Slide 1 
Is a whole mount preparation showing a histologic section of chronic ulcerative colitis with pseudopolyps at the top and a classic case of Crohn's disease of the colon at the bottom. A number of differences are readily apparent. The first obvious difference is that the colonic wall in Crohn's disease is considerably thicker than that in chronic ulcerative colitis. This is for a number of reasons. First of all, the inflammation in Crohn's disease is transmural; that is, it involves all coats of the colonic wall whereas the inflammation in ulcerative colitis is usually limited to the mucosa and submucosa. The collections of chronic inflammatory cells in all levels of the colonic wall, as well as the inflammatory edema and subsequent fibrosis of the submucosa and serosa in Crohn's disease, account for the increased thickness of the colonic wall. Linear ulcers are present in Crohn's disease as well as in ulcerative colitis, and you can see one in the center of the colonic mucosa in the Crohn's disease below. Whereas the ulcers in ulcerative colitis are usually shallow, the ulcers in Crohn's disease are often fissuring, knife-like ulcers that penetrate deep into the colonic wall. Pseudopolyps are much more common in ulcerative colitis than in Crohn's disease.

Slide 2
The transmural inflammation in Crohn's disease results in a fibrous reparative response in the submucosa and quite often in the serosa. This Trichrome preparation shows a marked increase in fibrous connective tissue (blue) in the submucosa (SM).

Slide 3
Gross - ileum: The fibrous reparative reaction in Crohn's disease causes marked thickening of the bowel wall and stenosis of the lumen (Rubber hose or lead pipe bowel). This is a classic example of Crohn's disease of the terminal ileum showing marked thickening with a loss of pliability of the wall and narrowing of the lumen.