Tissue Layers of the Gastrointestinal Tract

Throughout the gastrointestinal (GI) tract, walls are comprised of the same four fundamental tissue layers. From the lumen of the GI tract, these layers are the mucosa, submucosa, muscularis, and serosa.

The lining of the small intestine. Image courtesy of Dr. Allan Wiechmann, University of Oklahoma Health Sciences Center

Tissue layers of the GI tract. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).

The Mucosa The mucosa is a mucous membrane that makes up the inner lining of the GI tract. It has three layers: (1) the epithelium, made of closely packed cells without a blood supply or nerves in direct contact with the foodstuffs that enter the GI tract; (2) the lamina propria, a layer of connective tissue that supports the epithelial cells; and (3) a thin smooth muscle layer called the muscularis mucosae. In the mouth, pharynx, esophagus, and distal portion of the anal canal, the epithelium is primarily nonkeratinized-stratified squamous epithelium. In the stomach and intestines, it is simple columnar epithelium, whose cells

participate in secretion and absorption. Every five to seven days, the harsh chemical and mechanical environment of the GI tract causes epithelial cells to be sloughed off and replaced by new ones. Epithelial cells are interspersed with exocrine cells that secrete mucus and digestive fluid into the lumen (interior space) of the alimentary canal and with enteroendocrine cells that secrete hormones and paracrines. In addition to connective tissue, the lamina propria of the mucosa contains numerous blood and lymphatic vessels, which transport nutrients absorbed into the GI tract to the liver. The lamina propria also contains most of the immune cells that make up the mucosa-associated lymphatic tissue (MALT). The muscularis mucosae layer is not responsible for movement of material through the GI tract, but it controls the exposed surface area. Small alterations in the many small folds in the mucous membrane of the stomach and small intestine increases the surface area available for digestion and absorption. Of these folds, the rugae in the stomach are temporary structures while the plicae circulares of the small intestine are permanent. When this muscle layer contracts, rugae and plicae circulars are pushed together exposing less SA to the lumen. When it relaxes more of the mucosal surface is revealed and in contact with the digestive products in the lumen.

The Submucosa The submucosa binds the mucosa to the muscularis externa. It is composed of areolar connective tissue and includes blood and lymphatic vessels (which transport absorbed food molecules) and the submucosal plexus (which is part of nervous system control).

The Muscularis In the mouth, pharynx, and superior and middle esophagus, the muscularis externa contains skeletal muscle that we use for voluntary swallowing. The external anal sphincter is also made of skeletal muscle, giving us voluntary control of defecation. In the rest of

the GI tract, the muscularis externa is smooth muscle, which contracts involuntarily to break down food, mix it with digestive juices, and move it along the GI tract. Complementary muscles in longitudinal (along the length of the tract) and circular layers create peristalsis – the wave-like muscular movements to move good from the esophagus to the anus.

The Serosa The serosa is the superficial layer of the intestine that covers the parts of the GI tract that is exposed to the abdominal cavity. This serous membrane is made up of areolar connective tissue and simple squamous epithelium (mesothelium). The esophagus has a single layer of tough connective tissue called theadventitia; it does not have a serosa.