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To what extent is cell polarity an essential and unique component of epithelial cells?
Cell polarity means that the shape, functional areas and organelle distribution of a cell are asymmetric. Polarity of epithelial cells is due to the fact that they have at least two specialised membrane surfaces - apical and basal - and that the distribution of organelles and macromolecules is asymmetrical with regard to the two different surfaces. The main function of epithelial cells is to line tracts, vessels and ducts, and in doing so provide protection, structure, insulation, selective permeability, absorptive and secretory functions and a barrier between different compartments of the body. Cell polarity is an inherent and universal property of epithelia and arises from variation along their apicobasal axis, although some cells show planar polarity as well. This is important to the cell in several ways. The first is that it allows the correct positioning of epithelia relative to its function in the parts of the body where it is found. Secondly, the different membranes allow for selective and specialised absorption and secretion of substances. In addition, polarity is often vital to cell function, such as receiving signals and transporting substances along tracts. The difference in membranes also allows for different proteins to be found on each surface, which has a role in cell function due to receptors and enzymes being expressed on different surfaces according to where they are needed. Cell polarity is also present in other cell types such as neurons, fibroblasts, white blood cells and unicellular organisms, where it can enable such vital roles as electrochemical signalling and motility. Cell polarity is controlled tightly during development by many growth factors, the mechanisms of which are not yet well understood. It is maintained by the existence of a cytoskeleton within the cell. This is mainly composed of a network of actin filaments and is not fixed, but somewhat flexible: it is able to change conformation in response to stimuli. In addition, sorting mechanisms exist to maintain the distribution of proteins on the apical and basal surfaces. Once proteins are synthesised in the Golgi body, it is believed that sorting occurs in the trans-Golgi network; vesicles containing substances are then transported to the apical or basal surface, possibly along cytoskeletal tracks such as the motor myosin IIA. Transcytosis also occurs to correctly localise proteins from one membrane to another. Due to the fluid nature of the lipid bilayers, components such as embedded proteins could disperse over the membrane, and so to maintain polarity there are junction complexes beneath the apical membranes which, in addition to holding the cells together, form a physical barrier between the two membranes all the way around the cell. These are primarily tight junctions, but there are also gap junctions, an adhesive layer named the zonula adherens, septate junctions and desmosomes. Planar cell polarity refers to an innate ability of cells to give rise to processes which all angle in the same plane with respect to the sheet of epithelium. For example, asymmetrical projections on the wing cells of Drosophila all angle in the same direction; this reduces friction in flight. Epithelia in the vertebrate ear have specialised hair-like protrusions called stereocilia which are electrically sensitive to the angles at which they are tilted. These are all aligned in the same direction; if they were not, they would be useless for detecting balance. Epithelium is found lining the alimentary canal, gland ducts (e.g. mammary and salivary), respiratory and genito-urinary tracts and in many organs of the body. The key to the positioning of epithelia in the body is the polarity of the outer membranes of the cell. The apical membrane is that which faces the lumen or surrounding medium, whereas the basal membrane adjoins the basement membrane and adjacent cells, and may sit upon connective tissue that is termed the lamina propria. Interactions between cells based upon the relative orientations of these membranes (probably based on marker molecules on the membranes) can be vital to the arrangement of tissues: cultured epithelial cells have been shown to
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