Published online by Cambridge University Press: 06 June 2020
The lower gastrointestinal tract consists of ileum, vermiform appendix, colon, and rectum. The main epithelial cell types are absorptive cells, goblet cells, Paneth cells, and endocrine cells. Structurally the ileum resembles the duodenum, and includes villi and crypts. Gut-associated lymphoid tissue plays an important role in immune defence and has focal and diffuse elements in the small bowel. Lymphoid tissue is particularly prominent in the most distal part of the ileum. The mucosa of the large bowel is less complex than that of the small bowel, with parallel crypts and a smooth surface. In the normal colorectal mucosa, the density of plasma cells is highest in the upper one third and lowest in the lower one third. Eosinophilia in the ileum and large bowel is difficult to diagnose unless numbers are greatly increased. There may be a few neutrophils in the normal lamina propria but intraepithelial neutrophils are very infrequent. Apoptosis is a normal finding but the number of well-developed crypt epithelial cell apoptosis should be small. The intraepithelial lymphocyte count in the ileum is 0–9 per 100 surface epithelial cells and in the colon 0–5 per 100. Knowledge of the range of normality is important and helps pathologists to avoid overdiagnosis of inflammatory changes and of neoplasia.
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