During mammalian embryogenesis, cardiac progenitor cells constituting the second heart field (SHF) give rise to the right ventricle and primitive outflow tract (OFT), the latter of which is particularly vulnerable to developmental malformations that underlie common congenital heart defects. In zebrafish, SHF progenitors produce half of the single ventricle's myocardium and contribute significantly to the OFT. Previous lineage tracing and mutant analyses suggested that SHF ventricular and OFT progenitors co-migrate to the arterial pole of the zebrafish heart tube soon after their specification in the nkx2.5+ field of anterior lateral plate mesoderm. Here, we employ additional prospective lineage tracing to demonstrate that while nkx2.5+ SHF ventricular progenitors migrate directly to the arterial pole, OFT progenitors become temporarily sequestered in the mesodermal cores of pharyngeal arch 2 (PA2) where they downregulate nkx2.5 expression. While there, they intermingle with precursors for PA2-derived head muscles (HM) and hypobranchial artery endothelium, which we demonstrate are co-specified with SHF progenitors in the nkx2.5+ anterior lateral plate mesoderm. Soon after their sequestration in PA2, OFT progenitors migrate to the arterial pole of the heart where they differentiate into OFT lineages. The observation that SHF ventricular and OFT progenitors take unique developmental trajectories to the heart is supported by their differential sensitivities to a mutation in fgf8a. Our data highlight novel aspects of SHF, OFT and HM development in zebrafish that will inform mechanistic interpretations of cardiopharyngeal phenotypes in models of human congenital disorders.
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