Figure 2. The apical junctional complex and calcium-switch model of junction assembly. (A) The tight junction and adherens junction, and their associated actin cytoskeleton, comprise the apical junctional complex. The tight junction strands form a semi-permeable barrier, and ZO-1 functions as a tight junction scaffolding protein. At the adherens junction, the cadherin-catenin complex contributes to cell adhesion. The apical junctional complex separates the apical and basolateral domains. (B) In the calcium-switch model, junctions are disassembled by removing calcium (left, low Ca²⁺). Upon calcium re-addition (center), the junctions begin to assemble, and radial actin cables are observed at early cell-cell contacts. Cells are fully polarized in a mature monolayer (right).