Myotomal myogenesis inBombina variegata L.

InBombina variegata, striated myofibrils first appear in G2 uninucleated primary myoblasts. Multinucleated muscle fibres form later as a result of the fusion of primary myobasts with secondary myoblasts of mesenchymal origin. The nuclei of the polykaryocytes vary in size and DNA content (nuclear dimorphism). The larger nuclei of the primary myoblasts retain tetraploid quantities of DNA, whereas the smaller nuclei of the secondary myoblasts are diploid. From this we conclude that fusion can take place between cells that are in different phases of the cell cycle (G1-G2). Our findings are compared with those on myogenesis in other chordate species and are confronted with the current commonly accepted model of vertebrate muscle differentiation.