Polarized traffic towards the cell surface: how to find the route.

Polarity is the structural and functional hallmark of epithelia. The apical plasma membrane, facing the organism's exterior (the lumen of the gut, renal tubule and glandular duct), differs in many important respects from the basolateral plasma membrane that is apposed to the interior of the organism. The generation and maintenance of epithelial polarity require a highly specialized subcellular machinery to bring proteins to their appropriate sites of action. This is a dynamic process involving the interpretation of sorting signals, vectorial delivery mechanisms, membrane-specific fusion and retention processes. Here, we will provide a review of the field, highlighting recent advances within a historically relevant context.