Organization of polarized epithelial cells and the apical junctional complex. a, Polarized epithelial cells form a monolayer in which the apical (unbounded surface) is separated at the boundary with the basal and lateral membranes (bounded surfaces) by the apical junctional complex (top). The main part of the panel shows molecular organization of the apical junctional complex. In vertebrates, the apical junctional complex is separated into structurally and functionally different sub-domains comprising membrane proteins (Crumbs, JAM (junctional adhesion molecule), nectin, occludin/claudin and cadherin) linked to modular protein scaffolds, which in turn bind mostly to the actin cytoskeleton, although links to microtubules are possible. In invertebrates (C. elegans and Drosophila), the apical junctional complex is similarly organized, except that the ‘tight junction’ function is provided by the septate junction localized below the cadherin (adherens) junction. b, Simplified scheme for how different protein complexes in the apical junctional complex regulate cell–cell adhesion (cadherin complex), and apical membrane (Bazooka and Crumbs complexes) and lateral membrane (Lethal giant larvae, Scribble and Disc large complex) identity. For details see text.