The expression of fgf2 and wnt9b, two cardiomyocyte mitogens regulated by RA signaling during mouse heart development (Merki et al., 2005), was strongly decreased in epi-KO hearts (Figure 4H), whereas that of igf2a and igf2b, factors regulated by liver-derived erythropoietin (Brade et al., 2011), was unaffected (Figure 4H). Interestingly, the expression of fgf9, a gene encoding a key FGF regulated by RA signaling in the mouse heart (Lavine et al., 2005), as well as fgf20a and fgf20b was unaffected in epi-KO hearts, whereas fgf16 expression levels was strongly decreased (Figure 4H). These findings suggest that although FGF regulation by RA signals is evolutionary conserved, different FGF proteins may regulate cardiogenesis in zebrafish and mammals. We also analyzed the expression of fgf2, fgf16, and wnt9b in purified cardiomyocytes and epicardial cells (Figure 4A), finding that they are primarily expressed in the epicardium during zebrafish heart development (Figure 4I). Thus, epicardium-specific inactivation of shha expression revealed that Shha plays a crucial role in RA synthesis and mitogen expression in the epicardium during heart development in zebrafish.