Collagenous colitis is a type of microscopic colitis in which there is a thick collagen band below the surface epithelium.
Specimen: random biopsies from grossly normal colonic mucosa
Histologic features of collagenous colitis:
(1) lymphocytes within the epithelial cell layer along the surface and within the crypts
(2) degenerative changes in the epithelial cells (flattening, drop-out, etc)
(3) increase in inflammatory cells in the lamina propria consisting mostly of mononuclear cells
(4) presence of a thickened subepithelial collagen layer (layer > 10 microns thick)
• The first 3 items are the same as for lymphocytic colitis.
Features of the collagen band:
(1) The collagen band entraps cells (erythrocytes, inflammatory cells, fibroblasts, etc) and/or capillaries.
(2) The bottom of the band is irregular with collagen projecting into the lamina propria and around gland crypts.
(3) The overlying epithelial cell layers often shows an artifactual detachment from the collagen band.
Measurement of the collagen should be done on a trichrome stain in an area where the biopsy is well-oriented.
Additional histologic findings may include:
(1) inflammatory pseudomembrane
(2) acute inflammation, occasionally with crypt abscesses
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