I will talk about two project involving oriented tissue flows.

1) Most animals display one or more body axes (e.g. head-to-tail, left-right, ventral-dorsal), which usually emerge during early embryogenesis. In our work, we demonstrate that large-scale tissue flows can play an important role for the formation of body axes. To this end, we study aggregates of mouse embryonic stem cells, called gastruloids. Gastruloids are initially spherically symmetric, but later form an axis defined by the polarized expression patterns of specific proteins such as T/Brachyury or E-cadherin. While such early embryonic pattern formation is usually studied in the context of genetic and bio-chemical interactions, we show that also advection of cells with tissue flows contribute appriximately 1/3 to the overall polarization. We then more closely analyzed the flow field in the gastruloids and found that the dominant component was a recirculating flow. We further showed that this large-scale flow could be understood as a Marangoni flow, i.e. driven by interface and surface tension differences. We further independently confirmed the existence of differential interface and surface tensions though aggregate fusion experiments. Taken together, we found that in polarized gastruloids, differential tensions drive recirculation flows, which in turn further amplify polarization. We expect such a feedback loop to act also in many other systems in vitro and in vivo.

2) Oriented tissue deformation is a fundamental process omnipresent during animal development. However, how exactly cells coordinate to achieve robust oriented deformation on the tissue scale remains elusive. From a physics perspective, deforming tissues can be described as oriented active materials. However, it is known that oriented active materials can inherently exhibit instabilities such as the Simha-Ramaswamy instability. This instability destroys the homogeneously deforming state of active materials. This raises the question of how robust anisoropic tissue deformation can be possible during animal development. In particular, we ask whether the presence of a chemical signaling gradient (e.g. a morphogen gradient) can help stabilize oriented tissue deformation. Using a combination of vertex and hydrodynamic models, we find that stability depends on whether the signaling gradient acts to extend or contract the tissue along the gradient direction. In particular, gradient-extensile coupling can be stable, while gradient-contractile coupling is generally unstable. Intriguingly, developing tissues seem to exclusively use the gradient-extensile and not the unstable gradient-contractile coupling, suggesting that nature might just never have evolved the latter. Our work points to a potential new developmental principle that is directly rooted in active matter physics.