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Epithelial Cell Polarity

Most cell types are polarized with distinct structural orientations or protein localization patterns that allow cells to perform specialized functions. Epithelial cell polarity is characterized by cells with apical and basolateral membrane domains separated by adherens and tight junctions. The apical and basolateral membranes have unique protein and phospholipid compositions. For instance, apical membrane markers include atypical PKC and other proteins of the Crumbs and PAR complexes, while the basolateral domain contains Scribble, Discs Large, and Lethal Giant Larvae. The basolateral membrane also contains Integrins, which connect the cytoskeleton to extracellular matrix proteins within the basement membrane. Given that many tissues are lined by epithelia with apical cell membranes facing the lumen, polarization allows epithelial cells to transport molecules across the surface in a directional manner. Loss of epithelial cell polarity is associated with cell plasticity, or the ability to differentiate into another cell type. Cell plasticity is a feature of epithelial to mesenchymal transition (EMT), whereby epithelial cells lose apical-basal polarity and cell junctions and acquire a migratory mesenchymal phenotype.