The surface area of the small intestine is enhanced by three morphologic features that are peculiar to the gut: plicae circulares, the villi and the microvilli. The plicae circulares (circular folds) consist of mucosal/submucosal invaginations that are predominantly located in the duodenum and jejunum. These infoldings are visible on gross inspection. The intestinal villi, finger-like projections that protrude into the intestinal lumen, are approximately 0,5-1,5 mm long and cover the mucosal surface. They can be viewed by close inspection of the mucosa under low-power microscopy. Their microscopic appearance varies: duodenal villi are characteristically broad and leaf-shaped, jejunal villi are tall and thin, and ileal villi are short and broad. The length and shape of the villi also vary with geographic region. At the base of the villi, the epithelium enters the lamina propria and forms the crypts of Lieberkühn, which extend almost to the muscularis mucosae. The microvilli are sub-light microscopic tubular projections that are extensions of the apical cell membrane and compose the brush border. There are the enzymes and receptors in these structures which are required for terminal digestion and absorption

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