Induction of skeletal muscle progenitor cells from adult fibroblasts with Pax7, Mef2b, and MyoD. (a) Representative morphology of iSkM progenitor cells generated from TTFs. Low-power field of iSkM progenitor cells (left; scale bar: 500 µm) and high-power field of TTFs, iSkM progenitor cells, and cultured SCs (right; scale bar: 200 µm). n > 3. (b) Representative western blot for exogenous expression of MyoD by dox-inducible lentivirus vector (dox concentration was 0 or 100 ng/ml). O.E., overexpression. n = 3. (c) Analysis of TFs essential for generation of iSkM progenitor cells from TTFs. (d) Representative expression of M-cadherin analyzed by FACS in TTFs (upper), mixture of TTFs and iSkM progenitor cells (middle), and purified iSkM progenitor cells (lower). n > 4 for each group. (e) Immunocytochemical analysis of Pax3, Pax7, Myf5, MyoD, and Myogenin in iSkM progenitor cells. Scale bar: 200 µm. n > 4. (f) Expression of exogenous genes in iSkM progenitor cells. n = 6. (g) Endogenous expression of several marker genes in iSkM progenitor cells, TTFs, cultured SCs, and myotubes. n = 6–8. (h) Representative immunostaining of MyHC and Myogenin in iSkM cells 4 days after induction of differentiation. n > 6. Scale bar: 200 µm. (i) Representative images of Dystrophin-positive myofibers in iSkM progenitor cell-transplanted mdx mice. Scale bar: 100 µm. (j) Quantitative analysis of numbers of Dystrophin-positive fibers in transplanted tibialis anterior/extensor digitorum longus muscles. n = 4–9. Error bars indicate SEM. **P < 0.01, ***P < 0.001 by ANOVA with Tukey–Kramer test in (g,j).