The most conspicuous tissue specializations of the small intestine are the mucosal evaginations (plicae and villi) which increase surface area for nutrient absorption.
Layers of the small intestine
The mucosa of the small intestine is characterized by evagination into plicae and villi, which increase the surface area for nutrient absorption, and by short tubular invaginations, the crypts, which provide a protected site for stem cells.
The structure of the intestinal mucosa may be altered by pathological processes. Examples include ischemic enteritis or necrotizing enterocolitis, both characterized by necrosis of the mucosa (see WebPath and WebPath), and malabsorption enteropathy (celiac sprue), characterized by blunting or loss of villi with concommitant loss of absorptive surface area, increased crypt length, and inflammatory infiltrate in lamina propria (see WebPath or Milikowski & Berman's Color Atlas of Basic Histopathology, p. 256, or Robbins Pathologic Basis of Disease.)
Cell types of the intestinal mucosa
Small intestinal mucosa is lined by a simple columnar epithelium which consists primarily of absorptive cells (enterocytes), with scattered goblet cells and occasional enteroendocrine cells. In crypts, the epithelium also includes Paneth cells and stem cells.
Functional integrity of the mucosal epithelium is critical for normal function. The cholera toxin kills the epithelial cells, so that cell-to-cell junctions no longer maintain the integrity of the fluid barrier between lumen and lamina propria. As a consequence, bodily fluid moves freely into the lumen and hence out through the intestine as thin watery diarrhea, leading to rapid, massive dehydration and death. Most patients may be saved simply by keeping them hydrated for just a few days, either with an intravenous plasma substitute (expensive) or with a properly balanced solution of salt and sugar delivered orally (cheap). By that time, the cholera vibrios (bacteria) will have cleared, and the epithelium will be replaced by stem cells dividing in the crypts.
Absorptive cells (enterocytes) are responsible for absorbing nutrients from the intestinal lumen and transporting them across the epithelium to the lamina propria, whence they diffuse into capillaries.
The brush border of the intestinal epithelium not only facilitates absorption, it also provides a site of attachment for pathogens. For an example (cryptosporidia), see WebPath.
Goblet cells secrete mucus to promote movement and effective diffusion of gut contents. The proportion of goblet cells generally increases as one progresses down the GI tract, with the highest proportion found in the lower tract.
Enteroendocrine cells (not pictured) secrete hormones to regulate secretion into the GI tract.
Tuft cells (not pictured) orchestrate immunity against parasites.
Paneth cells, located at the bottoms of the crypts, secrete lysosomal enzymes and other factors into the crypt lumen. These agents presumably help protect the crypt epithelium with its vital stem cells.
Stem cells line the walls of the crypts and continually replenish the intestinal epithelium, completely replacing all the absorptive and goblet cells approximately once every four days.
Lamina propria of each villus is richly supplied with capillaries as well as a single lacteal, for transporting absorbed nutrients. Lamina propria also includes thin strands of smooth muscle (presumably allowing some motility for individual villi, to encourage thorough fluid mixing at the absorptive surface) and numerous white blood cells.
The muscularis mucosa of the small intestine forms a thin layer (only a few muscle fibers in thickness) beneath the deep ends of the crypts.
The submucosa of the small intestine is relatively unspecialized, except in the duodenum where it is packed with the mucous-secreting Brunner's glands.
Muscularis externa of the small intestine has the standard layers of inner circular and outer longitudinal smooth muscle, with ganglia of Auerbach's plexus scattered in between.
Over most of the small intestine, the outer layer is a serosa attached to mesentery. The exception is the duodenum, which is retroperitoneal.
Regions of the small intestine
The small intestine is divided, somewhat arbitrarily, into three regions: the duodenum, the jejunum, and the ileum.
These anatomical regions display several histological differences. Some (Brunner's glands in the duodenum, Peyer's patches in the ileum) are conspicuous. Others (the distribution of cell types, the shapes of the villi, the numbers of plicae, the amount of lymphoid tissue) are more subtle.
The proportion of goblet cells generally increases as one progresses down the GI tract, with the highest proportion found in the lower tract.
Brunner's glands provide abundant alkaline mucus to neutralize the acid contents entering from the stomach. Brunner's glands and the stomach's pyloric glands thus share a similar function. In fact, Brunner's glands look like pyloric glands that have enlarged and expanded into the submucosa of the duodenum.
Villi are rather flatter in the duodenum than in the jejunum, and plicae are less frequent.
Villi are rather longer and more finger-like in the jejunum than in the duodenum, and plicae are usually apparent.
The lymphoid tissue of Peyer's patches may bulge out toward the lumen, displacing villi, and inward across the muscularis mucosae into the submucosa.
The epithelium overlying this lymphoid tissue is cuboidal (rather than columnar as on villi) and includes M cells which transport antigen across the epithelium to immune cells.
The lymphoid tissue of Peyer's patches is similar to that of tonsils and appendix. These structures, together with other more diffuse lymphoid tissue, constitute the Gut-Associated Lymphoid Tissues, or GALT.
For more on GALT (or, more generally, MALT for Mucosa-Associated Lymphoid Tissues), consult your histology text (e.g. pp. 134-5 in Stevens & Lowe).
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SIUC / School
of Medicine / Anatomy / David
Last updated: 14 May 2022 / dgk