Intestinal epithelium surface

The surface epithelial cells of the small intestine are renewed rapidly and regularly. It takes about two days for the cells of the duodenum to be renewed completely. As a result of its rapid renewal rate, the intestinal epithelium is susceptible to various factors that may influence proliferation. Exposure of the intestine to ionizing radiation and cytotoxic drugs (such as folic acid antagonists and colchicine) reduces the cell renewal rate. 

The Cryptosporidium parasite attaches to the host s intestinal epithelium, becomes intracellular but remains extracytoplasmic. In vitro studies suggest that attachment is mediated by a Cryptosporidium parvum sporozoite ligand and an intestinal epithelial cell surface protein interaction [83, 84],... 

Thus, the apparent membrane permeability characteristics of hydrophilic compounds listed in Table 3.4 indicate that colonic epithelium is different from small intestinal epithelium in selectivity, or size or density distribution of the paracellular pathway. This is further complicated because of the possible involvement of unidentified carriers or channels for some compounds, as suggested for glycerol and D-xylose. However, the colon-to-SI ratios of the apparent membrane permeability are generally comparable with (or lower than) those calculated considering the morphological surface area, suggesting that such factors are not in favor for colonic absorption in most cases. Matching... 

Carbohydrates mainly occur in food in the form of polymers (starches and glycogen). They are cleaved by pancreatic amylase into oligosaccharides and are then hydrolyzed by glycosidases, which are located on the surface of the intestinal epithelium, to yield monosaccharides. Glucose and galactose are taken up into the enterocytes by secondary active cotransport with Na"" ions (see p. 220). In addition, monosaccharides also have passive transport systems in the intestine. 

Obver, R.E., Jones, A.E. and Rowland, M. (1998) What surface of the intestinal epithelium is effectively available to permeating drugs Journal of Pharmaceutical Sciences, 87, 634—639. 

It has long been established that all cell membranes in the body are composed of a fundamental structure called plasma membrane. This boundary surrounds single cells such as epithelial cells. More complex membranes such as intestinal epithelium and skin, are composed of multiples of this fundamental structure, which has been visualized as a bimolecular layer of lipid molecules with a monolayer of protein adsorbed into each surface. Cell membranes are further interspersed with small pores that can be protein line channels through the lipid layer or, simply, spaces between the lipid molecules. In membranes composed of many cells, the spaces between the cells constimte another kind of membrane pores (2). 

Fig. 2.2. Structural analogy between the tegumental cells of cestodes (right) and the mammalian intestinal epithelium (left) both of which show surface amplification (see text).

Fig. 5.1 Pathogenesis of Salmonella Typhimurium and Salmonella Typhi. Upon ingestion, Salmonella travel to the small intestine. The bacteria invade Microfold (M) cells and other intestinal epithelial cells through the apical surface to cause infection. (A). Salmonella Typhimurium remains localized in the small intestine and induces an inflammatory host immune response, resulting in bacterial clearance from immunocompetent individuals. (B). Salmonella Typhi escapes from intestinal epithelial cells at the basolateral surface of the intestinal epithelium, enters phagocytes, and evades the host innate immune response, resulting in systemic infection...

The slope of the curve corresponds to 7 A. In most experimental setups, the surface area A is known and flux J can be derived. Constant flux is maintained as long as C/j is much larger than Ca and as long as both are practically unchanged. In biologic systems, a good example for such conditions may be the uptake of xenobiotics from the Gl-tract, where the GI tract acts as a reservoir for the molecules, which are transported through the intestinal epithelium into the blood, which acts as a sink. 

MONOAMINE-OXIDASE-INHIBITORS(MAOIs) acton monoamine-oxidase (MAO) enzymes that are involved in the degradation of monoamines in the peripheral and central nervous system. Monoamine oxidase occurs within cells bound to the surface of the mitochondria. It is found not only within monoaminergic neurons, but also in the liver and intestinal epithelium. The enzyme converts amines to their corresponding aldehydes, which in the periphery are converted to their carboxylic acids by aldel e dehydrogenase. Neurotransmitters degraded by monoamine oxidase include dopamine. 5-hydnngrtryptamine and noradrenaline. 

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