SoftBio Theory Seminar: Pattern formation by living droplets in chemoattractant gradients

Gradients of extracellular signals organise cell behaviour in tissues. Although we have good models for chemotaxis of isolated cells in signal gradients, it is not clear how cells react to gradients when the cell population is undergoing 3D morphogenesis, in which cell-cell interactions and cell-signal interactions undergo extensive emergent behaviour. Using light sheet imaging to simultaneously monitor signalling, single-cell and population dynamics in Dictyostelium cell populations, we show that these cells migrate towards nutritional gradients in swarms. As the swarm advances, it deposits clumps of cells at the rear, triggering their differentiation. Clump deposition is explained by a model in which the cell swarms behave as living droplets, with cell proliferation and signal gradient remodelling opposing surface tension to promote droplet shedding. The model predicts vortex motion of the cells within the droplet emerging from active forces, which was validated by 3D tracking of single cells in the swarms. Our data and modelling show how the emergent dynamics of multicellular communities cause qualitative differences in chemotaxis from isolated or non-interacting cells.