Nonmuscle stem cells fail to significantly contribute to regeneration of normal muscle

Whole, normal extensor digitorum longus muscles (EDL) were orthtopically transplanted into transgenic mice, expressing nuclear localing β-galactosidase (nlsβ-gal) under control of a muscle-specific promoter, in order to determine the extent to which nonmuscle derived, multipotent stem cells (which under experimental conditions exhibit myogenic potential) are spontaneously recruited from distal, nonmuscle organs to participate in the graft's regeneration. The host's contribution to the graft's regeneration was determined by evaluating the number and distribution of β-gal positive myonuclei in regenerated grafts. Fibers with β-gal positive nuclei accounted for ∼1% of the long-term (28- and 56-day) graft's myofibers. All were confined to the graft's periphery, adjacent to host's muscles. Failure to find myofibers with β-gal positive nuclei across the revascularized graft's girth demonstrated that there was no meaningful recruitment of nonmuscle stem cells from distal host organs, which must arrive at the graft via the circulation. Rather, stem cells residing in the graft at the time of transplantation accounted for ∼99.9% of the regenerated graft's myonuclei, with a minor contribution from the surrounding host muscles’ myosatellite cells (that occurred when the epimysia of graft or host muscles were damaged during transplantation). The significance of these findings to gene therapy for Duchenne muscular dystrophy is discussed.