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Ulcerative colitis.

limited to the mucosa and submucosa
begins in the rectum and extends proximally in a continuous fashion
migratory polyarthritis, sacroiliitis, ankylosing spondylitis, uveitis, erythema nodosum, and hepatic involvement (pericholangitis and primary sclerosing cholangitis
Differences from Crohn’s disease
Well-formed granulomas are absent.
There are no skip lesions.
The mucosal ulcers rarely extend below the submucosa.
Mural thickening does not occur
high risk of carcinoma development

Morphology of UC:

rectum and sigmoid are invariably involved and may involve the entire colon-pancolitis
hyperemia, edema, and granularity with friability and easy bleeding
islands of regenerating mucosa bulge upward to create pseudopolyps
pericolonic abscess formation
Exposure of the muscularis propria and neural plexus to fecal material also may lead to complete shutdown of neuromuscular function-toxic megacolon
A diffuse, predominantly mononuclear inflammatory infiltrate in the lamina propria
crypt abscesses due to neutrophils
In late stages, granulation tissue fills in the ulcer craters, followed by regeneration of the mucosal epithelium
Submucosal fibrosis and mucosal architectural disarray and atrophy remain

Clinical features and complications:

mucoid diarrhea that may persist for days, weeks, or months and then subside, only to recur
severe diarrhea and electrolyte derangements
massive hemorrhage
severe colonic dilation (toxic megacolon) with potential rupture
perforation with peritonitis
Inflammatory strictures of the colorectum-must be differentiated from cancer.
sequential mucosal changes from dysplasia to invasive carcinoma- surveillance with repeated colonoscopies and multiple biopsies aimed at detecting dysplasia for possible prophylactic colectomy.

Crypt Abscess

Toxic Megacolon