Fig. 7. Model for forming the cell rear and front during fibroblast polarization. Prior to cell polarization in these cells, delocalized cell protrusions occur around the entire cell periphery (A,B, blue band). F-actin is enriched in protrusions, as expected. Prior to cell polarization (A,B), within the cell body, actin initially assembles to form non-oriented meshwork/bundles (A, black lines) and subsequently forms oriented actin-filament bundles (B, black lines). Oriented actin-bundle formation triggers localized cell-edge retraction, the first visual break in cell symmetry (C, thin arrow). Cell-edge retraction elaborates around one end of oriented actin-filament bundles (D, jagged edge) to form the cell rear, leaving a stable protrusion (D, blue band) at the opposing end of the bundles. Once cell-edge retraction is elaborated (D,E), net protrusion at the front and cell migration occurs (E). AC (A-E, red dots) localizes to polymerized actin within the cell body early in this process (A) and is appropriately localized, spatially and temporally, to promote formation of oriented actin-filament bundles (B, black lines) prior to cell polarization.