Additional Supporting Information may be found in the online version of this article.

STEM_1399_sm_SuppFig1.tif2205KFigure S1. (A) PCR of genomic DNA from p38αfl and p38αKO mice. 411 bp band confirms Cre-mediated deletion of exons 2 and 3 of MAPK14. (B) Whole mouse bodyweights (male): means ± SEM; n≥15 for each timepoint. **p<0.01, significantly different from p38αfl, assessed by ANOVA. (C) Total myofiber number per TA/EDL cross-section: means + SEM.
STEM_1399_sm_SuppFig2.pdf770KFigure S2. Inhibition of p38α/β increases primary myoblast proliferation at the expense of differentiation. (A) Immunofluorescent staining against Pax7 and Myogenin (upper panels) or MHC (lower panels) in primary myoblasts 120 h after isolation, co-stained with DAPI. Treated with 5 μM SB203580 (SB treated – right panels) or DMSO control (left panels). Scale bars, 100 μm. (B) Pax7+ cells per field of view (40x): means + SEM. (C) Pax7+/Ki67+ cells as a percentage of total Pax7+ cells per field of view (40x): means + SEM. **p<0.01, assessed by Student's t-test. Scale bars, 100μm. (D) Number of multinucleated myotubes (MHC expressing myotubes with ≥3 nuclei) per field of view (40x): means + SEM. (E) Western blot of primary myoblasts 120 h after isolation, treated with DMSO control or 5 μM SB203580, with antibodies against MHC, Pax7, phospho-p38 (pp38) and GAPDH loading control.
STEM_1399_sm_SuppFig3.pdf820KFigure S3. (A) Representative Pax7 immunofluorescence 5 days post injury, co-stained with DAPI. Scale bars, 100 μm. (B) Percentage of centrally nucleated myofibers per field of view (40x), assessed from TA/EDL sections immunostained for laminin α-2 and DAPI: means + SEM. (C) Quantitation of the proportion of myofibers per cross-sectional area (average crosssectional area also shown) 25 days post injury (DPI). ∼700 myofibers assessed per n, n=3. ***p<0.005, assessed by ANOVA.
STEM_1399_sm_SuppFig4.tif1632KFigure S4. (A) p38 isoform expression (relative to expression of p38α) in FACS isolated satellite cells, measured by real-time qPCR: means + SEM. (B) Ki67+ cells per field of view (40x) as a percentage of total nuclei, in BIRB-treated and control cultures: means + SEM. **p<0.01, assessed by ANOVA.
STEM_1399_sm_SuppTab1.pdf69KSupporting Information Table 1.
STEM_1399_sm_SuppTab2.pdf70KSupporting Information Table 2.
STEM_1399_sm_SuppTab3.pdf120KSupporting Information Table 3.
STEM_1399_sm_SuppTab4.pdf117KSupporting Information Table 4.

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