Normally old muscles regenerate two to three times more poorly than young muscles. Old muscles grafted into young hosts regenerate as well as young muscles, whereas young muscle grafted into old hosts regenerate as poorly as old into old grafts. Carlson and his group conclude that the age of the host determines to a large extent the success of muscle regeneration. Follow-up research has shown that in the absence of motor nerve damage, even muscle in old rats can regenerate well, but that when nerves are damaged along with muscle, the success of muscle regeneration in old rats is greatly reduced.
Current research involves the regenerative capacity of muscles in extremely old rats.
Another major area of recent research involves the restoration of long-term denervated muscle by regeneration. Carlson's group has identified a specific time window during which denervated rat muscles lose their ability to become restored through regeneration. Analysis of long-term denervated muscle has shown a significant loss of myonuclei, accompanied by first a gain and then a progressive loss of satellite cells over 7 months of denervation. In addition to changes in muscle fibers, the capillary supply to denervated muscle undergoes an almost tenfold reduction over the first seven months of denervation. Current research involves the analysis of muscle fiber neoformation in atrophying long-term denervated muscle and conditions surrounding the activation of satellite cells.