Gastrointestinal wall

This article is one of a series on the
Gastrointestinal wall
General structure Epithelium Mucosa Submucosa Circular muscle Longitudinal muscle Serosa Adventitia
Specific Meissner's plexus Auerbach's plexus
Organs Esophagus Stomach Intestines
v t e

The gastrointestinal wall of the

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] the epithelium, where most digestive, absorptive and secretory processes occur; the lamina propria, a layer of connective tissue, and the muscularis mucosae, a thin layer of smooth muscle.

The submucosa contains nerves including the

The muscular layer surrounds the submucosa. It comprises layers of smooth 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 the myenteric plexus (also called Auerbach's plexus).

The

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:^[1]

• The epithelium is the innermost layer. It is where most digestive, absorptive and secretory processes occur.
• The lamina propria, a layer of connective tissue within the mucosa.
• The muscularis mucosae, a thin layer of smooth muscle.

The epithelium, the most exposed part of the mucosa, is a

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.

• In the
  oesophagus, pharynx and external anal canal
  the epithelium is stratified, squamous and non-keratinising, for protective purposes.
• In the stomach, the epithelium is simple columnar, and is organised into gastric pits and glands to deal with secretion.^[1]
• In the
  Brunner's glands are found in the duodenum but not in other parts of the small intestine.^[1]
• In the
  colon, epithelium is simple columnar and without villi. Goblet cells, which secrete mucous, are also present.^[1]
• The appendix has a mucosa resembling the colon but is heavily infiltrated with lymphocytes.

Transition between the different types of epithelium occurs at

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

The

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

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 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 transmural fistulae.

Invasion of tumours through the layers of the gastrointestinal wall is used in

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]

Additional images

• 
  General organisation of GI tract
• 
  The wall of the stomach.
• 
  Cross-section histology of

  terminal ileum

  .

References