The active rheology of gastrulation

In the early embryo, tissue layers deform, flow, and exert forces in a tightly coordinated sequence to build the developing body. During convergence-extension flows, which occur in gastrulation and germ band extension, tissues contract and then elongate perpendicular to the tissue stress, i.e. with an apparent negative shear modulus. This rheology emerges through active T1 transitions, where quartets of cells rearrange against the stress, in addition to normal passive T1 transitions that yield along the principal stress direction. I will present a vertex model formulation of such a tissue [1], where the activity enters through a catch-bond mechanism feedback loop with stress of the actomyosin on individual junctions. We reliably find and characterise active T1 transitions in an optimal stress range, and compute flow curves for this material. We match T1 orientational distributions and flows with experimental data from chick embryo gastrulation. I will complement this result with a continuum model of the process [2] that highlights how the fixed points of the myosin feedback dynamics, together with boundary stresses, are responsible for this striking rheology.

[1] R. Sknepnek, I. Djafer-Cherif, M. Chuai, C. Weijer, S. Henkes, eLife, 12, (2023)
[2] A. Ioratim-Uba, T. B. Liverpool and S. Henkes, arXiv:2303.02109

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