Histology of the GI Tract

Three segments come out of this embryonic gut, called the foregut, the midgut, and the hindgut. These develop a blood supply and parts of it will expand to form wider structures, like the colon and the stomach. The foregut represents the esophagus, the stomach, the first and second parts of the duodenum, the gallbladder, the liver, and the pancreas. The midgut begins in the lower duodenum and extends down to the first two-thirds of the transverse colon. It is basically defined as being that part of the intestinal tract that is supplied by the superior mesenteric artery. The hindgut starts at the last third of the transverse colon and ends at the anal canal. It is supplied exclusively by the inferior mesenteric artery.

The gastrointestinal tract has a relatively consistent pattern of having four major layers to its lining. The inner layer is the mucosa, which is the part that has contact with the lumen of the gut. This layer comes in direct contact with the chyme as it is passing through the gut. There are three layers to the mucosa: the epithelium (inside layer that is mainly absorptive and secretory), the lamina propria (which is a connective tissue layer), and the muscularis mucosa (the smooth muscle layer).

Figure 3 shows the different layers of the GI tract:

Figure 3 The submucosa is just outside the mucosa. It consists of blood vessels, connective tissue, nerves, and lymphatic vessels that supply the other layers of the GI tract lining. It contains an enteric nerve plexus and a submucosal plexus, located just inside the muscular layer. The muscular layer is outside of the