(A) Segmentation schematic of the cortical (green) and the medial (red) region of a MCC within the plane of the exposed apical area. (B) Representative image sequence of automatically segmented cortical and medial region within apically expanding MCC (visualized by α-tubulin UtrCH-GFP). (C) Representative - effective actin (ratio of mean medial to mean cortical actin) dynamics (red) during the apical area (black) expansion process. Crosses, data points; line, data smoothed by factor 5. (D) Effective apical actin concentration for the consecutive apical domain sizes, categorized by binned area, in control cells, n=9 cells from 9 embryos. Data represent mean and variance. (E) Effective apical actin concentration as a function of apical area in control cells, n=9 cells from 9 embryos. Scale bar, 10µm. a.u., arbitrary units.