Extended Data Figure 6.

Orientation of neighbor exchanges in the MPZ and PSM. a, Sketch of a dorsal view of the elongating body axis, with the AP and ML directions defined (top). Sketch showing the orientation of a cell-cell contact (black thick line) before undergoing a neighbor exchange (bottom left). The angle θ corresponds to the angle between the cell-cell contact before undergoing the neighbor exchange and the AP axis (bottom). Four equal bins are defined (bin 1: 0 < θ < 22.5°; bin 2: 22.5° < θ < 45°; bin 3: 45° < θ < 67.5°; bin 4: 67.5° < θ < 90°) between the AP and ML orthogonal directions (bottom right). b, Frequency of neighbor exchanges along different angular regions (n=18 in 4 embryos for PSM and n=23 in 3 embryos for MPZ, with n being the number neighbor exchanges analyzed). Mean ± SD. Neighbor exchanges are largely randomly oriented in the MPZ. In the PSM, neighbor exchanges occur predominantly along either the mediolateral (ML) direction or along the AP axis, with neighbor exchanges occurring slightly less frequently for angles in between these orthogonal orientations. The more frequent occurrence of neighbor exchanges along the AP and ML axes in the PSM is consistent with the measured directions and extent of ellipsoidal droplet deformation (Fig. 1f), as the persistent and larger supracellular stresses in the PSM may bias neighbor exchanges in these directions. Since neighbor exchanges occur equally frequently along the ML and AP directions in the PSM, and are uniformly oriented in the MPZ, our results indicate no systematic alignment of neighbor exchanges along a single spatial direction that could potentially contribute to the elongation of the body axis.