Histology of the Alimentary Canal

From the oesophagus down to the rectum, the wall of the alimentary canal is built, from the innermost to the outermost layer, of four basic tissue layers — mucosa, submucosa, muscularis and serosa — though the exact structure of each layer is modified in different regions to suit the organ's function.

• Mucosa: the innermost layer, lining the lumen (cavity) of the canal. Throughout its length it contains goblet cells, which secrete mucus that lubricates the lumen. Its structure changes regionally: in the stomach it is thrown into irregular folds called rugae, and forms gastric glands that secrete gastric juice; in the small intestine it is raised into finger-like foldings called villi, lined by an absorptive brush border of epithelial cells bearing microvilli on their free surface. Each villus is supplied with a network of blood capillaries and a central lymph vessel called a lacteal. Between the bases of neighbouring villi, the mucosa dips down to form crypts of Lieberkuhn — intestinal glands.
• Submucosa: a layer of loose connective tissue carrying blood vessels, lymph vessels and nerves; in the duodenum specifically, the submucosa additionally contains glands (Brunner's glands). …