The wall of the human alimentary canal consists of four distinct layers: the mucosa, sub-mucosa, muscularis and serosa.
Fig: Transverse section of gut (diagrammatic representation)
Mucosa – The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen (open space within the tube). This layer comes in direct contact with digested food. This layer forms rugae in the stomach and villi in the small intestine. The mucosa is made up of epithelium – innermost layer, responsible for most digestive, absorptive and secretory processes. Mucosa also contains goblet cells which produces mucus that protects the epithelial surface.
Sub-mucosa – The sub-mucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymph vessels and nerves. In duodenum, glands are also present in it. Sub-mucosa supports the mucosa.
Muscularis/Muscular layer – Muscularis is a thin layer of smooth muscles arranged into an outer longitudinal layer and an inner circular layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The coordinated contraction of these layers is called peristalsis (alternate contraction and relaxation, which pushes ingested food). At a number of points along the gut the circular muscle thickens into structures called sphincters.
Serosa – Serosa is the outermost layer of the human alimentary canal. It is made up of a thin layer of secretory epithelial cells, with some connective tissues underneath. The epithelial layer, produce the lubricating serous fluid. This fluid has a consistency similar to thin mucus. These cells are bound tightly to the underlying connective tissue. The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells, and also serves as the binding layer which allows the serosa to adhere to organs and other structures.