Modeling fluidity in stellate mesenchymal tissues

In many developmental and disease processes, tissues shift from solid-like to fluid-like mechanical behavior to enable large-scale tissue flows. A key unresolved question is how different organisms regulate this transition by controlling cell-scale properties. In both zebrafish and chick, a fluid-to-solid transition occurs in the presomitic mesoderm, the driving force behind posterior body axis elongation. In zebrafish, this transition is well explained by a soft particle model that undergoes a jamming/unjamming transition, driven by small changes in global volume fraction and active fluctuations, without considering cell shape or deformation. However, the tissue architecture in chick is distinct from zebrafish, with large extracellular gaps and stellate cells with distinct arm junctions, indicating that even closely related species may have evolved different mechanisms to cross a fluid/solid transition. Here, we develop a computational model to understand the essential features needed to predict the unique properties of low density, but highly connected, stellate tissues, which tissue rounding experiments demonstrate are fluid-like on long timescale. We compare short-time retraction velocities and tissue relaxation due to laser ablation between experiment and simulation to determine whether the mesenchyme is under tension. Additionally, we propose novel glassy dynamics can be controlled not via density changes but instead by cell-cell adhesion unbinding kinetics coupled with contact inhibition of locomotion, and propose new experiments to test these ideas.