These include absorptive cells (colonocytes, enterocytes), goblet cells, enteroendocrine cells, Paneth cells (of the small intestine), tuft cells, and M cells ( 8). Each of these differentiated epithelial cell subtypes possess specific functions in maintenance of the barrier’s digestive, neuroendocrine, and immune functions. These stem cells give rise to progenitor cells (transit amplifying cells) and multiple differentiated epithelial cell subsets as they migrate through division up the crypt-villus axis before undergoing a program of apoptosis and luminal shedding. The tubular crypt and epithelial lining that defines the structure of both the small intestine and colon is continuously regenerated (every 3–5 days) by a population of long-lived Lgr5+ stem cells that reside at the base of the crypt ( 8). The intercellular junctions consist of desmosomes, gap junctions, and adherent junctions made up of various integral proteins such as claudins, occludins, and zonula occludens (ZO-1, ZO-2) which operate in concert to maintain the integrity of the epithelial barrier by regulating the paracellular transport of ions, metabolites, and macromolecules ( 4– 7). The underlying layer of intestinal epithelial cells (IECs) functions as a physical barrier facilitated by tight junctions ( 4, 5). The renewal of the mucus layer (colon consists of an inner more viscous and outer less viscous layer, small intestine a single layer only) is achieved through the secretion of mucus by goblet cells (primarily via the MUC2 gene) ( 3). It also acts to shield the host from digestive enzymes and microorganism epithelial penetration. The first layer of physical defense, the mucus layer, plays an important role in reducing adherence of pathogenic microbes while providing a rich-layer of sustenance for slow-growing anaerobic commensal organisms ( 1, 2). In order to achieve this fine balance the intestinal epithelium acts as mucosal barrier to micro-organisms while permitting a pathway to protein antigens and small molecule metabolites. The intestinal epithelium provides a controlled homeostatic system for molecular transit in order to mediate the balance of the multiple functions of the intestinal tract digestion, immunity and tolerance, and a repository for the various known and unknown symbiotic functions of the microbiota.

The intestinal epithelium lining forms the luminal surface to the external environment of both the small and large intestines. Of smooth muscle, an inner circular and outer longitudinal, forĬontinuous peristaltic activity of the small intestine.Introduction to the Intestinal Epithelial Barrier The muscularis externa layer contains two layers The lymphatic capillariesĪre called lacteals, and absorb lipids. Products, and there is a muscularis mucosae layer Has a rich vascular and lymphatic network, which absorbs the digestive The lamina propria which underlies the epithelium Of Lieberkuhn, which extend down to the muscularis Between the villi there are crypts, called crypts Together, these folds provide a huge surface area forĪbsorption. the lining columnar epithelial cells have fine projections on their apical surfaces called microvilli.smaller folds called villi, which are finger like mucosal projections, about 1mm long.large circular folds called plicae circulares (shown in the diagram to the right), most numerous in the upper.The pancreas and liverĪlso deliver their exocrine secretions into the duodenum. the small intestine secretes enzymes and has mucous producing glands.

The small intestine is 4-6 metres long in humans. The main functions of the small intestine are digestion, absorption of food and production of gastrointestinal hormones. This is a diagram which shows the villi of the small intestine, as indicated by the arrows in the diagram above, at higher magnification.

Lumen of small intestine