Digestive System: GIT
The Alimentary Canal has the same basic construction end to end. There are four distinct layers:
- Mucosa, which is made of epithelium, lamina propria, and muscularis mucosae
- The epithelium secretes digestive enzymes (found in the apical [free] plasma membrane), hormones, mucous, and antibodies (which it receives from connective tissue [diffuse lymphatic tissue]). The epithelium absorbs products of digestion and transports them to the vascular system. Absorption occurs in the small and large intestines.
- The lamina propria is made of areolar (loose) connective tissue under the epithelium. It contains glands, vessels to receive absorbed substances (fenestrated type), numerous lymphatic capillaries in the small intestine to receive lipids and some proteins, and components of the immune system (i.e. diffuse lymphatic tissue and lymphatic nodules that make up gut-associated lymphatic tissue [GALT] and Peyer’s patches).
- The muscularis mucosae has layers of smooth muscles that form a boundary between the mucosa and the submucosa. It consists of two layers, which are the inner circular and outer longitudinal that can produce movement of the mucosa independent of movement of the entire gut wall.
- Submucosa, which is made of dense irregular connective tissue, lymphatics, and larger blood vessels that send branches to the mucosa, muscularis externa, and serosa. It also contains nerves plexuses (aka submucosal [meissner’s] plexus), which are nerve networks that contain cell bodies (ganglion cells) of postganglionic neurons. They represent the third (enteric) division of the autonomic nervous system, which innervates the smooth muscle of the alimentary canal. Glands occur in the submucosa of the esophagus and initial part of the duodenum. Presence of these glands aids in identification of particular regions of gut.
- Muscularis (externa), which usually consists of two concentric thick layers of smooth muscle. The inner layer forms tight spiral seens as a circular layer. The porter loose spiral is described as a longitudinal layer. Located between the two muscle layers is a thin connective tissue layer that contains the myenteric plexus (aka auerbach’s plexus), part of the enteric division of the autonomic NS. It also contains blood vessels and lymphatic vessels.
- Serosa/adventitia. The serosa is a membrane of simple squamous epithelium (the mesothelium) and underlying connective tissue (equivalent to visceral peritoneum). It is continuous with the mesentery, which holds the digestive tract in place, and contains large blood and lymphatic vessels that travel to and from the mesentery to the gut. Large amounts of fat can accumulate in the serosa. The adventitia is found where the wall of the gut is attached to a structure. These are areas where the gut has no serosa (such as the esophagus of the neck region, duodenum, ascending colon, and descending colon). The adventitia blends with connective tissue of the surrounding structures.