The gastrointestinal wall of the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans a ...

is made up of four layers of specialised tissue. From the inner cavity of the gut (the lumen) outwards, these are: # Mucosa # Submucosa # Muscular layer #

Serosa The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...

or adventitia 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: the epithelium, where most digestive, absorptive and secretory processes occur; the lamina propria, a layer of

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tissue ...

, and the muscularis mucosae, a thin layer of

smooth muscle Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...

. The submucosa contains nerves including the submucous 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 of

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 the

myenteric plexus The myenteric plexus (or Auerbach's plexus) provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input (although present ganglion cell bodies belong to parasympathetic innervation ...

(also called Auerbach's plexus). The

serosa The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...

/ adventitia are the final layers. These are made up of loose

and coated in

mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

so as to prevent any friction damage from the intestine rubbing against other tissue. The serosa is present if the tissue is within the peritoneum, and the adventitia if the tissue is retroperitoneal.

Structure

When viewed under the microscope, the gastrointestinal wall has a consistent general form, but with certain parts differing along its course.

Mucosa

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: * The epithelium is the innermost layer. It is where most digestive, absorptive and secretory processes occur. * The lamina propria, a layer of

within the mucosa. * The muscularis mucosae, a thin layer of

. The epithelium, the most exposed part of the mucosa, is a glandular epithelium with many

goblet cell Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their se ...

s. Goblet cells secrete

, 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 a lacteal, a vessel connected to the lymph system that aids in the removal of

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids incl ...

s and tissue fluids. Microvilli are present on the epithelium of a villus and further increase the surface area over which absorption can take place. Numerous intestinal 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 the muscularis mucosa which is a layer of smooth muscle that aids in the action of continued peristalsis and catastalsis along the gut.

Epithelium

The lining of the mucosa, called the epithelium, differs along the gastrointestinal tract. 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. * In the

oesophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an Organ (anatomy), organ in vertebrates through which food passes, aided by Peristalsis, peristaltic contracti ...

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its ...

and external anal canal the epithelium is stratified, squamous and non-keratinising, for protective purposes. * In the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

, the epithelium is simple columnar, and is organised into gastric pits and glands to deal with secretion. * In the

small intestine The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through t ...

, epithelium is simple columnar and specialised for absorption. It is organised into plicae circulares and villi, and the enterocytes have microvilli. The microvilli create a brush border that increases the area for absorption. In the ileum there are occasionally Peyer's patches in the lamina propria. Brunner's glands are found in the

duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine ...

but not in other parts of the small intestine. * In the colon, epithelium is simple columnar and without villi.

Goblet cell Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their se ...

s, which secrete mucous, are also present. * The appendix has a mucosa resembling the colon but is heavily infiltrated with lymphocytes. Transition between the different types of epithelium occurs at the junction between the oesophagus and stomach; between the stomach and duodenum, between the ileum and caecum, and at the pectinate line of the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

Submucosa

The submucosa consists of a dense and irregular layer of connective tissue with

blood vessel Blood vessels are the structures of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from ...

s, lymphatics, and nerves branching into the mucosa and muscular layer. It contains the submucous plexus, and enteric nervous plexus, situated on the inner surface of the muscular layer.

Muscular layer

The muscular layer (also known as the ''muscularis propria'' ) consists of two layers of muscle, the inner and outer layer. 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. Between the two muscle layers are the myenteric or 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 the autonomic 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 called peristalsis and propels the food through the tract. Food in the GI tract is called a bolus (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 as chyme. 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: * In the colon, for example, the muscular layer is much thicker because the faeces are large and heavy and require more force to push along. The outer longitudinal layer of the colon thins out into 3 discontinuous longitudinal bands, known as taeniae coli (bands of the colon). This is one of the 3 features helping to distinguish between the large and small intestine. * Occasionally in the large intestine (2-3 times a day) there will be mass contraction of certain segments, moving a lot of faeces along. This is generally when one gets the urge to defecate. * The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. Alone among the GI tract, the stomach has a third layer of muscular layer. This is the inner oblique layer and helps churn the chyme in the stomach.

Serosa and adventitia

The outermost layer of the gastrointestinal wall consists of several layers of

and is either ''adventitia'' or ''serosa''. Regions of the gastrointestinal tract within the peritoneum (called Intraperitoneal) are covered with

. 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 the

, first part of the

, all of the

, caecum and

appendix Appendix, or its plural form appendices, may refer to: __NOTOC__ In documents * Addendum, an addition made to a document by its author after its initial printing or publication * Bibliography, a systematic list of books and other works * Index (pub ...

, transverse colon, sigmoid colon and

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the l ...

. In these sections of the gut there is clear boundary between the gut and the surrounding tissue. These parts of the tract have a mesentery. Regions of the gastrointestinal tract behind the peritoneum (called retroperitoneal) are covered with adventitia. They blend into the surrounding tissue and are fixed in position (for example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane). The retroperitoneal regions include the

oral cavity In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on t ...

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

, pylorus of the stomach, distal

, ascending colon, descending colon and anal canal.

Clinical significance

The gastrointestinal wall can be affected in a number of conditions. An ulcer is something that's eroded through the epithelium of the wall. Ulcers that affect the tract include peptic ulcers and perforated ulcer is one that has eroded completely through the layers. The gastrointestinal wall is inflamed in a number of conditions. This is called esophagitis, gastritis, duodenitis, ileitis, and colitis depending on the parts affected. It can be due to infections or other conditions, including coeliac disease, and

inflammatory bowel disease Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine, Crohn's disease and ulcerative colitis being the principal types. Crohn's disease affects the small intestine and large intestine, as wel ...

affects the layers of the gastrointestinal tract in different ways.

Ulcerative colitis Ulcerative colitis (UC) is a long-term condition that results in inflammation and ulcers of the colon and rectum. The primary symptoms of active disease are abdominal pain and diarrhea mixed with blood ( hematochezia). Weight loss, fever, an ...

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 transmural

fistula A fistula (plural: fistulas or fistulae ; from Latin ''fistula'', "tube, pipe") in anatomy is an abnormal connection between two hollow spaces (technically, two epithelialized surfaces), such as blood vessels, intestines, or other hollow or ...

e. Invasion of tumours through the layers of the gastrointestinal wall is used in staging of tumour spread. This affects treatment and prognosis. The normal thickness of the small intestinal wall is 3–5 mm, Updated: Sep 22, 2016 and 1–5 mm in the large intestine. Focal, irregular and asymmetrical gastrointestinal wall thickening suggests a malignancy. Segmental or diffuse gastrointestinal wall thickening is most often due to ischemic, inflammatory or infectious disease.

Additional images

File:GI Organization.svg, General organisation of GI tract File:Illu stomach2.jpg, The wall of the stomach.

References

{{Tissue layers Membrane biology Digestive system