Rigidity transitions during embryonic development

During embryonic development, an organism begins as a single cell and seamlessly regulates viscoelastic properties to maintain tissue integrity while enabling extensive growth and morphological changes. A growing body of research shows that this tight control is achieved through rigidity transitions between solid-like and fluid-like states, driven by distinct cellular and subcellular properties. In this talk, I will show how a simple mechanical model, incorporating key biological features, can explain mechanisms underlying these transitions. Motivated by experimentally observed rigidity transition in the elongating zebrafish tailbud, I developed the Active Foam model. This general framework identifies three different types of rigidity transitions governed by specific cellular parameters, and the model reveals that the observed transition in the tailbud is governed by non-equilibrium tension fluctuations. By extending the model to include the nucleus as a stiff particle within each cell, we find that increased nuclear crowding induces jamming transition distinct from cellular jamming where cell movements progressively slow down while tissue structure becomes more ordered for isotropic nuclei. Quantitative measurements in developing zebrafish retinal tissues confirm the presence of a nuclear jamming transition. This work highlights the versatility of rigidity transitions in guiding robust embryonic development and demonstrates how simple mechanical models provide fundamental insights into complex biological processes.