

Background: The metaplastic polyp of the colon is a non-neoplastic lesion t hat 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 occasion ally larger than 5 mm in diameter, and forms a swelling. The aims of the pr esent study were to clarify the factors that determine the morphology of pr otruding metaplastic polyps. More specifically, we investigated whether the metaplastic polyp forms as a result of an abnormality in cell proliferatio n or inflammation of the region. Methods: We examined 15 endoscopically resected metaplastic polyps of the c olon having a longitudinal diameter of more than 5 mm. To study aspects of cell proliferation, we used proliferating-cell nuclear antigen (PCNA) stain ing. For examination of histological changes caused by inflammation, we use d alpha-smooth muscle actin (alpha SMA) staining, in which myofibroblasts a re specifically stained. Results: Metaplastic polyps showed significantly higher expression of PCNA, not only in the deep layer, but also in the intermediate and superficial l ayers, compared with the normal mucosa of the colon. In the protruding meta plastic polyps, anti-alpha SMA staining revealed bundle-like myofibroblasts in the interstitium. Conclusions:The factors responsible for the formation of non-neoplastic met aplastic polyps larger than 5 mm with a protruding morphology are: an incre ased number of epithelial cells due to the movement of these cells toward t he ductal epithelium in the proliferating zone; and expansion of interstiti al tissues due to infiltration of myofibroblasts and other inflammatory cel ls in response to inflammation. (C) 1999 Blackwell Science Asia Pty Ltd.