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An in-depth exploration of epithelial structures and functions, including the different types of epithelia, adhesion between cells, the role of the basement membrane, epithelial cell polarity, and cell renewal. the functions of tight junctions and adhering junctions, the importance of the apical and basal surfaces, and the role of stem cells in epithelial tissue.
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Epithelia serve a number of functions. They offer protection to organs such as the skin. The absorb material in the intestine. They secrete material in glands. And they allow for passive diffusion of gases in the lung and blood vessels.
To accomplish these different functions, epithelia come in a variety of structures. Most epithelia are classified based on two criteria: shape and layers of cells. A single layer of cells is called simple whereas a epithelium with two or more layers of cells is called stratified. If the most superficial layer of cells is flat, the epithelium is referred to as squamous. Squamous epithelia facilitate diffusion of gases and other small molecules. Epithelia where the cells are about as tall as they are wide are called cuboidal. If the cells are taller than they are wide, then they are called columnar. Cuboidal and columnar cells are usually involved in secretion and/or absorption and need more cytoplasmic volume to accommodate the organelles needed for these activities. Two special types of epithelia are pseudostratified and transitional.
Epithelial cells are held together by three junctional complexes. All epithelia will have adhering junctions, but only some will have desmosomes. Epithelia also contain tight junctions which control the diffusion of material between epithelia cells.
Adhering junctions have characteristic arrangement in epithelial cells. They form continuous belt around the circumference of epithelial cells. The adherins junction are linked to bundles of actin filaments that also wrap around the cell. Myosin filaments can pull on the actin filaments to contract the cell. Because the adherins junctions are located closer to the apical surface, contraction causes the apical surface to shrink. This allows epithelia to form a tube from a sheet of cells.
Lumen Epithelia usually separate two compartments. Often one compartment is an external space or lumen of a tube and the other is the rest of a tissue or organ. The apical side of the epithelial cells faces the external space or lumen and the basal side faces the rest of the organ. One role for epithelia is to control the mixing of material between the two compartments. To accomplish this, epithelia must control the ability of small molecules and ions to diffuse between cells or paracellular diffusion. Tight junctions that link adjacent epithelial cells determine how permeable an epithelia is to ions and small molecules. Some epithelia are very permeable (e.g. intestine) where as others are restrictive (e.g. bladder).
Cell 1 Cell 2 Tight junctions contain more than 50 different types of proteins. Claudins are the proteins that primarily determine the permeability of the tight junction. Claudins contain four transmembrane domains and interact with cluadins in adjacent cells. The intracellular domains of claudins are linked indirectly to actin filaments by a set of proteins called ZO proteins. The interaction with actin filaments help stabilize the claudins in the cell membrane. Also, myosin filaments can generate tension on the actin filaments to loosen the tight junctions.
Tight junctions contain more than 50 different types of proteins. Claudins are the proteins that primarily determine the permeability of the tight junction. Claudins contain four transmembrane domains and interact with cluadins in adjacent cells. The intracellular domains of claudins are linked indirectly to actin filaments by a set of proteins called ZO proteins. The interaction with actin filaments help stabilize the claudins in the cell membrane. Also, myosin filaments can generate tension on the actin filaments to loosen the tight junctions.
The extracellular domains of claudins interact to form size-restrictive pores. The permeability of an ion or molecule is inversely related to size.
claudin- claudin- claudin- claudin-
The permeability of epithelia varies between organs up to 10,000 fold difference. The epithelia of the small intestine is more permeable that the epithelia of the bladder. There are 24 different claudin genes and each encodes a protein with different permeability properties. Some claudins are selective for certain ions. For example, mutations in claudin 16 lead to magnesium wasting because the kidney epithelia becomes permeable to magnesium. Epithelia generate different degrees of permeability by expressing different combinations of claudin genes.
All epithelial cells are attached on their basal surface to a basement membrane. The basement membrane provides some mechanical support as it tethers together a sheet of epithelial cells. It also supports the growth and survival of the epithelia as it controls the access of epithelia to nutrients, ions, proteins and oxygen. Epithelia lack their own blood supply and rely on the capillaries in the underlying tissues. All the nutrients from the blood must cross the basement membrane to reach the epithelial cells. The basement membrane also regulates the growth and division of epithelial cells.