Fig. 9. The dynamic location of events in accretion of myocardium to the outflow tract from the secondary heart field. The splanchnic mesoderm expresses Gata-4 and Nkx2.5. The outflow tract moves craniocaudally across this mesoderm. As the outflow tract approaches, Gata-4/Nkx2.5-positive cells begin to express HNK-1, move into the outflow tract, and begin to express MF20. FGF-8 expressed at the lateral sides of the pharyngeal arches may cause proliferation of the cells in the secondary heart field, while BMP-2 expressed by the distal outflow myocardium and secondary heart field near the outflow tract induce expression of the contractile proteins in the cells being incorporated into the outflow myocardium.