Origin of yield stress and mechanical plasticity in biological tissues*

During development and under normal physiological conditions, biological tissues are constantly exposed to significant mechanical stresses. In response to large deformations, cells within a tissue must undergo collective rearrangements to maintain integrity and resilience. However, the temporal and spatial connections between these events remain unclear. In this study, through computational and theoretical modeling, we explored the mechanical plasticity of epithelial monolayers under substantial deformation. Our findings reveal that the jamming-unjamming (solid-fluid) transition in tissues varies markedly with the level of deformation, highlighting that tissues behave as highly unconventional materials. Through analytical modeling, we uncovered the underlying mechanisms driving this behavior. Additionally, we demonstrate that tissues accommodate large deformations through a series of collective rearrangements, resembling avalanches seen in non-living materials. These "tissue avalanches" are regulated by stress redistribution and the spatial arrangement of vulnerable regions. Finally, we present a straightforward and experimentally accessible framework to predict these avalanches and estimate mechanical stress in tissues using static images.