Figure 2. A 2D vertex model reveals that the alternate, carefully timed contractions are a general feature of cell intercalation.

(A) Initial cell arrangement used in the simulations. Vertical axis (x) indicates antero (A) - posterior (P), and horizontal axis (y) indicates medio (M) – lateral (L). (A’) Example of junction contractions. The cell-cell junctions aligned along the mediolateral axis (x-axis) can contract in the model. The edge color indicates the magnitude of the line tension of the cell and cell junction (red=maximum strength, blue=minimum strength). (B) Example of the junction remodeling in the simulations. After the cell-cell junction shows emergence of a new junction, and is counted as one intercalation, as shown in Figures 2G and 2H. (C) Simulation of CE with coincident, continuous contraction without oscillation generates no intercalation. Top panel shows cells at 1 simulation time t = 1 (A.U); middle panels at t = 250 and lower panels at t = 555. (D) Synchronous oscillating contractions (In-phase oscillation) generate modest intercalation. (E) Alternate oscillating contractions generate robust intercalation. (F) Continuous contraction without oscillating at 1.5 times the line tension generates modest intercalation. (G) Graph of intercalation quantified by the number of newly formed cell-cell junctions after complete v-junction shortening as shown in Figure 1B, representing the efficiency of cell intercalation. Blue: continuous contraction (Λ = 0.10); Black: Synchronous oscillation (Λ = 0.10); Green: alternating oscillation (Λ = 0.10); Orange: continuous contraction (Λ = 0.15). Frequency of oscillation is defined by angular frequency ω [radian/simulation time unit] (ω = 2π/T, See experimental procedures). For both synchronous and alternating oscillations, 2π/T is fixed to 0.114. The phase shift to generate alternating oscillation is defined by θ_x=0.7854, θ_y=0.7854 (see experimental procedure). (H) Effect of oscillating frequency on intercalation efficiency for alternating oscillation. Purple: ω = 0.057 (slowest), Green: ω = 0.114, Red: ω = 0.228, Pink: ω = 0.342 (Fastest). The phase shift is fixed to Δθ = 2.5π × 10⁻¹ for the three groups.