Small Intestine Histology
- The small intestine possesses all of the basic histological layers of the GI Tract (See: GI Tract Histology). The entire small intestine is relatively uniform throughout its length across the duodenum, jejunum, and ileum. The only major differential histology is the presence of Brunners Glands which solely appear in the duodenum.
- The small intestine mucosa is architecturally arranged as a forest of intestinal villi whose bases are termed the Crypts of Lieberkuhn. The surface of the villi and crypts of lieberkuhn are defined by the intestinal epithelium while their flesh is composed of the mucosal lamina propria. The bottom border of the mucosa is defined by the muscularis mucosa which does not extend into the fingers of the villi but rather is arranged as a flat surface.
- Small Intestine Epithelium:
- The small intestinal epithelium is composed of a tall simple columnar epithelium made up of mostly enterocytes which possess extensive subcellular microvilli that substantially increase their resorptive surface area and appear as a fuzzy brush border on microscopy. Occasional goblet cells which secrete protective mucus are also observed within the epithelium as well as intraepithelial lymphocytes which appear to be T-cells as well as B-cells that secrete IgA. The Crypts of Lieberkuhn are densely populated with highly mitotic stem cells that rapidly replenish the population of enterocytes and goblet cells within the villous epithelium.
- Small Intestine Lamina Propria:
- The Lamina Propria is a loose collagenous matrix lying at the center of the intestinal villi which contains a dense capillary network necessary for the rapid transport of absorbed nutrients. Additionally, a single large lymphatic vessel, termed the lacteal, lies at the center of each villous that drains chylomicrons secreted by enterocytes. Finally, throughout the lamina propria of the small intestine lie lymphoid aggregates more commonly known as Peyer's Patches.
- Small Intestine Muscularis Mucosa
- The small intestinal muscularis mucosa is a thin, flat layer of smooth muscle cells which lies just under the bottom edge of lieberkuhn's crypts and forms the border of the small intestinal mucosa.
|Small Intestinal Submucosa|
- The duodenal submucosa is uniquely wide in order to accommodate the mass of Brunners glands whose necks pass through the intestinal mucosa and open into the duodenal lumen. The epithelium of the Brunners glands is continuous with that of the intestinal epithelium and is composed of a simple columnar epithelium that secretes an alkaline mucus critical for protecting the intestinal mucosa from stomach acid.
- Jejunum and Ileum
- The submucosa of the jejunum and Ileum is much thinner than that of the duodenum due to the absence of Brunners glands. It is a largely collagenous layer which conducts vasculature, lymphatics, and nerves.
|Muscularis Propria and Adventitia|
- Muscularis Propria
- The small intestine muscularis propria is composed of the traditional inner circular layer and outer longitudinal layer of smooth muscle cells and actuates propulsion of intestinal contents through peristalsis.
- The small intestine adventitia is a thin layer covered with the serosa of the peritoneum.