Dual role for neural crest cells during outflow tract septation in the neural crest-deficient mutant Splotch2H

Splotch^2H ( Sp^2H ) is a well-recognized mouse model of neural crest cell (NCC) deficiency that develops a spectrum of cardiac outflow tract malformations including common arterial trunk, double outlet right ventricle, ventricular septal defects and pharyngeal arch artery patterning defects, as well as defects in other neural-crest derived organ systems. These defects have been ascribed to reduced NCC in the pharyngeal and outflow regions. Here we provide a detailed map of NCC within the pharyngeal arches and outflow tract of Sp^2H/Sp^2H embryos and fetuses, relating this to the development of the abnormal anatomy of these structures. In the majority of Sp^2H/Sp^2H embryos we show that deficiency of NCC in the pharyngeal region results in a failure to stabilize, and early loss of, posterior pharyngeal arch arteries. Furthermore, marked reduction in the NCC-derived mesenchyme in the dorsal wall of the aortic sac disrupts fusion with the distal outflow tract cushions, preventing the initiation of outflow tract septation and resulting in common arterial trunk. In around 25% of Sp^2H/Sp^2H embryos, posterior arch arteries are stabilized and fusion occurs between the dorsal wall of the aortic sac and the outflow cushions, initiating outflow tract septation; these embryos develop double outlet right ventricle. Thus, NCC are required in the pharyngeal region both for stabilization of posterior arch arteries and initiation of outflow tract septation. Loss of NCC also disrupts the distribution of second heart field cells in the pharyngeal and outflow regions. These secondary effects of NCC deficiency likely contribute to the overall outflow phenotype, suggesting that disrupted interactions between these two cell types may underlie many common outflow defects.