Collagenous colitis. Light micrograph of a section through a colon (large intestine) affected by collagenous colitis, showing a strip of submucosal collagen (yellow). Collagenous colitis (inflammation of the colon) is an inflammatory bowel disease affecting the colon that is characterised by the deposition of collagen in the lamina propria, the area of connective tissue between colonic glands. It causes watery intermittent diarrhoea, usually without rectal bleeding, with more than 10 bowel movements per day. Magnification: x560, when printed 10 centimetres wide.