o Undamaged epithelia form effective barriers because of intercellular connections that involve large area of opposing plasma membranes or specialized attachment sites. o Cell Junctions Tight junction - lipid layers of adjacent plasma membranes are tightly bounded together by interlocking membrane proteins Prevent passage of water and solutes between cells Common between epithelial cells exposed to harsh chemicals or powerful enzymes Gap junction - two cells are held together by embedded membrane proteins called connexons
3 Chapter 4 : The Tissue Level of Organization Connexons form a narrow passageway the lets small molecules and ions pass cell to cell. Most abundant in cardiac muscle and smooth muscle tissue, where they are essential to coordination of muscle contractions o Desmosome , the plasma membrane of two cells are locked together by CAMS and proteoglycans between the opposite dense areas of each cell. Desmosomes that form a small disc are called spot desmosomes Hemidesmosomes resemble half of a spot desmosome and attach a cell to the basement membrane Abundant cells in superficial layers of the skin The Epithelial Surface o Apical surface is exposed to an internal or external environment o Many epithelia that line internal passageways have microvilli on exposed surfaces Abundant on epithelial surfaces where absorption and secretion take place, such as portions of the digestive system and kidneys Microvilli causes larger surface area of a cell more transport proteins are exposed to extracellular environment The Basement Membrane – lies between epithelium and underlying connective tissues o Firmly connect to rest of body o Consists of a network of protein fibers o Epithelial cells are attached to basement membranes by hemidesmosomes o Restricts proteins and other large molecules from moving from underlying connective tissue into epithelium Epithelial Renewal- stem cells (unspecialized cells or germinative cells) which are found in deepest layer of epithelium near basement membrane; needed to maintain epithelium structure 4-3 Describe the relationship between form and function for each type of epithelium.
4 Chapter 4 : The Tissue Level of Organization Simple epithelium consists of a single layer of cells covering the basement membrane o they are thin, fragile, and cannot provide much mechanical protection and are found only in protected areas inside body ex. they line internal compartments and passageways, including the pleural, pericardial, and peritoneal cavities, the heart chambers, and blood vessels o characteristic of regions where secretion or absorption occurs ex. lining of intestines and gas-exchange surfaces of lungs thinness is an advantage where it reduces the time for materials to cross the epithelial barrier Stratified epithelium provides greater protection because it has several layers of cells above the basement membrane o Found in areas exposed to mechanical or chemical stresses Ex. surface of skin and lining of mouth
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- Winter '08