20.2: Histology of the Alimentary Canal

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Rosanna Hartline
West Hills College Lemoore

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Histology of the Alimentary Canal

The organs of the GI tract are made from four layers, the inner lining or mucosa, the submucosa containing blood vessels and lymphatics, the muscularis externa or smooth muscle layer, and the outermost layer or serosa/adventitia. Each tissue layer plays a vital role in the digestive system ranging in their capacity to form a protective barrier from the highly acidic contents of the stomach to supplying hormones, producing muscular contractions and draining lymph. For example, specialized cells found the in epithelium lining the inside of the stomach are supporting cells which produce a protective layer of mucus and gastric acid for digestion. Additionally, other supporting cells, in the stomach and the acinar cells of the pancreas, release zymogens, inactive forms of digestive enzymes that become triggered to become active digestive enzymes.

Generalized tissue layers of the alimentary canal. SEROSA Longitudinal muscle MUSCULARIS EXTERNA Circular muscle SUBMUCOSA Muscularis mucosa MUCOSA Lamina propria Epithelium Lumen Gland in submucosa

Above: Tissue layers of the organs of the alimentary canal.

Layer of the Alimentary Tissue	Location	Substructures	Functions
mucosa	innermost layer; lines the alimentary canal	1. epithelium (mostly simple columnar) 2. lamina propria (connective tissue; may contain MALT; may contain lacteal capillaries) 3. muscularis mucosa (smooth muscle)	produces mucus, digestive enzymes, digestive zymogens, hormones and specialized materials necessary for digestion; protects from infectious agents; absorption of nutrients from food
submucosa	between the mucosa and muscularis externa	contains blood and lymphatic vessels (may contain lacteals) and nerves	provides structure; absorption of nutrients in blood vessels and lacteals
muscularis externa	between the submucosa and the serosa / adventitia	1. circular layer, 2. longitudinal layer; deep to the circular layer is a third layer in the stomach only, the oblique layer	contractions and relaxations in the canal that push digesting food through the canal (peristalsis) breaking it up [click here to see a gif of peristalsis]
serosa / visceral peritoneum / adventitia	outermost layer; lines the outside of the alimentary canal	1. areolar connective tissue, 2. epithelium (simple squamous)	production of peritoneal fluid to reduce friction between the organs during movements

MALT = mucosa-associated lymphatic tissue; immune functions

lacteal = fat-absorbing lymphatic vessels in the small intestine

How tissues in different regions of the alimentary canal differs

Above: The major diagnostic features of the components of the tubular digestive tract are highlighted in these drawings. All images show the tubular digestive organs cut in cross section, except for the small intestine illustrated in the two smaller panels located in the lower left quadrant. These small intestine sections depict longitudinal sections of this organ. Mucosae are pink, submucosae are orange, muscularis externa are red, and serosa / adventitia are blue-gray.

The esophagus is lined by stratified squamous moist epithelium. Scattered glands are present in lamina propria of the upper and lower esophagus and in the submucosa. Muscularis externa begins as skeletal muscle, transitions to skeletal plus smooth muscle and finally to only smooth muscle in the lower portion. An adventitia surrounds the organ.

The stomach is characterized by a thick mucosa lined by a surface sheet gland. The lamina propria is filled with gastric glands, each opening into a gastric pit. The muscularis externa is very thick with three, rather than the usual two, subdivisions. The stomach is covered by a serosa.

Villi are prominent features of small intestine. Intestinal glands open at the bases of villi and extend to muscularis mucosae. Glands in submucosa distinguish duodenum from the remainder of small intestine. Most of the organ is covered by a serosa, although a portion of duodenum is retroperitoneal and possesses an adventitia.

The large intestine (colon) lacks villi and possesses very straight intestinal glands in lamina propria. No submucosal glands are present. Most of the outer longitudinal subdivision of muscularis externa is relegated to three longitudinal strips termed the taeniae coli. The large intestine is covered by either an adventitia or a serosa, depending on its location.

Attributions

"Digital Histology" by Department of Anatomy and Neurobiology and the Office of Faculty Affairs, Virginia Commonwealth University School of Medicine and the ALT Lab at Virginia Commonwealth University is licensed under CC BY 4.0
"Human Anatomy Lab Manual" by Malgosia Wilk-Blaszczak, Mavs Open Press, University of Texas at Arlington is licensed under CC BY 4.0
"Mucosa.jpg" by National Cancer Institute is in the Public Domain
"Peristalsis.gif" by Auawise is licensed under CC BY-SA 4.0