Crohn Disease : Microscopic
Non-caseating epithelioid granulomas are one of the diagnostic hallmarks of Crohn disease. They are found in 40-60% of cases; therefore, absence of granulomas does not exclude Crohn disease. They may be present in any layer of the intestine, inflamed or non-inflamed regions, and even in mesenteric lymph nodes. This image shows a large epithelioid granuloma within muscularis propria in a colectomy specimen. In chronic cases, repeated cycles of crypt destruction followed by repair/healing lead to distortion of crypts and mucin depletion in the lining epithelium. Instead of appearing as symmetrical, vertically lined and evenly spaced cylinders (like a parallel row of test tubes), the crypts appear misshapen, distorted or branching. Chronic cases may also show epithelial metaplasias, including pseudopyloric metaplasia (lining epithelium resembles gastric antral mucosa) and Paneth cell metaplasia in the left colon (where Paneth cells are normally absent). Crypt distortion and metaplastic changes persist after resolution of active inflammation. Eventually, there is mucosal atrophy with loss of crypts altogether.