Figure 1. Matrix viscoelasticity determines symmetry breaking, tissue branching, and epithelial to mesenchymal transition.

a, Schematic demonstrating how simultaneously changing the polymer molecular weight and extent of crosslinking allows for constant gel stiffness but altered viscoelastic properties. b, Quantification of the storage modulus of resulting alginate hydrogels (n=5,9 gels per condition). c, Quantification of the timescale at which an initially applied stress is relaxed to half its original value (n=19 gels per condition). d, Examples of growth of MCF10A spheroids in elastic versus viscoelastic hydrogels over 5 days. Phalloidin in cyan, Hoechst in magenta. e-f, Quantification of the spheroids area (e) and circularity (f), respectively (error bars, s.e.m). n=19–43 spheroids/condition/day. g, Representative examples of phosphorylated FAK focal adhesions in MCF10A celll spheroids growing in elastic and viscoelastic gels. pFAK in gray and hoechst in magenta (n=3,4 images per condition). h. Representative examples of phalloidin, Hoechst (upper row) and YAP (lower row) stainings of spheroids in elastic and viscoelastic gels (spheroids core cells and branch leader cells). i. Quantification from stainings of the percentage of cells with nuclear YAP per image for the indicated regions (n=8,11,17 images per condition). Statistical analysis was performed using Kruskal–Wallis test followed by post hoc Dunn’s test. For panels b,c,e,f, statistical analysis was performed using two-sided Mann-Whitney U-test. All data are mean ± s.d. except where indicated, all scale bars are 75 μm.