Temporally Distinct Six2-Positive Second Heart Field Progenitors Regulate Mammalian Heart Development and Disease.

The embryonic process of forming a complex structure such as the heart remains poorly understood. Here, we show that Six2 marks a dynamic subset of second heart field progenitors. Six2-positive (Six2) progenitors are rapidly recruited and assigned, and their descendants are allocated successively to regions of the heart from the right ventricle (RV) to the pulmonary trunk. Global ablation of Six2 progenitors resulted in RV hypoplasia and pulmonary atresia. An early stage-specific ablation of a small subset of Six2 progenitors did not cause any apparent structural defect at birth but rather resulted in adult-onset cardiac hypertrophy and dysfunction. Furthermore, Six2 expression depends in part on Shh signaling, and Shh deletion resulted in severe deficiency of Six2 progenitors. Collectively, these findings unveil the chronological features of cardiogenesis, in which the mammalian heart is built sequentially by temporally distinct populations of cardiac progenitors, and provide insights into late-onset congenital heart disease.