Collagenous colitis. Ultrastructural study and collagen immunotyping of four cases.

We studied four cases of collagenous colitis (CC), which is characterized by thickening of the subepithelial collagenous layer of the colorectal mucosa and is associated with chronic watery diarrhea. Eighteen cases of CC have been previously reported in the literature, to our knowledge. In our four patients, ultrastructural study and collagen immunotyping demonstrated that the basement membrane over the thickened collagen was normal. We found no similar collagenous thickening in a series of 96 biopsy specimens of chronic inflammatory bowel diseases, but collagenous thickening was noted in 17 (25%) of 68 colonic metaplastic polyps. The cause of CC is unknown. The pericryptal fibroblastic sheath of the colonic mucosa has a probable role in the development of CC and metaplastic polyps. The role of the numerous mast cells present in our four cases of CC is currently unknown.