Srf controls SC fusion. (A) Number of nuclei (DAPI) within the dystrophin⁺ sarcolemma per myofiber before (SO) and after 1, 3, and 5 wk of CH in control and Srf mutant plantaris muscles (n = 6–11 muscles from n = 4–6 mice). Data are mean ± SEM. *, P < 0.05 versus SO; **, P < 0.01 versus SO; ^§§, P < 0.01. (B) Phase-contrast representative pictures of FACS-sorted control and mutant SCs cultured in rich medium (D0) or 1 (D1) and 3 (D3) days after differentiation induction. (C) Proportion of nuclei within multinucleated cells (fusion index) in control and mutant cells 3 d after differentiation (n = 5–6). (D) Mean number of nuclei per MyHC⁺ cell in control and mutant cells induced to differentiate for 3 d (n = 4–5). (E) Control and mutant MB were labeled with Orange Cell Tracker and mixed with control or mutant MBs labeled with Deep Red Cell Tracker. After 48 h of co-culture in differentiation medium, the percentage of dual-labeled MTs per total number of nuclei was scored (n = 3–6). (F) MT control were labeled with Orange Cell Tracker and mixed with MB control or MB mutant labeled with Deep Red Cell Tracker. After 48 h of co-culture, the percentage of dual-labeled MTs per total number of cells was scored (n = 4–5). For C–F, data are mean ± SEM. *, P < 0.05; **, P < 0.01.