Composition and specialised functions of intestinal epithelial cells (IECs). (IMAGE)
Caption
Crypt-base-columnar cells are continuously dividing intestinal stem cells that generate IECs, which are composed by distinct specialised cell types that play different functions: Enterocytes (in the small intestine) known as colonocytes in the colon, are the most numerous and function primarily for terminal digestion of nutrient substrates, and nutrient, ion and water absorption. Enterocytes express many catabolic enzymes on their exterior luminal surface to break down molecules to size appropriate for uptaking into the cell and solute transporters to mediate transepithelial absorption. Examples of molecules taken up by enterocytes are: ions, water, simple sugars, vitamins, lipids, peptides and amino acids. Single-cell studies discovered several types of enterocytes with distinct functions at specific positions along the crypt-villus axis. Goblet cells secrete mucin as a primary function to create a protective mucus layer which protects the epithelium from the luminal contents. Goblet cells are also involved in immunoregulation, since they can internalise and deliver luminal antigens to dendritic cells to induce tolerance. Goblet cells are the most numerous among the secretory cell types lining the mucosal surface. Enteroendocrine cells secrete hormonal products, they regulate intestinal motility, satiety, insulin secretion, immune responses, or release of digestive enzymes. For example, enteroendocrine cells secrete the gastrointestinal hormones secretin, pancreozymin and entero-glucagon among others. Subsets of sensory IECs synapse with nerves and are known as neuropod cells. Paneth cells produce antimicrobial peptides and proteins and other components that are important in host defence and immunity such as human alpha-defensin. Paneth cells contain secretory granules that are filled with antimicrobial agents (lysozyme, alpha-defensins, and phospholipase A2) and secreted at basal levels. Proinflammatory stimuli (ie, IFN gamma drastically increase their secretion. Paneth cells also contribute at the base of small intestinal crypts to the primary niche for intestinal stem cells. Microfold cells or M cells sample antigens from the lumen and deliver them to the lymphoid tissue associated with the mucosa. In the small intestine, M cells are associated with Peyer’s patches, secondary myeloid organs involved in immune surveillance. Tuft cells are low-abundant IEC components that play parts in the immune response, and are involved in chemosensation. They are closely related to taste receptor cells and use their chemosensory ability to initiate type II immune responses in the intestinal epithelium on detection of parasites. Stem cells are crypt-base-columnar cells that are continuously dividing to generate all other epithelial cell types. They reside exclusively at the bottom of crypts wedged between Paneth cells. Intestinal stem cells divide every day and produce two equipotent daughter cells.
Credit
By Claudio Hetz, Juan Francisco Silva-Agüero, Lisa M Ellerby
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CC BY-NC