Supplementary Figure 1: The incisor epithelium is in steady state between 8 and 16 weeks of age.

a,b, In vivo micro-computed tomography (µCT) images of mouse hemi-mandibles immediately after a small notch was made in the left incisor (a) and 4 days after (b). Boxed areas (orange) are enlarged on the right. The change in distance between the notch and the tooth’s labial surface (red and green lines) corresponds to the amount of enamel growth in 4 days. Scale bars, 3 mm. c, µCT images showing the left hemi-mandible at different ages and the associated changes in size of the incisor growth region. Scale bars, 5 mm in left panels; 100 μm in right panels. d, Immunostaining for BrdU in the incisor growth region. Mice were dosed three times with BrdU over 3 days before harvesting to assess cell division. Nuclei were counterstained with DAPI. Scale bars, 100 μm. e, Schematic models showing broadly distributed cycling cells in the incisor growth region of young mice (<5 weeks, top) versus the restriction of cycling cells in the IEE and the adjacent SR in older mice (bottom). f, TUNEL staining revealed few (1-2 cells/section) apoptotic cells in cervical loops from 8 and 16 weeks old mice. Yellow arrowheads mark TUNEL signals. Scale bars, 100 μm. g, Quantification of the rate of incisor growth per week and the incisor growth region volume, both assessed by µCT. N=3 animals. h, Percentage of BrdU⁺ cells/section. Dashed lines outline epithelium. N=3 animals. Scatter plots indicate the mean of samples and error bars represent ± S.D. of the mean. Normally distributed data were analyzed using parametric tests including one-way ANOVA with Tukey–Kramer post hoc test. Significance was taken as P<0.05 with a confidence interval of 95%. * P< 0.05, ** P < 0.01, ** P < 0.01, *** P < 0.001.