Changes in UC usually begin in the rectum and may extend proximally to involve a variable length of and sometimes, the entire colon (pancolitis). The mucosa shows intense and diffuse inflammatory cell infiltrate, crypt abscesses, mucin depletion and surface ulceration, especially in active colitis where it is characterised predominantly by cryptitis (neutrophils into crypt epithelium), and crypt abscesses (neu-trophils within the crypt lumen) . Superficial broad ulcerations occur and, when severe, can extend into the muscularis propria. Paneth cell metaplasia is also a feature. The inflammation is mainly mucosal and the infiltrate is mainly composed of mononuclear cells, including lymphocytes (many of which are activated T lymphocytes), plasma cells with occasional eosinophils and mast cells together with neutrophil polymorphs [60, 61]. There is marked mucin depletion of the crypts (goblet cell depletion), resulting from both atrophic and regenerative changes [54,62].
It is important to realize that polymorph nuclear infiltrate in the crypts or the surface epithelium is associated with mucous depletion and this is indicative of the disease activity index and has no specific diagnostic attributes. Architectural distortion of crypts is the hallmark of IBD in which the surface attains villiform configuration with the crypts exhibiting irregularity with abnormal branching. The degree of architectural distortion is usually more severe in UC than in CD.
Architectural disarray, Paneth cell metaplasia, pseudo-pyloric metaplasia, villiform configuration of the surface epithelium, dense chronic inflammatory cell infiltrate of the deeper aspect of the lamina propria and the so-called 'ilealisation' of the colonic epithelium, constitute the cardinal features of chronicity in IBD but also can be seen to a variable degree in some other forms of chronic irritation and injury like in radiation, diversion, chronic ischemia.
Epithelioid granulomas together with multinucle-ated giant cells are characteristically not seen in UC. However, intramucosal leakage granulomas in close association with crypt rupture can be evident in this disease. On the other hand, isolated giant cells and well-defined epithelioid granulomas that are distant from the crypts in a biopsy showing features of chronicity are a strong indicator of a diagnosis of Crohn's disease . The mucosa sometimes contains dilated blood vessels some of which may contain thrombi . Features of endarteritis obliterans are seen in submucosal vessels and the muscularis mucosa may appear reduplicated . In quiescent disease the mucosa may appear nearly normal with slight crypt distortion and presence of Paneth cells and very occasional neutrophils in the lamina propria . As mentioned earlier, if the rectal biopsy appears normal, a diagnosis of UC is unlikely. However, subtle inflammatory changes can occur, especially if the patient is a child or if the disease has been treated with steroids [67, 68].
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