The intestinal epithelium is the single cell layer that forms the luminal surface (lining) of both the small and large intestine (colon) of the gastrointestinal tract. Composed of simple columnar epithelium its main functions are absorption, and secretion. Useful substances are absorbed into the body, and the entry of harmful substances is restricted. Secretions include mucins, and peptides. Absorptive cells in the small intestine are known as enterocytes, and in the colon they are known as colonocytes. The other cell types are the secretory cells – goblet cells, Paneth cells, enteroendocrine cells, and Tuft cells. Paneth cells are absent in the colon. 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.
Structure
The intestinal epithelium is part of the mucosal lining. The epithelium is simple cuboidal epithelium composed of a single layer of cells, while the other two layers of the mucosa, the lamina propria and the muscularis mucosae, support and communicate with the epithelial layer. To securely contain 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:
Circular folds are transverse folds that slow the passage of the luminal contents and serve to expand the total surface area threefold. Villi and intestinal glands serve to increase the mucosal surface area tenfold. (Intestinal villus) Microvilli covering the apical surface of the enterocytes increase the absorptive surface twentyfold. These numerous microscopic (100 nanometers in diameter) finger-like projections form an undulated brush border. 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.
Cell types 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. The cell types produced are: enterocytes (small intestine) (known as colonocytes in colon), Goblet cells, enteroendocrine cells, Paneth cells, microfold cells, cup cells and tuft cells. Their functions are listed here:
Enterocytes (in the small intestine) known as colonocytes in the colon, are the most numerous and function primarily for nutrient digestion and absorption. Enterocytes express many catabolic enzymes on their exterior luminal surface to break down molecules to sizes appropriate for uptake into the cell. Examples of molecules taken up by enterocytes are: ions, water, simple sugars, vitamins, lipids, peptides and amino acids. Goblet cells secrete the mucus layer which protects the epithelium from the luminal contents. Enteroendocrine cells secrete various gastrointestinal hormones including secretin, pancreozymin (also called cholecystokinin or CCK), enteroglucagons (GLP-1 and GLP-2),serotonin, among others. Subsets of sensory intestinal epithelial cells synapse with nerves, and are known as neuropod cells. Paneth cells produce antimicrobial peptides such as human alpha-defensin. Microfold cells (commonly referred to as M cells) sample antigens from the lumen and deliver them to the lymphoid tissue associated with the mucosa (MALT). In the small intestine, M cells are associated with Peyer's patches. Cup cells are a distinct cell type that produces vimentin. Tuft cells play a part in the immune response. Throughout the digestive tract, the distribution of the different types of epithelial cells varies according to the function of that region.
Structural components of cellular junctions
Important for the barrier function of intestinal epithelium, its cells are joined securely together by four types of cell junction which can be identified at the ultrastructural level:
Gap junctions Desmosomes Adherens junctions Tight junctions
Gap junctions Gap junctions bring the adjacent cells within 2 nanometers of each other. They are formed by several homologous proteins encoded by the connexin gene family coming together to form a multiprotein complex. The molecular structure of this complex is in the form of a hexamer. The complex, which is embedded in the cell membranes of the two joined cells, forms a gap or channel in the middle of the six proteins. This channel allows various molecules, ions and electrical impulses to pass between the two cells.
Desmosomes These complexes, consisting of transmembrane adhesion proteins of the cadherin family, link adjacent cells together through their cytoskeletons. Desmosomes leave a gap of 30 nanometers between cells.
Adherens junctions Adherens junctions, also called zonula adherens, are multiprotein complexes formed by proteins of the catenin and cadherin families. They are located in the membrane at the contact points between the cells. They are formed by interactions between intracellular adapter proteins, transmembrane proteins and the actin cytoskeletons of the cells . Besides their role in linking adjacent cells, these complexes are important for regulating epithelial migration, cell polarity, and the formation of other cell junction complexes.
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