Download Tissue Layers of the Gastrointestinal Tract

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 mucosaassociated 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.