Figure 6A Schematic drawing of MuSC transplantation and in vivo bioluminescence experiments. Briefly, 10 000 MuSCs isolated by FACS from CAG-Luc-eGFP mice were injected into the irradiated hindlimb of immunocompromised WT or Myf6-KO mice. Bioluminescent imaging was performed at day 1, 7, 14 and 21 post donor MuSC injections. B Representative images of bioluminescent signal from hindlimbs of Myf6-KO and WT counterparts at day 1, 7, 14, and 21 post transplantation. Scale bar=1cm C Time course of average bioluminescent signal (total flux in radiance) in Myf6-KO and WT mice at 1, 7, 14 and 21 days post MuSC transplantation showing engraftment dynamics of donor MuSCs (mean across 4-5 animals per group ± SEM at each time point, two-tailed t-test). D Bioluminescent signal (radiance) in Myf6-KO and WT mice at each time point normalized to the bioluminescent signal at day 1 post MuSC transplantation. (n=4-5 animals per group ± SD) two-tailed t-test, bars represent mean +/- SD. E-F Immunostaining of TA cross-sections of injured muscle from 3 month-old WT (E) and Myf6-KO (F) mice stained with laminin (LAMA1) and PAX7 antibodies. Acute muscle injury was performed by injection of Cardiotoxin (CTX) into the TA muscle of Myf6-KO and WT mice (n=5 animals per group). Arrowheads indicate PAX7+ MuSCs. Scale bar=100μm G Quantification of the number of PAX7+ cells per unit area of TA muscle cross sections between Myf6-KO and WT mice (n= 5 animals per group) two-tailed t-test, bars represent mean +/- SD H Immunofluorescent analysis of Myf6-KO and WT EDL myofibers cultured in growth media for 48 hours and stained with PAX7 and MYOD antibodies. Scale bar=20μm IQuantification of the number of PAX7+ MuSCs per fiber in Myf6-KO (n=27 fibers from n=3 mice) and WT (n=29 fibers from n=3 mice) mice after 48 hours in culture. Two-tailed t-test, error bars = +/- SD. J Immunofluorescence staining of WT EDL myofibers cultured in growth media for 48 hours and treated with LIF (1000U/ml) or EGF (80ng/ml) every 8 hours. Fibers were stained with antibodies against PAX7 and MyoD. Scale bar=30μm K Quantification of the number of PAX7+ and MYOD+ cells on the EDL myofiber treated with recombinant LIF (n=80 fibers), EGF (n=50 fibers) or vehicle control (n=61 fibers). Two-tailed t-test, bars represent mean +/- SD EdU staining of cultured primary myoblasts isolated by FACS (see materials and methods) from Myf6-KO and WT mice. Staining was analyzed after 6, 12 and 24 hours of EdU incorporation. Scale bar=100μm M Quantification of the percentage of EdU+ myoblasts after 6 hours (WT n=75 fields of view from n=3 mice, Myf6-KO n=72 fields of view from n=3 mice), 12 hours (WT n=82 fields of view from n=3 mice, Myf6-KOn=82 fields of view from n=3 mice), and 24 hours (WT n=70 fields of view from n=3 mice, Myf6-KO n=65 fields of view from n=3 mice) of EdU incorporation. Scale bar=100μm Representative images of a co-culture assay between WT myotubes and either WT or Myf6-KO myoblasts. Briefly, WT myoblasts were differentiated for 2 days before adding equal numbers of either WT or Myf6-KO myoblasts for an additional 2 days, followed by immunostaining for MF20 and DAPI. Scale bar=400μm O Quantification of the average number of myoblasts per mm2 that have remained mononuclear after their addition to 2DM WT myotubes, for 2 additional days (n=3 biological replicates). Two-tailed t-test, bars represent mean +/- SD.