The mucosal surface of a portion of sigmoid colon seen here demonstrates diverticula. There are areas of mucosal erythema around some of the diverticula, consistent with inflammation causing diverticulitis. Colonic diverticula tend to have narrow openings that can become eroded and inflamed and, rarely, perforated. If erosion of small mucosal vessels occurs, there can be bleeding. Colonic diverticula become increasingly prevalent with aging in persons who do not have a diet high in fiber. The presumed pathogenesis is increased intralumenal pressure that leads to small 0.5 to 1 cm outpouchings of mucosa through the muscularis, which in the colon is mostly the inner circular layer, since the longitudinal layer is composed of three separate bands--the taenia coli. Only about 20% of persons with diverticula have symptoms related to them.