Atlas of plant and animal histology

As the other regions of the gut, the small intestine consists of four layers: the mucosa, submucosa, muscularis, and adventitia.

Mucosa

TThe mucosa consists of three layers: the epithelium, lamina propria, and muscularis mucosae. The mucosa has structures to increase the inner surface of the tube, thereby increasing the overall absortive surface. In humans, there are long folds that are arranged circularly or helically known as circular folds or valves of Kerckring. These folds involve the entire mucosa and are found in the duodenum and jejunum, but they disappear towards the end of the ileum.

The surface of the small intestine is formed by many expansions of the mucosa, showing glove-finger-like shapes and range from 0.5 to 2 mm in length. These expansions are known as intestinal villi. Each expansion (villus) is made up of simple columnar epithelium on the surface, supported by loose connective tissue under the epithelium, forming the lamina propria of the mucosa. The lamina propria contains many immune cells, some smooth muscle cells, and fibroblasts. There is also a dense network of blood vessels with fenestrated endothelium (with gaps in the endothelial cells) in the interior of each expansion. The blood network allows an efficient removal of absorbed substances by the epithelium. A network of lymphatic vessels is also found in the villi.

At the base of the villi, the excretory openings of the intestinal glands are found. These glands are known as crypts of Lieberkühn. Their secretory parts of these glands are located within the muscularis mucose, and their excretory ducts run through the lamina propria, Finally, these ducts connect with the intestinal epithelium.

As mentioned above, the connective tissue forms the lamina propria, which is found in the interior of the villi as well as at their base, surrounding the intestinal glands. The lamina propria contains many immune cells, which may be either scattered or grouped into nodules. In the ileum region, the nodules are found clustered together, forming the intestinal Peyer's patches..

The muscularis mucosae consists of two layers of smooth muscle cells: an inner layer with circularly arranged smooth muscle cells and an outer layer with smooth cells arranged longitudinally.

The intestinal epithelium is responsible for nutrient and water absorption, a function primarily carried out by enterocytes. Other cell types present in the intestinal epithelium are goblet cells, Paneth's cells, enteroendocrine cells, and M cells.

Submucosa

The submucosa is a layer of dense connective tissue, containing some scattered adipocytes. In the duodenum, the duodenal glands, or glands of Brunner, are specialized in releasing an alkaline secretion to neutralize the very low pH of the partially digested food coming from the stomach.

Muscularis propria

This layer is made up of two sublayers of smooth muscle: one inner layer oriented circularly and an outer layer oriented longitudinally. Between the two sublayers lies a plexus of nerve terminals called the Auerbach's plexus (or myenteric plexus). Each muscle sublayer performs a different type of contraction with a particular role. The inner one moves the digested food both forward and backward to mix properly with digestive substances and to renew the liquid in contact with the surface of the epithelium to enhance absorption. The outer longitudinal layer contracts the gut tube and moves the digested material toward the large intestine.

Adventitia

The adventitia consists of connective tissue that forms the outer layer of the small intestine. In some regions, it is covered by the peritoneum.