Emergence of tissue shape changes from collective cell behaviours.

Anyone watching a movie of embryonic development immediately appreciates the importance of morphogenetic movements and cell flows that reshape tissue. Dynamic tissue shape changes are genetically choreographed, but their execution is essentially a mechanical event. How the interplay between genetics and tissue mechanics controls tissue shape is a fundamental question. Key insights into this problem have emerged from studies in different model organisms as well as in cultured epithelia. These studies have revealed how gene expression patterns can generate patterns of planar cell polarity that orient cellular force generation and give rise to anisotropic mechanical properties of cells and tissues. These can autonomously bias the rate and orientation of cellular events such as cell divisions, extrusions, neighbor exchanges and shape changes that drive morphogenesis. However recent studies also highlight how autonomously controlled cell dynamics lead to tissue-wide stress patterns framed by mechanical constraints such as cellular connections to extracellular matrices. These stress patterns themselves can orient the cell behaviours underlying morphogenesis. As a result of this interplay, tissue shape emerges in a mechanical process that tightly couples mechanics and genetics.