Sparse mesenchymal cell networks as a fluid under tension

Sparse mesenchymal cellular networks are ubiquitous across animals, shaping both embryonic and adult structures through dynamic interactions with epithelia. Yet, the physical principles underlying their collective behaviors remain elusive, as their stellate cells and large extracellular spaces--filled with matrix or interstitial fluid--pose significant experimental and computational challenges. Here, we demonstrate that the avian presomitic mesoderm (PSM), a canonical embryonic mesenchymal tissue, behaves as a fluid under tension, exhibiting structural organization that cannot be explained by simple Brownian-like cell motion. Through quantitative modeling, we identify contact inhibition of locomotion (CIL)--where cells actively retract and move away upon contact--as a key mechanism that enables sparse mesenchymal networks to sustain macroscopic tension while flowing like a fluid. Simple continuum equations relate observable cell-scale parameters to the emergent remodeling dynamics observed in both experiments and simulations. Together, these findings put forward an unrecognized mechanical role for CIL, extending its influence beyond collective migration, and establish the fluid-under-tension state as a distinct class of tissue behavior that describes key developing embryonic tissues and may illuminate how matrix-rich adult tissues become fluidized during tumorigenesis.