The construction of complex three-dimensional tissue structures during embryogenesis requires precise control of cell and tissue mechanics. In the Xenopus embryo, dorsal tissues progressively stiffen during gastrulation, but the interrelationship between this stiffening, the emerging features of tissue architecture and the forces generated by the constituent cells is poorly understood. Using dorsal tissue explants and integrating drug treatments, physical measurements and advanced microscopy, Lance Davidson and colleagues now test hypotheses generated by the cellular solids model (CSM), a framework that links a material's macro properties to microstructures in its constituent parts. The authors confirm that embryonic explants stiffen considerably during neurulation, a process that coincides with the generation of a complex three-dimensional architecture. Remarkably, a complete loss of this architecture following tissue ‘scrambling’ experiments does not lead to a large-scale change in tissue stiffness – consistent with prerequisites of the CSM – although scrambled tissues did fail to match stiffening of native tissues over time when cultured. Increasing or decreasing cell density leads to a respective increase or decrease in stiffness, again compatible with CSM predictions. During the process of tissue stiffening, F-actin increases at the cell cortex, and actin crosslinking by α-actinin-1 increases its stability and density, leading to a concurrent increase in tissue stiffening. This work thus sheds light on the control of tissue mechanical properties during morphogenesis, and leads the way for further studies testing predictions of mechanical models in the embryo.
