Intestinal epithelium cells, proliferation

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 body s cells are normally subject to strict social control. They only divide until they come into contact with neighboring cells cell division then ceases due to contact inhibition. Exceptions to this rule include embryonic cells, cells of the intestinal epithelium (where the cells are constantly being replaced), cells in the bone marrow (where formation of blood cells takes place), and tumor cells. Uncontrolled cell proliferation is an important indicator of the presence of a tumor. While normal cells in cell culture only divide 20-60 times, tumor cells are potentially immortal and are not subject to contact inhibition. 

The intrinsic barrier of the gastrointestinal epithelium is characterized by intercellular junctions at the apical (luminal) side of differentiated epithelial cells, the so-called tight junctions (TJ), and the maintenance of epithelial integrity based on the balance between cellular proliferation and cell death, as described above. Barriers against drug absorption by the intracellular and paracellular routes, their modulation, and maintenance will be discussed in the following, with the focus on the intestinal epithelium. 

Several mechanisms have been proposed for the protective action of the dietary hber found in whole grains. Increased fecal bulk and decreased transit time allow less opportunity for fecal mutagens to interact with the intestinal epithelium. Secondary bile acids are thought to promote cell proliferation, thus allowing increased opportunity for mutations to occur and abnormal cells to multiply. The effect of fiber on the actions of bile adds may be attributable to the binding or diluting of bile adds. 

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