Fig. 5. Wnt signaling regulates expansion of Isl1⁺ SHF progenitors through regulation of Isl1 and FGF signaling. (A) Multiple reports have demonstrated that Wnt/β-catenin signaling (red) is required upstream of Isl1 and FGF signaling to promote expansion of SHF stem/progenitor cells (green) (Ai et al., 2007; Cohen et al., 2007; Kwon et al., 2007; Qyang et al., 2007b). (B,C) Expression of an activated β-catenin protein in the secondary heart field (SHF) using the SM22-cre transgenic line leads to a dramatic increase in the number of Isl1⁺ SHF progenitors in the anterior pharyngeal mesoderm (bracket) and the outflow tract of the mouse embryonic heart (arrowheads). (B,C) Reproduced, with permission, from Cohen et al. (Cohen et al., 2007).