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Abstract:
The folding of cellular monolayers pervades embryonic development and disease, and is often caused by cell shape changes such as cell wedging. However, the function and mechanical role of different active cellular changes in different regions of folding tissues remain unclear in many cases, at least partially because the quantification of out-of-plane mechanical stresses in complex three-dimensional tissues has proved challenging. The gastrulationlike inversion process of the green alga Volvox provides a unique opportunity to overcome this difficulty: Combining laser ablation experiments and a mechanical model, we infer the mechanical properties of the curved tissue from its unfurling on ablation. We go on to reproduce the tissue shapes at different developmental timepoints quantitatively using our mechanical model. Strikingly, this reveals out-of-plane stresses associated with additional cell shape changes away from those regions where cell wedging bends the tissue. Moreover, the fits indicate an adaptive response of the tissue to these stresses. In this way, our paper provides not only the experimental and theoretical framework to quantify out-of-plane stresses in tissue folding, but it also shows how additional cell shape changes can provide another source of out-of-plane stresses in development complementing cell wedging.
