Journal ArticleFree volume theory explains the unusual behavior of viscosity in a non-confluent tissue during morphogenesisDas, Rajsekhar; Sinha, Sumit; Li, Xin; Kirkpatrick, TR; Thirumalai, D<p>A recent experiment on zebrafish blastoderm morphogenesis showed that the viscosity (<italic>η</italic>) of a non-confluent embryonic tissue grows sharply until a critical cell packing fraction (<italic>ϕ</italic>_{<italic>S</italic>}). The increase in<italic>η</italic>up to<italic>ϕ</italic>_{<italic>S</italic>}is similar to the behavior observed in several glass-forming materials, which suggests that the cell dynamics is sluggish or glass-like. Surprisingly,<italic>η</italic>is a constant above<italic>ϕ</italic>_{<italic>S</italic>}. To determine the mechanism of this unusual dependence of<italic>η</italic>on<italic>ϕ</italic>, we performed extensive simulations using an agent-based model of a dense non-confluent two-dimensional tissue. We show that polydispersity in the cell size, and the propensity of the cells to deform, results in the saturation of the available free area per cell beyond a critical packing fraction. Saturation in the free space not only explains the viscosity plateau above<italic>ϕ</italic>_{<italic>S</italic>}but also provides a relationship between equilibrium geometrical packing to the dramatic increase in the relaxation dynamics.</p>eLife2024-01-1910510205eLife12RP879662050-084Xhttps://doi.org/10.7554/eLife.879662310639National Science Foundation