a, Example images of ear wounds of young mice, fast-healing old mice (fast old) and slow-healing old mice (slow old) at the indicated time points (2 independent experiments). Ink circles depict initial size of wounds. b, Ear wound healing curve from young (3–4 months, n = 26) and old (24–26 months, n = 28) mice (2 independent experiments). Full thickness wounds were induced on the dorsal side of both ears (see ‘Wounding and wound healing experiments’ for details) and the size of the wounds was assessed by imaging ear wounds every second day for 20 days. For each mouse, the average of both ear wounds was calculated. Graph depicts the average percentage of wound area remaining at the indicated time points. Data are mean ± s.e.m. c, Ear wound healing curves of the five fastest and the five slowest healing young and old mice. Graph depicts the average average of wound area remaining at the indicated time points. Data are mean ± s.e.m. d, FACS analysis as described in Fig. 3b to assess the percentage of THY1+PDGFRα+Lin− (THY1+) cells in ears of young and old mice during basal conditions and at 7 days after induction of wounds. Results are shown as a percentage of THY1+PDGFRα+Lin− cells over PDGFRα+Lin− cells. Data shown are from young basal (3–4 months, n = 9 replicates, each with 2–3 mice), young wounded (3–4 months, n = 8 replicates, each with 2–3 mice), old basal (24–26 months, n = 10 replicates, each with 2–3 mice) and old wounded (24–26 months, n = 8 replicates, each with 2–3 mice) (3 independent experiments). Each dot represents a replicate with cells pooled from 2–3 mice. Line depicts median percentage. P values, two-tailed Wilcoxon rank-sum test. Note that the percentage of THY1+PDGFRα+Lin− in young and old basal conditions is also presented in Fig. 3b. e, Pathway enrichment analysis based on population RNA-seq of young wounded (3–4 months, n = 6 replicates, each with 2–3 mice) and old wounded (24–26 months, n = 6 replicates, each with 2–3 mice) THY1−PDGFRα+Lin− and THY1+PDGFRα+Lin− cells in vivo (1 experiment). The graph shows a subset of KEGG pathways that were found to be significantly enriched (FDR-adjusted P < 0.05). For a complete list of differentially expressed genes and pathways, with corresponding specific P values, see Supplementary Table 5c, d. **P < 0.01, ***P < 0.001; two-sided nominal P value with Benjamini–Hochberg correction. f, Comparison between the transcriptomic changes that occur in fibroblasts with age in vitro (as described in Fig. 2b) and in vivo (as described in Fig. 3b), as well as changes that occur upon wounding in young and old ears (as described in d). The heat map depicts the enrichment of the KEGG pathways that are present in at least two of the conditions described. For the complete list of differentially expressed genes and significant KEGG terms with specific P values, see Supplementary Tables 2b, c, 4f, g, 5a, b. The scale for enrichment is indicated on the left. g, Heat map of expression of a subset of cytokine genes from population RNA-seq of fibroblasts from young and old ears during basal and wounded conditions. Expression is shown as VST-transformed read counts, scaled row-wise. The scale for expression fold changes is indicated on the left. Basal and wound signatures refer to the average expression of the genes that are significantly downregulated or upregulated with wounding, respectively, in this dataset.