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The intestinal epithelium is the cell tissue that lines the large and small intestine. It serves primarily to take part in the digestive process, but it also takes part in the function of the immune system. The epithelium of the intestine is simple columnar, and is nonciliated.
Epithelium is a tissue that is made up of a variety of types of cells. It covers the surface of the body and organs, as well as forming the inner lining of body cavities and hollow organs. All epithelium has an exposed surface, called a free surface or apical surface, and is attached to connective tissue on its underside by a thin layer called a basement membrane. Epithelium cells divide readily, so injuries to the tissue will heal quickly. Intestinal epithelium cells are constantly being damaged and then replaced.
There are many different classifications of epithelium cells. Intestinal epithelium is primarily made up simple columnar epithelium. This type of tissue is a single layer of cells that are elongated and resemble columns. The nuclei of these cells are usually at about the same level as the basement membrane.
The simple columnar epithelium is either ciliated or nonciliated. Cilia are extensions of the cell and are always moving. Intestinal epithelium is nonciliated.
Main functions of simple columnar epithelium are absorption, secretion, and protection. The simple columnar cells secrete digestive fluids and absorb nutrients. These cells are also longer, and that makes the tissue thicker, making it protective.
Simple columnar epithelium cells are particularly specialized for absorption because they have thousands of tiny, cylinder-shaped projections called microvilli. These microvilli increase the surface area of the cell membrane where the cells are exposed, and allow for greater exposure to whatever substance is being absorbed. In the case of intestinal epithelium, this is most important in the small intestine because the majority of the digestive process takes place there. These types of cells are called brush border cells.
Absorbing the remaining water in digested food and excreting the remaining waste is the primary responsibility of the large intestine. Its intestinal epithelium is not as complex or varied as the small intestines. Both the small and large intestine, however, have cells called toll-like receptor 4 (TR4) cells that contribute to the immune system by recognizing pathogens and forming a barrier to stop them.
The intestinal epithelium also contains goblet cells. They are shaped like a flask and are scattered among the other cells. These cells secrete mucus, a protective fluid, onto the free surface of the epithelium tissue.