Light and electron microscopic studies of 14 cases of collagenous colitis are reported. Comparative electron microscopic examinations were carried out on 13 cases of ulcerative colitis in the chronic fibrotic stage of the disease. Separation of pericryptal fibroblasts which showed enhanced fibre-forming activity, proliferation of myofibroblasts, accumulation of mast cells and a pericapillary collagen accumulation were noted in both groups. Based on these results, collagenous colitis was considered to result from scar formation secondary to previous superficial inflammation.

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