Confocal time-lapse 3D imaging of ring cells at E12.5. (A) A representative mammary bud from a K14-Cre/mTmG embryo at the beginning, at 2 h 45 min, and at the end of the imaging (5 h 30 min). White and orange dashed line mark the perimeter of the invaginating mammary bud in the beginning and at the indicated time point, respectively. Scale bar, 30 µm. (B) Quantification of the epidermal contact area of individual mammary buds during imaging. (C) Individual ring and epidermal cell tracks in three mammary buds. Dashed line marks the perimeter of the invaginating mammary bud. Scale bar, 30 µm. See also Video 3. (D) Rose plots of the escape angles (the angle between cell trajectories at the beginning and end of the time lapse in respect to the center of the mammary bud). Rayleigh test for nonuniformity, H0 = random; n = 96 ring cells and 95 epidermal cells from three biological replicates. Statistical significance was assessed with Watson’s U2 test.