Muscle differentiation and morphogenesis in the regenerating tail of lizards

The differentiation of muscles in the lizards Anolis and Lampropholis with tails that had regenerated for 21-50 d was investigated by light and electron microscope autoradiography using tritiated thymidine. At the apex of the regenerating tail, groups of 4-8 myoblasts of the promuscle aggregates fused to produce bundles of myotubes whose multiple labelled and unlabelled nuclei appeared to be distributed at random. The formation of the first myotubes and their growth is responsible for the formation of the myotome primordia and their separation from the intermuscular connective myosepta. More nuclei were added with the lengthening of the myotubes--up to 14-18 nuclei in the oldest proximal myotubes. At 4-5 h after injection labelled nuclei were found outside the myotubes while at 2-6 d after injection many labelled nuclei were observed in the myotubes, particularly near the two ends of the myotubular sarcoplasm contacting the myoseptum. This change from the initial distribution suggests that the growth of the myotubes takes place mostly at their terminals. There is an apparent correlation between the number of nuclei and the final length of the myotubes and myotome. The insertion of fibres with a similar number of nuclei and lengths into the pinnated connective myoseptum of the original musculature, the autotomy plane, probably determines the wave-like shape of the muscles within the regenerated myotomes.