Muscle fiber branching--difference between grafts in old and young rats.

Large numbers of branched muscle fibers occur in the freely grafted rat extensor digitorum longus muscle. The ratio of branched/non-branched muscle fibers in grafts is much higher in old (24 months) than in young (4 months) host rats. Cross-age transplants show that the proportion of branched muscle fibers is related to the age of the grafted muscle and not to the age of the host. This is in contrast to mass and maximum isometric tension, in which the age of the host, rather than the age of the grafted muscle, is the determinant of the success of the muscle graft.