Supracellular actomyosin assemblies: master coordinators of development.

Most movement in biological systems is driven by assemblies of actomyosin, be it in the form of sarcomeres in muscles or as actomyosin networks in non-muscle cells. Actomyosin has several key functions within epithelial cells, the cells that will form most of the organs of an animal during development. One such function is to support cellular shape through an actomyosin cortex just underneath the plasma membrane. In addition, actomyosin accumulates apically at adherens and tight junctions, supporting cell-cell adhesion and epithelial tightness. Evidence over recent years has shown that apical actomyosin can also organise into 'supracellular' networks that seemingly span many cells. These large-scale assemblies either form interlinked networks of apical-medial actomyosin just underneath the free apical plasma membrane or form linear actomyosin cables at the level of adherens junctions. Both types of supracellular assemblies appear to be conserved across evolution, though were characterised in Drosophila. In this Review, I discuss the formation of these supracellular structures, the tissues in which they are known to function during development, their functional roles, and the remaining unknowns regarding their components and potential emergent properties.