Intestinal epithelium

The intestinal epithelium is the single cell layer that form the luminal surface (lining) of both the small and large intestine (colon) of the gastrointestinal tract. Composed of simple columnar epithelial cells, it serves two main functions: absorbing useful substances into the body and restricting the entry of harmful substances. As part of its protective role, the intestinal epithelium forms an important component of the intestinal mucosal barrier. Certain diseases and conditions are caused by functional defects in the intestinal epithelium. On the other hand, various diseases and conditions can lead to its dysfunction which, in turn, can lead to further complications. The intestinal epithelium is part of the intestinal mucosa layer. The epithelium is composed of a single layer of cells. The other two layers of the mucosa, the lamina propria and the muscularis mucosae, support and articulate the epithelial layer. To securely contain of the contents of the intestinal lumen, the cells of the epithelial layer are joined together by tight junctions thus forming a contiguous and relatively impermeable membrane. Epithelial cells are continuously renewed every 4–5 days through a process of cell division, maturation, and migration. Renewal relies on proliferative cells (stem cells) that reside at the crypt (base) of the intestinal glands (epithelial invaginations into the underlying connective tissue). After being formed at the base, the new cells migrate upwards and out of the crypt, maturing along the way. Eventually, they undergo apoptosis and are shed off into the intestinal lumen. In this way, the lining of the intestine is constantly renewed while the number of cells making up the epithelial layer remains constant. In the small intestine, the mucosal layer is specially adapted to provide a large surface area in order to maximize the absorption of nutrients. The expansion of the absorptive surface, 600 times beyond that of a simple cylindrical tube, is achieved by three anatomical features: The brush border on the apical surface of the epithelial cells is covered with glycocalyx, which is composed of oligosaccharides attached to membrane glycoproteins and glycolipids. Seven different cell types are produced by the stem cells that reside at the base of the crypts. Each type matures according to its specific differentiation program as it migrates up and out of the crypt. Many of the genes necessary for differentiation into the different epithelial cell types have been identified and characterized (see this table). The cell types produced are: enterocytes, Goblet cells, enteroendocrine cells, Paneth cells, microfold cells, cup cells and tuft cells. Their functions are listed here: Throughout the digestive tract, the distribution of the different types of epithelial cells varies according to the function of that region. Important for the barrier function of intenstinal epithelium, its cells are joined securely together by four types of junctions (cell junctions), which can be identified at the ultrastructural level: