PROJECT SUMMARY Mesenchymal cells are major components of tissue stroma and best known for their role in supporting the physical structure of organs. They juxtapose to and communicate with epithelial cells, to control organ development in embryos, and to regulate tissue inflammation, injury repair and cancer progression in adults. Traditionally, mesenchymal cells in different organs were thought to perform similar functions. However, recent studies have revealed unexpected diversity of gene expression signatures among mesenchymal cells from different organs. Such diversity is likely to play a crucial role in regulating mesenchymal-epithelial interactions and thus organ development and physiology. The overarching goal of this proposal is to understand how epithelial-mesenchymal interactions control organogenesis across different organs. To achieve this goal, we will first address the question of how mesenchymal diversity arises as the consequence of distinct signaling histories. We will develop a novel high-throughput system to simultaneously test tens of thousands of signal combinations. We will then investigate how epithelial-mesenchymal interactions control tissue morphogenesis, by reconstituting and genetically perturbing such interactions ex vivo. We are especially interested in comparing the specificity and plasticity of epithelial-mesenchymal interactions across different organs, and identifying the role of extracellular matrices that are shared by epithelial and mesenchymal cells in controlling morphogenesis. We will closely integrate experimental measurements with computational models to produce a quantitative framework for understanding organ morphogenesis. By integrating tools and materials from diverse fields, we will gain a fundamental understanding about the developmental origin and functional implications of the long-overlooked mesenchymal cells. These insights and tools will enable the engineering of organoids to better model development and diseases, and will advance the field of tissue engineering.