Mechanical forces are known to play important roles during morphogenesis. However, our understanding of the spatial and temporal distribution of such forces in living tissues is incomplete, in part due to our limited ability to measure stress in vivo. Here, Alexandre Souchaud, François Graner, François Gallet and colleagues develop a pipeline of techniques to directly image and measure shear stress within developing tissues. Specifically, they use a microfluidics device to generate fluorescent droplets of an elastomer called polydimethylsiloxane. These droplets, which are cell-sized and exhibit a rigidity similar to that of tissues in vivo, can be embedded into tissues and used as stress sensors. The authors first use these droplets to infer and map the distribution of stress in aggregates of CT26 tumour cells, revealing that stress amplitude increases from the centre of the aggregate to its edge. They further use the droplets to measure the distribution, amplitude and orientation of stress in the prechordal plate (PPl) of zebrafish embryos at different stages of epiboly. Here, the shear stress amplitude appears higher in the centre of the PPl than at its front, supporting the hypothesis that stress gradients exist in the PPl. Overall, this new approach provides the means to quantitatively map local stresses in living tissues both in 3D and in real time.
