Fig. 9. Summary of changes to the microfilament network triggered by formation of cadherin-dependent cell-cell contact. (A) Mechanisms of formation and functions of junctional actin and thin bundles. After cadherin receptors cluster, two distinct processes are observed: new actin polymerization and filament reorganization. Actin assembly is a major contributor to junctional actin (thick brown arrow), but has a minor effect on thin bundles (dashed thin brown arrow). In contrast, myosin function is essential for bundle stability, but does not affect initial clustering of cadherin receptors or junctional-actin formation. All the above events do not occur in cells maintained in low calcium medium or in which cadherin function is blocked. Two separate functions are identified (red arrows): junctional actin stabilizes cadherin receptors at puncta and thin bundles are essential for development of maximum height of the lateral domains. At later time points, these two actin populations apparently colocalize at cell-cell contacts to form mature junctional actin (merged blue and yellow lines; see below). ^*Adams et al., 1996; Nelson, 2003. (B) Temporal and spatial events. After initiation of cell-cell adhesion, distinct cellular processes are detected at different time points (horizontal bars). Once initiated, each event increases until a plateau after 60 minutes of cell-cell adhesion. E-cadherin clustering at puncta is observed within a couple of minutes, and is followed shortly by new actin incorporation as junctional actin ( minutes, fast dynamics) and then by thin bundle labelling (approximately minutes; slower dynamics). Increased P-MLC localization at bundles and bundle reorganization occurs later (from 15 minutes). The end result is the formation of a cuboidal epithelial morphology, with the spatial organization of microfilaments as shown. A possible mechanism, supported by our results, is myosin-dependent contraction of thin bundles from a loose band of filaments at the periphery to filaments that are coincident with junctional actin. Please see text for functional implications and more details. Arrows indicate bundles; arrowheads indicate junctional actin, open arrow indicates mature junctional actin.