The interchange between fluid-like and solid-like tissue states is key for multicellular

morphogenesis. Recent biophysical models have quantitatively predicted and

experimentally verified tissue material phase transitions, setting tissue rigidity as a bona fide

example of a cell collective. The next step is to understand the function of cell collectives in

context, and specifically how they respond to- and feedback on- mechanical and

biochemical stimuli. In this talk, I will present how rigidity percolation theory can identify in

vivo rigidity phase transitions with high spatiotemporal precision during zebrafish

morphogenesis. I will further discuss how we can use optogenetics to spatiotemporally

engineer tissue rigidity within the developing embryo and showcase preliminary work on

how tissue rigidity is involved in cell differentiation and tissue patterning via regulating

morphogen signaling.