Small Intestine Properties

1. Villi. The presence of these epithelium-covered fingerlike mucosal projections into the lumen is the most diagnostic feature of small intestine structure. The villi increase the mucosal surface area about 10-fold and thus enhance absorption; their shape and abundance differ according to the region where they are located.
2. Intestinal glands (crypts of Lieberkuhn), These simple tubular glands (often coiled) extend into the lamina propria below the bases of the villi. They are lined by absorptive, goblet, Paneth's, enteroendocrine, and undifferentiated cells. Their secretions enter the lumen via small openings between the villi. Similar glands are seen in the large intestine, where they contain many more goblet cells.
3. Enterocytes (absorptive cells). These are the predominant cell type covering the villi. They occur in small numbers in the crypts. The approximately 3000 microvilli per cell give the cell-lumen border a striped appearance, referred to as a striated border.

4. Goblet cells. These lie between the absorptive cells, with more in the surface epithelium than in the crypts. They gradually increase in number from the duodenum to the ileum.

5. Paneth's cells. Lying in the bases of the crypts, these cells synthesize a protein polysaccharide complex. In addition to RER and Golgi complexes, they have many large acidophilic secretory granules that contain lysozyme, an antibacterial enzyme that may help control the intestinal flora.

6. Enteroendocrine cells. Most known types of enteroendocrine cells are found in the crypts of the small intestine. Those that occur mainly in this area produce hormones and amines such as secretin, which increases pancreatic and biliary bicarbonate and water secre tion; cholecystokinin, which increases pancreatic enzyme secretion and gallbladder contraction; gastric inhibitory peptide, which decreases gastric acid production; and motilin, which increases gut motility.

7. Undifferentiated cells. Mucosal epithelial cells undergo continual turnover. Replacement occurs through the mitosis of undifferentiated (stem) cells located near the base of the crypts. Products of these divisions differentiate into all the cell types described above; by a mechanism that is still unclear, they move toward the crypt base or toward the tips of the villi, from which they are finally sloughed into the lumen.

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