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.

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:

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.

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: