Muscle transplantation between young and old rats: age of host determines recovery.

As compared with age-matched controls, extensor digitorum longus (EDL) muscles autografted in young rats regenerated significantly greater mass (1.8 times) and developed greater maximum contractile force (2.6 times) than EDL muscles autografted in old rats. A cross-age transplantation study showed that the mass and maximum force of old muscles grafted into young hosts were not significantly different from those of young muscles grafted into the same young hosts. Conversely, young muscle grafted into old hosts regenerated no better than old muscles grafted into the same old hosts. We conclude 1) that chronological age alone is not a factor that limits the intrinsic ability of a muscle to regenerate and 2) that the poor regeneration of muscles in old animals is a function of the environment for regeneration provided by the old host.