Abstract
During epithelial morphogenesis, cell polarity aligns individual cell behaviors into collective motions that shape developing tissues. Here, we combine experiments with computational modeling to investigate how cell-scale forces oriented by Planar Cell Polarity (PCP) direct the collective, counter-rotational cell flows that occur during hair placode morphogenesis. We rule out that PCP directs apical neighbor exchanges, as junctional myosin and PCP protein localization are not co-correlated with junction shrinkage. Instead, we find that PCP directs anterior-directed crawling of placode cells along the basal surface of the tissue through a mechanism that requires cell crawling regulator Rac1. Modeling the placode as a continuum viscoelastic fluid, we find that active forces from cell crawling at the basal surface is sufficient to generate the experimentally observed counter-rotational cell motion at the apical surface. Our results show an unexpected role for PCP in epithelial morphogenesis, centering the basal surface as the site of force generation.