## Histological Distinction Between UC and Crohn's Disease **Key Point:** Ulcerative colitis is characterized by **mucosal and submucosal inflammation only**, with crypt abscesses and surface ulceration, while the muscularis propria remains intact. This is the hallmark feature that distinguishes UC from Crohn's disease. ### UC Histology - Inflammation limited to **mucosa and submucosa** - **Crypt abscesses** (neutrophils in crypt lumens) - Surface ulceration with granulation tissue - Muscularis propria **preserved** - No skip lesions - Continuous involvement (rectum → proximal) ### Crohn's Disease Histology - **Transmural inflammation** (all layers involved) - **Non-caseating granulomas** (present in ~30–50% of cases) - Fissuring ulcers - Skip lesions (patchy, discontinuous involvement) - Cobblestone mucosa - Strictures and fistula formation **High-Yield:** The **intact muscularis propria in UC** explains why toxic megacolon (when it occurs) is due to neuromuscular dysfunction rather than structural perforation. In Crohn's, transmural involvement predisposes to fistulas and strictures. **Clinical Pearl:** Crypt abscesses are so characteristic of UC that their presence on biopsy strongly supports the diagnosis, though they are not pathognomonic. | Feature | Ulcerative Colitis | Crohn's Disease | | --- | --- | --- | | **Depth of inflammation** | Mucosa + submucosa | Transmural | | **Granulomas** | Absent (or rare) | Present (30–50%) | | **Crypt abscesses** | Prominent | Less common | | **Skip lesions** | No | Yes | | **Fissuring ulcers** | No | Yes | | **Muscularis propria** | Intact | Involved | [cite:Robbins 10e Ch 17] 
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