Figure 8. Model of the forces and cell behaviours controlling streak formation.

a-) Diagrams depicting forces generating the tissue flows during streak formation. The active pulling forces - yellow arrows, the passive pushing forces - red arrows, the direction of tissue flows - green arrows. The sickle region is indicated in black, the Area Pellucida outline in blue. Light blue squares indicate scattered events of junctional contraction, while dark blue shapes indicate regions of ingression. b-) Schematic of sequential junctional contraction (in a region marked by the blue squares in A). The sequentially contracting junctions are indicated with different (red grey, green, magenta) colours. c-) Schematic of cells showing apical contraction (blue arrows, coupled to elongation (red arrows) along the apical axis followed by ingression (green arrow). d-) Propagation of the force generated by contracting/ingressing cell between symmetrically and e-) asymmetrically shaped cells. In case of symmetrical cells the magnitude of the force decreases strongly and symmetrically at every successive junction bifurcation. However, for asymmetric cells, the broken symmetry favours force transmission along the aligned junctions (red lines), while damping transmission in perpendicular directions (green lines). f-) Image of randomly oriented cells outside the sickle region, showing lack of junctional alignment. g-) Image of cells inside the sickle, showing many aligned junctions in neighbouring cells forming long chains (red line). The alignment of asymmetrically shaped cells inside the sickle region (Fig. S5) enables anisotropic force propagation by apical contraction and directional junctional shortening resulting in large scale directed motion. Scale bar in F and G, 25 µm.