Controlling organoid symmetry breaking uncovers an excitable system underlying human axial elongation

The human embryo breaks symmetry to form the anterior-posterior axis of the body. As the embryo elongates along this axis, progenitors in the tailbud give rise to tissues that generate the spinal cord, skeleton, and musculature. This raises the question of how the embryo achieves axial elongation and patterning. While ethics necessitate in vitro studies, the variability of organoid systems has hindered mechanistic insights. Here we developed a bioengineering and machine learning framework that optimizes symmetry breaking by tuning the spatial coupling between human stem cell-derived organoids. This framework enabled the reproducible generation of axially elongating organoids, each possessing a tailbud and neural tube. We discovered that an excitable system composed of WNT/FGF signaling drives elongation through induction of a neuromesodermal progenitor (NMP)-like signaling center. We discovered that instabilities in the excitable system are suppressed by secreted WNT inhibitors. Absence of these inhibitors led to ectopic tailbuds and branches.  Our results identify mechanisms governing stable human axial elongation. Single-cell mRNA sequencing of day 4 human ESC-derived organoids