STRUCTURE OF GASTRO-INTESTINAL WALL

From the mid-esophagus to the anus, the wall of the gastrointestinal tract has the general structure illustrated in Figure  below.Most of the luminal (inside) surface is highly convoluted, a feature that greatly increases the surface area available for absorption. From the stomach on, this surface is covered by a single layer of epithelial cells linked together along the edges of their luminal surfaces by tight junctions. Included in this epithelial layer are exocrine cells that secrete mucus into the lumen of the tract and endocrine cells that release hormones into the blood. Invaginations of the epithelium into the underlying tissue form exocrine glands that secrete acid, enzymes,water,ions, and mucus into the lumen.

Just below the epithelium is the lamina propria , which is a layer of loose connective tissue through which pass small blood vessels, nerve fibers, and lymphatic vessels.The lamina propria is separated from underlying tissues by the muscularis mucosa which is a thin layer of smooth muscle that may be involved in the movement of villi,described subsequently.The combination of these three layers—the epithelium,lamina propria, and muscularis mucosa—is called the mucosa. Beneath the mucosa is the submucosa ,which is a second connective-tissue layer. This layer also contains a network of neurons, the submucosal plexus,and blood and lymphatic vessels whose branches penetrate into both the overlying mucosa and the underlying layers of smooth muscle called the muscularis externa.Contractions of these muscles provide the forces for moving and mixing the gastrointestinal contents.

Except for the stomach, which has three layers, the muscularis externa has two layers: (1) a relatively thick inner layer of circular muscle , whose fibers are oriented in a circular pattern around the tube so that contraction produces a narrowing of the lumen;(2) and a thinner outer layer of longitudinal muscle whose contraction shortens the tube.Between these two muscle layers is a second network of neurons known as the myenteric plexus There are neurons projecting from the submucosal plexus to the single layer of cells on the luminal surface as well as to the myenteric plexus.The myenteric plexus is innervated by nerves from the autonomic nervous system and has neurons that project to the submucosal plexus.

Finally, surrounding the outer surface of the tube is a thin layer of connective tissue called the serosa .Thin sheets of connective tissue connect the serosa to the abdominal wall and support the gastrointestinal tract in the abdominal cavity.

 The macro- and microscopic structure of the wall of the small intestine is shown in Figure below.The circular folds (mucosa and submucosa) are covered with finger-like projections called villi.The surface of each villus is covered with a layer of epithelial cells whose surface membranes form small projections called microvilli  Interspersed between these absorptive epithelial cells with microvilli are goblet cells that secrete mucus that lubricates the wall of the small intestine. The combination of circular folds, villi, and microvilli increases the small intestine’s surface area about 600-fold over that of a flat-surfaced tube having the same length and diameter.The human small intestine’s total surface area is about 250 to 300 square meters, roughly the area of a tennis court.  The large surface area provided by the morphology of the small intestine allows for the highly efficient absorption of nutrients.  

Epithelial surfaces in the gastrointestinal tract are continuously being replacedbynewepithelialcells.Inthesmall intestine , new cells arise by cell division from cells at the base of the villi These cells differentiate as they migrate tothetopofthe villus, replacing older cells that die and are discharged into the intestinallumen. These dead cells release their intracellular enzymes into the lumen,whichthencontributetothe digestive process.About 17 billion epithelial cells are replaced each day, and the entire epithelium of the small intestine is replaced approximately every 5 days.

It is because of this rapid cell turnover that the lining of the intestinal tract is so susceptible to damage by treatments that inhibit cell  division, such as anticancer drugs and radiation therapy.Also at the base of the villi are enteroendocrine cells that secrete hormones that control a wide variety of gastrointestinal functions,including motility and exocrine pancreatic secretions.The center of each intestinal villus is occupied by both a single, blind-ended lymphatic vessel—a    lacteal   —and a capillary network. Most of the fat absorbed in the small intestine enters the lacteals.Material absorbed by the lacteals reaches the general circulation by eventually emptying from the lymphatic system into large veins through a structure called the thoracic duct.