Collagenous Colitis : Microscopic
Microscopically, the main findings in collagenous colitis are thickening of subepithelial collagen layer (10μ or greater) which appears as a hypocellular pink stripe or band; an infiltrate of mononuclear cells (lymphocytes, plasma cells, and eosinophils) in the lamina propria; and an intact crypt architecture. There are increased numbers of intraepithelial lymphocytes as well as damage to surface epithelium. Chronic cases, especially the ones refractory to therapy, may show Paneth cell metaplasia. The thickened collagen layer is best seen in the right colon and may not be apparent in the left colon or the rectosigmoid region. It may be missed on endoscopic biopsies since it is patchy with variable thickness. It should be evaluated in well-oriented sections. It merges imperceptibly with the basement membrane collagen. The normal basement membrane is 2 to 5 μ thick, whereas the collagen band in collagenous ranges from 10 to 30 μ in thickness. Image courtesy of: Sanjay Mukhopadhyay, MD; Dept. of Pathology, Cleveland Clinic, Cleveland, Ohio. USA.