Project Summary The development of epithelial organs involves a complex interplay between epithelial- mesenchymal interactions, cell fate induction, proliferation, and tissue polarity. In epithelial organs, intercellular signaling and mesenchymal interactions specify the spatial patterns of tubes, branches, and other specialized cell types. How these events are coordinated to generate organs with precise and reproducible patterns, dimensions, and sizes remains poorly understood. With recent advancements that allow patient-specific stem cells to be differentiated into organs in a dish, how these forms arise has become increasingly important to resolve. The goal of this proposal is to decipher how intercellular signaling, cell fate specification, and tissue polarity coordinate morphogenesis of developing the skin epithelium. The mammalian skin, which is decorated with spatially patterned, globally aligned hair follicles is an excellent system to explore how polarized architecture is established in multicellular structures. Recently we discovered an unexpected and novel collective cell behavior that directs morphological and cell fate asymmetry in developing hair placodes. The polarization of initially circular hair placodes is driven by dramatic cell rearrangements coordinated in a counter-rotational pattern of cell flows. To generate the pattern of cell flow, spatial patterning of radial cell fates cooperates with planar cell polarity to direct polarized cell neighbor exchanges through myosin-dependent junction disassembly. Using a combination of live imaging, genetic mosaics, biophysical manipulations and transcriptional profiling, the following proposal will test the hypothesis that differences in motility, contractile forces on intercellular junctions and cell-cell adhesion between inner and outer placode cells drive counter-rotational cell movements during placode polarization. Successful completion of these aims will provide an integrated view of how cell signaling, cell fate specification, polarity and adhesion cooperate to generate a novel mode of tissue morphogenesis. We anticipate that counter-rotating cell rearrangements may be in fact be widespread in epithelial organs, polarizing a wide array of branched and tubular structures. Thus, our findings are likely having a broad impact on the fields of regenerative medicine and tissue engineering.