Controlling human organoid symmetry breaking reveals signaling gradients drive segmentation clock waves

Axial development of mammals is a dynamic process involving several coordinated morphogenetic events, including axial elongation, somitogenesis, and neural tube formation. To gain insight into the signals control the dynamics of human axial morphogenesis, we generated hundreds of axially elongating organoids by inducing anteroposterior symmetry breaking of spatially coupled epithelial cysts derived from human pluripotent stem cells. Each organoid was composed of a neural tube flanked by presomitic mesoderm sequentially segmented into somites. Periodic activation of the somite differentiation gene MESP2 coincided in space and time with anteriorly traveling segmentation clock waves in the presomitic mesoderm of the organoids, recapitulating critical aspects of somitogenesis. Through timed perturbations of organoids, we demonstrated that FGF and WNT signaling play distinct roles in axial elongation and somitogenesis and that FGF signaling gradients drive the segmentation clock waves. By generating and perturbing organoids that robustly recapitulate the architecture and dynamics of multiple axial tissues in human embryos, this work offers a means to dissect complex mechanisms underlying human embryogenesis. Organoids were dissociated into a single-cell suspension using the Worthington Papain Dissociation System kit (Worthington Biochemical).