PROJECT SUMMARY Cells have a remarkable capacity to self-assemble into organ-like structures in vitro. However, current in vitro-derived organs lack proper size and higher-order patterning, features that require organism-level information not present in a dish. Notably, the collective polarization and unidirectional alignment of cells across a tissue, a phenomenon known as planar cell polarity (PCP), is lacking in vitro organs, yet is essential for proper organ formation and function. Thus, to fulfill the promise of tissue engineering to generate functional organs in vitro we must understand how cells establish long-range collective polarization. Our lab established the murine skin as a model system to investigate the multiscale coordination of PCP in an expansive and regenerative tissue. By developing methods to perform ex vivo culturing, long-term live imaging, biophysical perturbations, and organotypic reconstitution of the epidermis, we have made key discoveries about PCP establishment at the tissue, cellular and molecular scales. The long-term goal of this work is to build on these previous discoveries and technological developments to develop a fully-fledged research program aimed at understanding the angstrom-scale molecular interactions that ultimately build organism-scale tissue organization in the skin epidermis. Successful completion of the aims laid out in this NIAMS STAR award proposal will bring us one step closer to this vision, by defining how oligomeric clustering interactions drive asymmetric partitioning of PCP complexes to establish cellular polarity. Using technical innovations to perform live and super-resolution imaging of endogenously-tagged PCP proteins, and to spatiotemporally control PCP protein clustering with light-sensitive oligomerization modules, this work will provide fundamental new insights into the multiscale coordination of PCP.