Morphology and functional capac-
to the sham-injected muscle, the myoblast-injected ity of homotopically transplanted extensor digitorum muscles weighed 61% more and had 50% more myolongus muscles (EDL) of adult SCID mice that refibers and 82% more cross-sectional area occupied by ceived 1 1 10 6 myoblasts [stably transfected to exmyofibers at the muscles' widest girths. Their absolute press nuclear localizing b-galactosidase under the twitch and tetanic tensions were threefold and twofold control of the myosin light-chain 3F promoter/engreater, respectively, and their specific twitch and tehancer] 2 days posttransplantation were evaluated 9 tanic tensions were 71% and 50% greater, respecweeks after transplantation, to determine whether the tively, than those of sham-injected muscles. In many injection of exogenous myoblasts had an effect on parameters, the regenerating muscle subjected to muscle regeneration. Regenerated muscles that remyoblast transfer equaled or exceeded those of musceived exogenous myoblasts were compared to simicles that were transplanted only (received only one larly transplanted muscles that received (a) no furtrauma). Absolute twitch and tetanic tensions were ther treatment, or (b) sham injection of the vehicle 73% and 65% greater, respectively, and specific (without myoblasts) and to unoperated EDL. Nine twitch tensions of the muscles receiving myoblasts weeks after myoblast transfer, myofibers containing were 50% greater than forces generated by muscles donor-derived nuclei could be identified after staining subjected to whole-muscle transplantation only. with X-gal solution. Judging from its size and poor