Epithelial polarity

Epithelial polarity is one example of the cell polarity that is a fundamental feature of many types of cells. Epithelial cells feature distinct 'apical', 'lateral' and 'basal' plasma membrane domains. Epithelial cells connect to one another via their lateral membranes to form epithelial sheets that line cavities and surfaces throughout the animal body. Each plasma membrane domain has a distinct protein composition, giving them distinct properties and allowing directional transport of molecules across the epithelial sheet. How epithelial cells generate and maintain polarity remains unclear, but certain molecules have been found to play a key role. A variety of molecules are located at the apical membrane, but only a few key molecules act as determinants that are required to maintain the identity of the apical membrane and, thus, epithelial polarity. These molecules are the proteins Cdc42, atypical protein kinase C (aPKC), Par6, Par3/Bazooka/ASIP. Crumbs, "Stardust" and protein at tight junctions (PATJ). These molecules appear to form two distinct complexes: an aPKC-Par3-Par6 "aPKC" (or "Par") complex that also interacts with Cdc42; and a Crumbs-Stardust-PATJ "Crumbs" complex. Of these two complexes, the aPKC complex is the most important for epithelial polarity, being required even when the Crumbs complex is not. Crumbs is the only transmembrane protein in this list and the Crumbs complex serves as an apical cue to keep the aPKC complex apical during complex cellular shape changes. In the context of renal tubule physiology, the term basolateral membrane refers to the cell membrane which is oriented away from the lumen of the tubule, whereas the term apical or luminal membrane refers to the cell membrane which is oriented towards the lumen. The principal function of this basolateral membrane is to take up metabolic waste products into the epithelial cell for disposal into the lumen where it is transported out of the body as urine.