Cell movement is an active migratory process rather than displacement propelled by proliferation. (A and B) Representative captions of an 8-h time-lapse live imaging sequence depicting the elongation of branches expressing nuclear mCherry in the epithelium (K14-Cre;R26R-RG) after 2-h treatment with vehicle (A) or with 1 µg/ml of MMC (B). (C) A histogram enumerating the cell divisions detected in MMC-treated and control branches (n = four branches per condition of equal length on average, imaged in three experiments). (D and E) The tracks of cells generated between 0 and 8 h after treatment with vehicle (D) or MMC (E) where the mean velocity of the track is color-coded (fire LUT, black→white = low→high). (F) Cells were pooled within 100 µm of the leading edge and the velocity of tracked cells between frames per tip plotted as a function of time (n = four branches per condition from three experiments). Data shown represent average (line) ± SD (shaded region). (G) The leading edge of each tip was tracked frame by frame and the distance to the starting point plotted as a function of time. (H) Tracks within 100 µm of the leading edge were compared between control and MMC-treated branches (n^control = 539, n^MMC = 395, four branches each from three experiments). Data shown represent the median (line) with 25th and 75th percentiles (hinges) plus 1.5× interquartile ranges (whiskers). Statistical significance was assessed with the Wilcoxon rank sum test; ****, P ≤ 0.0001.