1. In epithelia, the differentiation of apical and basal specializations. The apical and basolateral regions of the plasma membrane may differ in lipid and protein composition, and tight junctions prevent exchange of membrane molecules from one domain to the other. The apical membrane may have active transport functions or may be the only site for exocytosis.
2. In motile cells the internal polarity that defines ‘front’ and ‘back’ and allows locomotion. The polarity probably derives from the microtubular cytoskeleton associated with the microtubule organizing centre, and is disrupted by drugs such as colchicine.