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Diverticulosis of the left colon is an acquired condition that is associated with aging. The diverticula arise at the points of penetration of the circular layer of the muscularis propria by the blood vessels supplying the submucosa and mucosa. They are outpouchings of mucosa invested by wisps of muscularis mucosae, which are believed to result from elevated intraluminal pressure in the left colon, associated with degenerative changes in the collagen and elastin content of the colonic wall. Pathologically, diverticulitis occurs when the mucosa of diverticula becomes inflamed. This may result from obstruction of the diverticulum, for example, by inspissated feces or a fecolith, leading to bacterial overgrowth, gas and toxin production, and mucosal injury. The flat mucosa of the colon between the orifices of the diverticula is also commonly inflamed, with changes of varying severity indistinguishable from those of ulcerative colitis or Crohn's disease. Such mucosal changes include a dense lymphoplasmacytic infiltrate, mucin depletion, cryptitis, crypt abscesses, architectural distortion, gland dropout, Paneth cell metaplasia, and ulceration. Complications of diverticulitis include bleeding (the diverticula are in intimate contact with the penetrating blood vessels which may become eroded), ulceration, abscess formation, penetration, perforation, fistulas, stricture formation, and pylephlebitis.