Nutritional Processes: Gastrointestinal Tract
The gastrointestinal tract (GI) is bordered by a layer of epithelial cells (with glands) sitting on a lamina propria (or basement membrane), comprising the mucosa and adjacent to the submucosa, which is penetrated by blood capillaries, lymphatics and nerves. Beneath the mucosa and submucosa are two layers of smooth muscle, lying in longitudinal and transverse directions, to allow contractions and peristalsis, Within the stomach , but particularly in the small intestine, the surface area of the mucosa is greatly increased. The mucosal and submucosal layer is folded into microscopic villi on the surface of larger folds or ridges. At the bases of the villi are the “crypts” where new epithelial cells are formed that migrate upward to the villi. These cells are sloughed off at a fairly rapid rate; the lifespan of villus cells in the small intestine is as little as 2 - 3 days (in man), that of colonic cells 3 – 8 days). Cells in the crypts include those with glandular and mucous-secreting functions, where as those in the villi are largely absorptive. Glandular cells are important in signaling the initiation and coordination of digestive processes, involving a large number of hormones neurotransmitters and paracrine factors. Mucous provided by “goblet” cells promotes lubrication within the lumen of the GI tract. In the small intestine, crypt cells are also the source of some digestive juices.
The epithelia cells of the mucosa have an apical (lumen –oriented) surface that is often additionally invaginated to form microvilli (or a brush border). In the small intestine the brush border contains transporter and some digestive enzymes. It is also more rigid than other parts of the cells membrane, a fact now attributed to high concentrations of sphingolipid in the outer half of the lipid bilayer. Surface cells are held together by tight junctions near the apical (top) parts of the cells. At the opposites (serosal) end, the cells membrane has a different (less rigid) structure (high in phosphatidyl-choline) and also serves different functions. Nutrients entering the blood or lymph for distribution to body tissues must first cross the brush border and ultimately the serosal surface of these cells to enter the intestinal fluid. Transport across either or both of these surfaces may be independently and/or differentially controlled, depending upon the nutrient. For there, capillaries and lymphatics take nutrients to the rest of the body. Nutrients not making it across the serosal membranes will remain with the mucosal cells until they are sloughed off, from whence they may be released by digestion and resorbed or lost with cell debris and bacteria in feces.
Nutritional Processes: Gastrointestinal Tract
