Ulcerative colitis. Light micrograph of a section through the bowel, showing the epithelial cells that make up the inner surface of the bowel (purple), dilated blood vessels (red) and inflammatory cells (leucocytes, dark). Ulcerative colitis is inflammation of the bowel, typically arising in the rectum and often extending to involve part of, or most of, the large intestine. This form of colitis is characterized by deep ulcers, which are usually superficial but may perforate the bowel wall in some cases. The affected bowel lining becomes haemorrhagic, contributing to diarrhoea and the passing of blood and mucus.