FUSION OF MYOGENIC CELLS TO THE NEWLY SEALED REGION OF DAMAGED MYOFIBERS IN SKELETAL-MUSCLE REGENERATION

In regenerating skeletal muscle, sarcoplasmic extensions containing va riable numbers of nuclei, widely referred to as 'buds' or 'stumps', ar e formed at the ends of damaged myofibres. In this paper we investigat ed whether the nuclei seen in the buds results from fusion of myogenic cells or from migration of myonuclei to the sealed ends of damaged my ofibres in murine muscle regenerating after crush injury. The fusion o f mononuclear and multinucleate myogenic cells to the buds was demonst rated by transmission electron microscopy. In order to elucidate the f requency and kinetics of cytoplasmic continuity between myotubes and s ealed myofibres, we labelled the damaged myofibres with carbocyanine d ye Dil (which inserts into the lipid bilayer and travels down continuo us 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, bu t significant fusion had occurred by the tenth day especially at the n ewly sealed region of the damaged myofibre. A scheme for the repair of damaged skeletal muscle is presented.