[Histopathology of microscopic colitis]

Microscopic colitis (MC) is recognized to be a common cause of chronic, non-bloody diarrhea with rising incidence in the last decade. The diagnosis can only be made by histology and the specific histological findings define two subtypes of MC: lymphocytic (LC) or collagenous colitis (CC). The key histological feature of LC is an increased number of surface intraepithelial lymphocytes (IEL). In the literature, >20 IELs/100 epithelial cells are required to warrant the diagnosis of LC. IELs are mostly cytotoxic CD8+ T-lymphocytes. The key histological criterion for CC is a continuous subepithelial fibrous band underneath the surface epithelium (>10 µm). Other hallmarks of CC are chronic mucosal inflammation, and the collagen band can contain entrapped capillaries, red blood cells, and inflammatory cells. Damaged epithelial cells appear flattened, mucin depleted, and irregularly oriented. Focally, small strips of surface epithelium may lift off from their basement membrane. In both subtypes of MC, the lamina propria shows increased numbers of plasma cells and lymphocytes with loss of the normal gradient, even eosinophilic and neutrophilic granulocytes may be present. But these histological features alone do not warrant the diagnosis of MC even though they may be responsible for the clinical symptoms. The term MCi (MC incomplete) is suggested for the subgroup of patients with diarrhea and an increase in cellular infiltrate in the colonic lamina propria and either an abnormal collagenous layer and/or intraepithelial lymphocytes coming but not fulfilling the criteria for CC or LC.