Heart formation and left-right asymmetry in separated right and left embryos of a newt

During vertebrate cardiac development, the heart tube formed by fusion of right and left presumptive cardiac mesoderms (PCMs) undergoes looping toward the right, resulting in an asymmetrical heart. Here, we examined the right and left PCMs with regard to heart-tube looping using right- and left-half newt embryos (Cynops pyrrhogaster ). In the half embryos, the rightward (normal) loop of the heart tube was formed from the left PCM, irrespective of the timing of its separation, while the leftward (reversed) loop of the heart tube was formed from the right PCM, separated by stage 18. In addition, the direction of the leftward loop was inverted to the rightward direction in right-half embryos bisected after stage 18. Incision or resection of the embryonic caudal region implicated interactions between the right and left sides of this region as crucial for inverting the direction of the heart-tube loop from leftward to rightward in the right-half embryos. In situ hybridization of CyNodal (Cynops nodal-related gene) suggested that the inversion of heart looping in the right-half embryos has no association with the CyNodal expression pattern. Based on these findings, we propose a mechanism for the rightward looping underlying normal amphibian cardiac development.