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.
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.
<h3>Cell types of the intestinal mucosa and its roles</h3>
- 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.
- 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 secrete hormones to regulate secretion into the GI tract.
- Tuft cells 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.
<h3>What is mucosa?</h3>
The mucosa is the inner layer of any epithelially-lined hollow organ (e.g., mouth, gut, uterus, trachea, bladder, etc.). The mucosa consists of the epithelium itself and also the supporting loose connective tissue, called lamina propria, immediately beneath the epithelium. Deeper connective tissue which supports the mucosa is called the submucosa. In the GI tract (but not in other tubular organs), there is a thin layer of smooth muscle, the muscularis mucosae, at the boundary between mucosa and submucosa.
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