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Content: Reflecting its absorptive function, the surface of the small intestine is amplified significantly at three levels: At a gross level, the small intestine is a long tube into whose lumen projects the plicae circularis, circular folds of the mucosal epithelium, lamina propria, muscularis mucosa, and submucosa. Villi, finger-like projections involving only the epithelium and lamina propria, project into the lumen. They have a central lymphatic vessel known as a lacteal, which is crucial for the absorption of lipids from the intestine. Microvilli make up a brush border on the surface of the columnar cells of the mucosal epithelium. The small intestine begins after the gastro-duodenal junction and is divided into three segments: duodenum, jejunum, and ileum. Duodenum
The duodenum contains the same wall layers seen in the previous portions of the GI tract: mucosa, submucosa, and muscularis externa. The epithelia and lamina propria of the mucosal layer are thrown into villi.