The timing between skeletal muscle myoblast replication and fusion into myotubes, and the stability of regenerated dystrophic myofibres: an autoradiographic study in mdx mice

In mdx mice, a model for Duchenne muscular dystrophy, the timing between th e replication of myoblasts and their incorporation into myotubes was determ ined autoradiographically. Thirty-eight mdx mice aged 23 d were injected wi th tritiated thymidine to label myoblasts replicating early in the dystroph ic process. At intervals from 8 h to 30 d after injection the tibialis ante rior muscles were removed, processed for autoradiography and analysed for l abelled central myonuclei (derived from the progeny of myoblasts which had been labelled at 23 d). At 8 h after injection there were no labelled centr al myonuclei, showing that the labelled myoblasts had not fused within this time. At 1 d, 2% of central myonuclei were labelled, at 2 d, up to 32% wer e labelled, at 3 d similar to 60% were labelled, and at 4 d the labelling p eaked at 74%. In the 27 mice sampled from 5-30 d after injection, the level s of central myonuclear labelling varied enormously: from 1-63%. However, t here was a consistent decrease in the numbers of labelled central myonuclei with time. This may have been due to dilution of the relative numbers of l abelled myonuclei due to other, nonlabelled, myoblasts replicating after th e availability of tritiated thymidine, and fusing. It was also possible tha t labelled myofibres underwent subsequent necrosis and were eliminated from the muscle. The proposal that a regenerated myofibre can undergo a subsequ ent cycle of necrosis and regeneration was supported by evidence of some ne crotic myofibres with labelled and unlabelled central nuclei. These results have implications for understanding the cellular biology and pathology of dystrophic muscle, particularly in relation to myoblast transfer therapy as a potential treatment of Duchenne muscular dystrophy.