An active torque dipole across tissue layers drives avian left-right symmetry breaking

Unlike in mice, frogs, and fish, left-right (L/R) body axis formation in avian embryos does not arise from the chiral beat of cilia. Instead, a counter-clockwise tissue rotation around Hensen’s node, the organizer of amniote development, repositions cells expressing L/R sidedness genes. Yet, the physical origin of this rotation remains elusive. Here, we provide evidence that in quail embryos, the node tissue generates an active chiral torque of ∼6µNµm to drive the chiral tissue flow. Microsurgery experiments reveal that this torque depends on actomyosin molecular activity, is generated within the dorsal node tissue, and requires the underlying ventral meso-/endoderm to act as a mechanical substrate sustaining the counter-torque. We conclude that a dorsoventrally oriented tissue-scale active torque dipole at the node translates cell-scale chirality to organismal L/R asymmetry, adding a mechanical dimension to the canonical function of embryonic organizers as signaling hubs.