Chapter 14. Histology of the Esophagus and Gastrointestinal Tract
The human alimentary canal is a true and complete digestive tract. A true digestive tract is defined as one in which digestion is completely extracellular, occurring entirely within the tract's lumen or on the surface of its lining. A complete digestive tract is defined as one that is a tubular, compartmentalized organ, with a lumen that lies outside of the body and with an entrance (a mouth) that is separate from its exit (the anus).
The human alimentary canal has fiver basic compartments, each of which is usually considered an organ: the esophagus, the stomach, the small intestine, the large intestine, and the rectum.
General Histology of the Alimentary Canal
The wall of the alimentary canal has the same basic structure throughout its length. It consists of four basic layers. Listed from the lining epithelium to the covering tissue, they are 1) the mucosa, 2) submucosa, 3) muscularis externa, and 4) the serosa or adventita.
The mucosa contains three sublayers:
- the lining epithelium,
- mucosa glands in most parts of the alimentary canal
- the lamina propria, which consists of a loose connective tissue supporting the epithelium, and a
- muscularis mucosa, which is a double-layer of smooth muscle surrounding the lamina propria. The inner layer is a layer of circularly-arranged smooth muscle cells, and the outer layer is a layer of longitudinally-arranged smooth muscle cells.
The submucosa is a relatively thick layer of dense irregular connective tissue. It not only binds and supports the other layers of the canal wall, but also supports vascular, lymphatic, and nerve supplies of the wall.
The muscularis externa consists of multiple layers of muscle. Contractions of the muscularis externae provide the movements that mix the luminal contents of the canal and move the contents along its length. Peristalsis, the common waves of contraction that move lumenal contents, come from the contractions of the muscularis externa.
The serosa is the outermost tissue layer of those portions of the alimentary canal that are suspended in the peritoneal cavity by mesentery. It is the visceral peritoneum of gross anatomy. It is continuous with mesentery and consists of a thin layer of loose connective tissue covered by mesothelium. The loose connective tissue layer supports large blood vessels, lymphatic vessels, and contains variable numbers of adipocytes.
Regions of the alimentary canal that are not suspended by mesentery, but rather are attached directly to the body wall, are surrounded by an adventitia rather than a serosa. These regions include the esophagus, which is actually physically attached to the aorta, the duodenum, the ascending colon, and the descending colon. The adventitia is a thicker layer of loose connective tissue that is continuous with the loose connective tissue of other structures.
The wall of the alimentary inside the muscularis externa is generally folded. Folds of the mucosa, the so-called mucosal folds, can be identified by their core of lamina propria. Submucosal folds can be identified by their core of submucosa.
The wall of the alimentary canal also contains a number of different exocrine glands that deliver substances to the lumen or the lining epithelium of the alimentary canal. These glands are classified according to their position in the wall. Mucosal glands are those that extend into and are supported by the lamina propria. Submucosal glands extend into and are supported by the submucosa.