Fusion of myogenic cells to the newly sealed region of damaged myofibres in skeletal muscle regeneration.

In regenerating skeletal muscle, sarcoplasmic extensions containing variable numbers of nuclei, widely referred to as 'buds' or 'stumps', are formed at the ends of damaged myofibres. In this paper we investigated whether the nuclei seen in the buds results from fusion of myogenic cells or from migration of myonuclei to the sealed ends of damaged myofibres in murine muscle regenerating after crush injury. The fusion of mononuclear and multinucleate myogenic cells to the buds was demonstrated by transmission electron microscopy. In order to elucidate the frequency and kinetics of cytoplasmic continuity between myotubes and sealed myofibres, we labelled the damaged myofibres with carbocyanine dye DiI (which inserts into the lipid bilayer and travels down continuous membranes) and the samples were then examined by confocal scanning microscopy. This technique showed that there was little fusion between myotubes and myofibres during the first 6 days after crush injury, but significant fusion had occurred by the tenth day especially at the newly sealed region of the damaged myofibre. A scheme for the repair of damaged skeletal muscle is presented.