Figure 4.

Transition between front–rear and apical–basal polarity. (A) Transition from migratory cells to epithelial structures. During development, migratory (mesenchymal) cells coalesce into cell aggregates through cell–cell adhesion, and following induction of the epithelial program, develop into epithelial structures (e.g., a tube). (B) Organization of signaling pathways at nascent cell–cell contacts. Migratory cells interact with each other through their leading edge (front). Cell–cell adhesion is induced by E-cadherin, and additional adhesion proteins (nectin, JAM-A). These adhesion proteins have direct (solid line) and indirect (dotted line) interactions with protein complexes important in apical–basal polarity, including the PAR complex (JAM-A) and the exocyst (directed exocytosis, E-cadherin/nectin); Scribble and LGL are also localized at the leading edge and play critical roles in regulating expocytosis through binding the t-SNARE syntaxin4. Microtubules are oriented toward the front of migrating cells through the localization of the APC (and DLG) complex at the leading edge; APC also plays a role in localizing microtubules to the AJC in the apical–basal axis in polarized epithelial cells.