(a) mRNA levels for Cdkn2a and Cdkn1a and SASP factors Il6, Mmp3 and Laminb1, normalized to Actb mRNA, determined by RT–PCR (n = 8 for young or old serum treatment for 3 days; n = 4 for young + old (50/50) serum treatment for 3 days). (b) Representative EdU (green; EdU negative non-proliferating SnCs in arrows), HMGB1 (red; SnCs marked by HMGB1 nuclear loss with arrows), Hoechst labelled nuclei (blue) visualized by fluorescence microscopy (3–6 images per n) and SA-β-gal staining (3–7 images per n) and (c) quantification of EdU-positive SnCs in MDFs 3 days after culturing in young, old or young + old (50/50) mouse serum (n = 4 for each group). (d) Bioluminescence from 3MR-expressing cells (Renilla luciferase assay) in non-senescent MDFs from cultured in young (4-month-old), old (32-month-old) or young+old (50/50) mouse serum for 6 days (A.U.) (n = 4 for young or old mouse serum; n = 6 for young+old mouse serum). Data are means ± s.e.m. of biologically independent samples. Statistical significance was tested using one-way ANOVA followed by Dunnett’s post hoc test for multiple comparisons with *, P < 0.05; **, P < 0.01; ***, P < 0.001. Scale bars, 100 μm. Rel, relative.