Figure 7.

β-Catenin regulates progenitor number and myofiber number and type during fetal myogenesis in the limb. In P0 Pax7^iCre/+;β-catenin^Δ/fl6;R26R^YFP/+ hindlimbs inactivation of β-catenin in Pax7-derived fetal muscle leads to fewer overall laminin⁺ myofibers with a lower proportion of slow myosin⁺ myofibers (A vs. B) but relatively the same number of Pax7⁺ progenitors (D vs. E) as compared with heterozygous Pax7^iCre/+;β-catenin^fl2–6/+;R26R^YFP/+. In P0 Pax7^iCre/+;β-catenin^fl3/+;R26R^YFP/+ constitutive activation of β-catenin leads to a disorganized pattern of myofibers, fewer overall laminin⁺ myofibers with a higher proportion of slow myosin⁺ myofibers (C vs. B), and more Pax7⁺ progenitors (F vs. E) as compared with Pax7^iCre/+;β-catenin^fl2–6/+;R26R^YFP/+. (Arrow in F) Dense concentrations of Pax7⁺ progenitors are frequently found in Pax7^iCre/+;β-catenin^fl3/+;R26R^YFP/+ limbs. (G) Model of embryonic and fetal limb myogenesis.