Arrested coalescence of multicellular aggregates

Multicellular aggregates are known to exhibit liquid-like properties. The fusion process of two cell aggregates is commonly studied as the coalescence of two viscous drops. However, tissues are complex materials, which usually exhibit viscoelastic behaviour. It is known that elastic effects can prevent the complete fusion of two drops, a phenomenon known as arrested coalescence. Here we report the presence of this phenomenon in aggregates of mouse embryonic stem cells and provide a theoretical framework which agrees with the experiments. In addition, agent-based simulations show that cell protrusion activity controls a solid-to-fluid phase transition, revealing that arrested coalescence can be found in the vicinity of an unjamming transition. By analyzing the dynamics of the fusion process it is possible to infer mechanical parameters of the aggregates, such as the viscoelastic relaxation time and the elastocapillary number. Our work provides a simple in vitro method to characterize the mechanical properties of 3D multicellular aggregates and sheds light on the impact of cellular activity on tissue mechanics.