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Thegastrointestinal wall of thegastrointestinal tract is made up of four layers of specialised tissue. From the inner cavity of the gut (thelumen) outwards, these are themucosa, thesubmucosa, themuscular layer and theserosa oradventitia.
The mucosa is the innermost layer of the gastrointestinal tract. It surrounds the lumen of the tract and comes into direct contact with digested food (chyme). The mucosa itself is made up of three layers:[1] theepithelium, where most digestive, absorptive and secretory processes occur; thelamina propria, a layer ofconnective tissue, and themuscularis mucosae, a thin layer ofsmooth muscle.
The submucosa contains nerves including thesubmucous plexus (also called Meissner's plexus), blood vessels and elastic fibres with collagen, that stretches with increased capacity but maintains the shape of the intestine.
The muscular layer surrounds the submucosa. It comprises layers ofsmooth muscle in longitudinal and circular orientation that also helps with continued bowel movements (peristalsis) and the movement of digested material out of and along the gut. In between the two layers of muscle lies themyenteric plexus (also called plexus).
Theserosa/adventitia are the final layers. These are made up of looseconnective tissue and coated inmucus so as to prevent any friction damage from the intestine rubbing against other tissue. The serosa is present if the tissue iswithin the peritoneum, and the adventitia if the tissue isretroperitoneal.
When viewed under themicroscope, the gastrointestinal wall has a consistent general form, but with certain parts differing along its course.
The mucosa is the innermost layer of the gastrointestinal tract. It surrounds the cavity (lumen) of the tract and comes into direct contact with digested food (chyme). The mucosa is made up of three layers:[1]
The epithelium, the most exposed part of the mucosa, is aglandular epithelium with manygoblet cells. Goblet cells secretemucus, which lubricates the passage of food along and protects the intestinal wall from digestive enzymes. In the small intestine,villi are folds of the mucosa that increase the surface area of the intestine. The villi contain alacteal, a vessel connected to thelymph system that aids in the removal oflipids and tissue fluids.Microvilli are present on the epithelium of a villus and further increase the surface area over which absorption can take place. Numerousintestinal glands as pocket-like invaginations are present in the underlying tissue. In the large intestines, villi are absent and a flat surface with thousands of glands is observed. Underlying the epithelium is the lamina propria, which contains myofibroblasts, blood vessels, nerves, and several different immune cells, and themuscularis mucosa which is a layer of smooth muscle that aids in the action of continuedperistalsis andcatastalsis along the gut.
| Cell type[2] | Location in the mucosa | Function |
|---|---|---|
| Absorptive cell | Epithelium/intestinal glands | Digestion and absorption of nutrients in chyme |
| Goblet cell | Epithelium/intestinal glands | Secretion of mucus |
| Paneth cell | Intestinal glands | Secretion of the bactericidal enzyme lysozyme; phagocytosis |
| G cells | Intestinal glands of duodenum | Secretion of the hormone intestinalgastrin |
| I cells | Intestinal glands of duodenum | Secretion of the hormonecholecystokinin, which stimulates release of pancreatic juices and bile |
| K cells | Intestinal glands | Secretion of the hormone glucose-dependent insulinotropic peptide, which stimulates the release of insulin |
| M cells | Intestinal glands of duodenum and jejunum | Secretion of the hormonemotilin, which accelerates gastric emptying, stimulates intestinal peristalsis, and stimulates the production of pepsin |
| S cells | Intestinal glands | Secretion of the hormonesecretin |
The epithelial lining of the mucosa, differs along the gastrointestinal tract.[1] The epithelium is described as stratified if it consists of multiple layers of cells, and simple if it is made up of one layer of cells. Terms used to describe the shape of the cells in it - columnar if column-shaped, and squamous if flat.
Transition between the different types of epithelium occurs atthe junction between the oesophagus and stomach; between thestomach and duodenum, between theileum and caecum, and at thepectinate line of theanus.[1]
The submucosa consists of a dense and irregular layer of connective tissue withblood vessels, lymphatics, and nerves branching into the mucosa and muscular layer. It contains thesubmucous plexus, andenteric nervous plexus, situated on the inner surface of the muscular layer.[1]
Themuscular layer consists of two layers of muscle, the inner and outer layer.[3] The muscle of the inner layer is arranged in circular rings around the tract, whereas the muscle of the outer layer is arranged longitudinally. The stomach has an extra layer, an inner oblique muscular layer.[1] Between the two muscle layers is themyenteric plexus (Auerbach's plexus). This controls peristalsis. Activity is initiated by the pacemaker cells (interstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can, of course, be modulated by the rest of theautonomic nervous system.
The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.
The coordinated contractions of these layers is calledperistalsis and propels the food through the tract. Food in the GI tract is called abolus (ball of food) from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to aschyme. In the large intestine the remaining semi-solid substance is referred to as faeces. The circular muscle layer prevents food from travelling backward and the longitudinal layer shortens the tract.
The thickness of the muscular layer varies in each part of the tract:
The outermost layer of the gastrointestinal wall consists of several layers ofconnective tissue and is either ofserosa (below the diaphragm) oradventitia above the diaphragm.[4][1][5]
Regions of the gastrointestinal tract within the peritoneum (calledIntraperitoneal) are covered withserosa. This structure consists of connective tissue covered by a simple squamous epithelium, called the mesothelium, which reduces frictional forces during digestive movements. The intraperitoneal regions include most of thestomach, first part of theduodenum, all of thesmall intestine,caecum andappendix,transverse colon,sigmoid colon andrectum. In these sections of the gut there is clear boundary between the gut and the surrounding tissue. These parts of the tract have amesentery.
Regions of the gastrointestinal tract behind the peritoneum (calledretroperitoneal) are covered withadventitia. They blend into the surrounding tissue and are fixed in position (for example, the retroperitoneal section of the duodenum usually passes through thetranspyloric plane). The retroperitoneal regions include theoral cavity,esophagus,pylorus of the stomach, distalduodenum,ascending colon,descending colon andanal canal.[citation needed]
The gastrointestinal wall can be affected in a number of conditions.
Anulcer is something that's eroded through the epithelium of the wall. Ulcers that affect the tract includepeptic ulcers andperforated ulcer is one that has eroded completely through the layers.
The gastrointestinal wall is inflamed in a number of conditions. This is calledesophagitis,gastritis,duodenitis,ileitis, andcolitis depending on the parts affected. It can be due to infections or other conditions, includingcoeliac disease, andinflammatory bowel disease affects the layers of the gastrointestinal tract in different ways.Ulcerative colitis involves the colonic mucosa.Crohn's disease may produce inflammation in all layers in any part of the gastrointestinal tract and so can result in transmuralfistulae.
Invasion of tumours through the layers of the gastrointestinal wall is used instaging of tumour spread. This affects treatment and prognosis.
The normal thickness of the small intestinal wall is 3–5 mm,[6] and 1–5 mm in the large intestine.[7] Focal, irregular and asymmetrical gastrointestinal wall thickening suggests a malignancy.[7] Segmental or diffuse gastrointestinal wall thickening is most often due to ischemic, inflammatory or infectious disease.[7]
This article incorporatestext available under theCC BY 4.0 license.Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (September 13, 2023).Anatomy & Physiology. Houston: OpenStax CNX. 23.5 The small and large intestines.ISBN 978-1-947172-04-3.