Histology - GI Tract Overview
Overview
4 continuous walls of the GI tract from inner to outer:
- Mucosa and submucosa, which, in the upper sections, is thrown into folds surrounding the lumen
- The muscularis (externa), which comprises an inner circular layer and outer longitudinal layer
- The adventitia (if the organ is directly adhered to the body wall) or serosa (if the organ is suspended by mesenteries).
- The plexus of Meissner (aka, submucosal plexus)
- The plexus of Auerbach (aka, myenteric plexus)
- Swallowed foods, liquids, and salivary juices pass from the esophagus to the stomach, then through the small and large intestines.
Details
Esophagus
- Mucosa (and part of the submucosa) of the esophagus is folded when relaxed; the longitudinal folds unfurl during swallowing.
- Mucosa comprises stratified squamous epithelia, which provides a physical barrier against swallowed foods, liquids, and salivary juices.
- Mucus-secreting glands in the mucosal and submucosal layers open to the lumen and provide protection and lubrication.
- The circular and longitudinal layers of the muscularis externa comprise both skeletal and smooth muscle fibers, depending on the location along the length of the esophagus:
- In the upper third, both the circular and longitudinal layers comprise skeletal muscle
- In the middle third, the inner circular layer comprises skeletal muscle, and the longitudinal layer comprises smooth muscle
- In the final third, both layers comprise smooth muscle.
- The outermost layer of the esophagus is adventitia until it passes through the diaphragm and becomes serosa.
- The surface of the stomach is marked by gastric folds, aka, rugae; the submucosa contributes to the core of the ruga.
- The rugae are covered in pits, which open to glands within the mucosa, where they produce protective mucus and gastric juices.
- The muscularis externa of the stomach comprises an additional layer of smooth muscle, the oblique layer, in addition to the circular and longitudinal layers.
- The outermost layer of the stomach is serosa.
Comprise the small intestine
- Villi are evaginations of the mucosa
- Crypts/glands - Villi are continuous with the intestinal glands, aka, crypts, which are invaginations of the mucosa.
- Microvilli, which form a brush border at the luminal surface of the small intestine;
Together, the microvilli, villi, and intestinal glands greatly increase the surface area of the mucosa of the small intestine, which is responsible for absorption of nutrients and water.
- Circular fold, aka, plicae circulare comprises folds of villi-covered mucosa and a portion of the submucosa. These folds, which are most prominent in the middle regions of the small intestine, also increase mucosal absorptive surface area.
- Blood and lymphatic vessels course through the submucosa
- Lacteals and capillaries branch off and extend through the mucosa in the villi, where the lacteals absorb lipids (lymphatic circulation delivers the lipids to the general circulation.)
- In each segment of the small intestine the muscularis comprises smooth muscle in both the circular and longitudinal layers.
- Submucosa of the duodenum house mucous-secreting Brunner's glands; because these are only found in this segment of the small intestine, they are also referred to as duodenal glands.
- Houses Peyer's patches, which are aggregations of lymphoid tissues.
- Immune tissues reside throughout the small intestine as GALT – gut-associated lymphoid tissues – but Peyer's patches are most commonly seen in the ileum.
- Straight tubular glands
- Abundant mucous-secreting goblet cells.
- Circular folds, villi, and microvilli are absent from the large intestine, which is responsible for water, salt, vitamin, and mineral absorption.
- Muscularis comprises two layers of smooth muscle, but it's longitudinal layer is scant, and exists as three thin ribbon-like structures called taeniae coli.
- Taeniae coli pull the colon into pouch-like structures called haustra.
- Colon segments that reside laterally (the ascending and descending colons) are adhered to the body wall by adventitia, while the segments that reside more medially (transverse and sigmoid colons) have serosal coverings.
Images:
Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)