Figure 1A. Young and old myofibers were isolated from hind leg mouse muscle at 3 days post injury by cardiotoxin and were cultured for 24 hours in Ham's F10 supplemented with 10% young or old mouse serum and 50% of the supernatant specified. 10 μM of MEK inhibitor was added to some wells, as indicated. Proliferating muscle progenitor cells that were generated by the activated satellite cells were immunodetected with anti-desmin (green) and anti-BrdU (red) antibodies; Hoechst (blue) was used to label all nuclei. Percent of proliferating myogenic cells was determined by CellProfiler. Typically poor myogenicity of old satellite cells cultured with old serum was rescued by hESC supernatant in a MAPK-dependent manner.