Epithelial tension in the second heart field promotes mouse heart tube elongation.

Extension of the vertebrate heart tube is driven by progressive addition of second heart field (SHF) progenitor cells to the poles of the heart. Defects in this process cause a spectrum of congenital anomalies. SHF cells form an epithelial layer in splanchnic mesoderm in the dorsal wall of the pericardial cavity. Here we report oriented cell elongation, polarized actomyosin distribution and nuclear YAP/TAZ in a proliferative centre in the posterior dorsal pericardial wall during heart tube extension. These parameters are indicative of mechanical stress, further supported by analysis of cell shape changes in wound assays. Time course and mutant analysis identifies SHF deployment as a source of epithelial tension. Moreover, cell division and oriented growth in the dorsal pericardial wall align with the axis of cell elongation, suggesting that epithelial tension in turn contributes to heart tube extension. Our results implicate tissue-level forces in the regulation of heart tube extension.