Towards understanding the topology of the avian left-right organizer during body axis specification

Body axis specification is critical for development. Unlike the well-studied cilia-based left-right (L/R) symmetry breaking mechanism in many organisms, L/R specification in others (e.g. avians, pigs, and cattle) is cilia-independent, instead requiring chiral cell flow. In avians, anticlockwise cell movement around the L/R organizer, or Hensen’s node, displaces initially symmetric L/R determinants. While the genetic pathways are known, the roles of mechanics and tissue geometry in the process remain unclear. Work from our lab in quail embryos has shown this chiral flow requires a tissue-scale, dorsoventrally-oriented torque dipole at the node, making avian L/R specification a three-dimensional (3D) process rather than 2D cell flow. Furthermore, cells at the node flow and ingress beneath the epiblast, forming a stable tissue-scale structure composed of transient cell-scale pieces. To address the node’s 3D structure and its connection to torque generation and stability, we determine defect structure and winding number of the coarse-grained nematic director fields of cell and nucleus elongation near the node. We find node-adjacent cells twist along their dorsoventral axis, with the basal side leading in the rotation direction, while distant cells are not noticeably twisted. Together with the node’s role in generating torque, these results provide further insight into the basis of chiral cell flow and how mechanics and geometry are coupled to break symmetry.