Figure 2: Cells undergo collective spreading displacement during cartilage growth.

(a,b) Cells in the proliferative zone (PZ) were segmented (red dots), and their net displacement vectors (blue lines in a) and trajectories (colored lines in b) were mapped, showing strong orientation of the cell displacement towards the distal end of the tissue. (c,d) Analysis of individual cell trajectories. (c) Cell displacement over time relative to its initial y position (colour-coded as shown in the top inset), showing the largest displacement along the y axis and smaller along the x and z axes. (d) Total cell displacement (t=55 h) along different axes and planes relative to their initial y positions. Cell displacements along the y axis account for most of the displacements in 3D with linear increase in magnitude according to their initial y positions (R²=0.986), as expected if the motion of the cells depends both on local changes and similar changes happening more proximally. (e,f) Analysis of cell-cell distance change. The mean of centre-to-centre distance between all possible pairs of cells at any given time was measured (as indicated in the insets), and the distributions of those means for all cells over time (colour-coded) were plotted along the x (e) and the y axes (f) on a semi-log scale (to amplify the increase in mean over time graphically). More increase in cell-cell distance along the y axis as compared with the x axis implies an anisotropic spreading behaviour of the PZ cells; n=472 cells in (c–f). Scale bars (a,b), 50 μm.