A model of myogenesis in vivo, derived from detailed autoradiographic studies of regenerating skeletal muscle, challenges the concept of quantal mitosis.

We have recently shown that myogenesis following severe injury is prolonged compared with minor injury (McGeachie and Grounds 1987). In this previous autoradiographic study 44 mice were injected with tritiated thymidine at various times after muscle injury (0 to 120 h), and samples were taken 9 d after injury to determine the percentage of labelled myotube nuclei. In the present study the same experimental data are analysed in detail to reveal how many times labelled muscle precursors divided before fusing to form myotubes. Additional mice were prepared and samples removed 1 h after injection of tritiated thymidine to determine the maximum grain counts of premitotic nuclei. When a labelled premitotic nucleus divides, each of the two daughter nuclei will contain half of the original label. The grain counts of nuclei resulting from sequential divisions of a maximally labelled premitotic nucleus, forms the basis for our detailed analysis which can reveal how many times a muscle precursor has divided after labelling. Nine days after injury the autoradiographic grain counts of labelled myotube nuclei were analysed in detail. The results describe an in vivo model of myogenesis which we use to evaluate quantitatively observations derived from tissue culture studies. The analysis shows that, at the onset of myogenesis in regenerating muscle (30 h after injury), muscle precursors divide only twice before fusing to form myotubes. This observation challenges the concept of quantal mitosis as defined by the tissue culture studies of Quinn et al. (1984, 1985).