The ability to compartmentalize function has enabled multicellular organisms to evolve sophisticated physiological systems. At the cellular level, compartmentalization of function is established by the maintenance of asymmetric structure and function between the apical and basolateral surfaces of the cell. This asymmetry (polarity) extends on a biochemical level to the membrane proteins of the apical and basolateral plasma membrane. Indeed, many of the polarized functions of epithelia are mediated by membrane proteins enriched or restricted to one particular cellular domain (1 ,2 ).
