Gastrointestinal System I: Tongue, Esophagus, Stomach, Small and Large Intestine  

Learning objectives:

1. Know how the fundamental organization of the GI tract is modified along its course from the esophagus through large intestine.
2. Know the cell types in the mucosa of the GI tract and their function. 
   
   

The digestive system consists of a long, tortuous tube that begins at the lips and ends at the anus.  During ontogeny, the digestive system initially develops as a closed, endodermal-lined mesenchymal tube, which secondarily gains communication with the external (ectodermal) surface of the embryo by the rupture of the oral and anal membranes.  Thus, in the adult, the digestive system is continuous with the external surface of the body only at the oral and anal openings.

The digestive system is a continuous series of organs that are involved in the intake and propulsion of food, its mechanical and chemical breakdown (digestion), the absorption of a portion of the digested substances, and the elimination of undigested material from the body.  It may be subdivided into: (1) the oral cavity; (2) the pharynx;  (3) the alimentary tract or canal and (4) the anal canal.  In addition to this elongate tubular portion of the digestive system, there are both intrinsic and extrinsic glands derived from the primitive tubular gut; these glands may elaborate digestive enzymes or mucus to facilitate the digestion and transport of foodstuffs.  The intrinsic glands lie within the mucosa or submucosa of their organ of origin, and will be discussed with each of these organs.  The extrinsic glands communicate with their organ of origin by means of excretory ducts.  A useful concept in the study of extrinsic glands is that the site of termination of their excretory ducts denotes their embryological origin.  Thus, the opening of the common bile duct into the duodenum is a reminder that both the liver and the pancreas develop as outgrowths from the foregut (duodenum).  The extrinsic digestive glands are: (1) the major salivary glands - the parotid, sublingual and submandibular (submaxillary) glands; (2) the pancreas; and (3) the liver.  These are the subject of the next laboratory.

The following is the general structural plan that is characteristic for most of the digestive system.  Typically, there are 4 concentric coats; proceeding outward from the lumen, these are: (1) the mucosa  (mucous membrane); (2) the submucosa; (3) the muscularis (muscularis externa); and (4) the adventitia or serosa. (Click here for diagram.)

1. The mucosa has three components: (a) the epithelium and its underlying basement membrane; (b) a thin underlying layer of loose, cellular connective tissue, the lamina propria; and (c) a relatively thin layer of smooth muscle, the muscularis mucosae. The latter may consist of both circular and longitudinally arranged layers.
2. The submucosa is composed of a layer of dense, irregularly arranged connective tissue that contains nervous tissue (the submucosal plexus of Meissner), as well as blood vessels.
3. The muscularis externa consists of at least 2 layers of smooth muscle; an inner circular and outer longitudinal layer.  Connective tissue, separating the muscle layers, contains nerves (myenteric plexus of Auerbach) and blood vessels.
4. The outermost layer or adventitia consists of a thin layer of loose connective tissue; where the digestive system is covered by peritoneum the adventitial layer is called the serosa.

Lab Activities

1. Oral Cavity
   
2. Esophagus
3. Stomach
4. Small Intestine
5. Large Intestine
6. Electron Micrographs
7. Questions
8. Answers