Endocardial differentiation in zebrafish occurs during early somitogenesis and is dependent on BMP and etv2 signalling

The endocardium and adjacent vascular endothelial network share a number of molecular markers however there are distinct physiological functions of these tissues. What distinguishes these lineages on a molecular level remains an important, unanswered question in cardiovascular biology. We have identified the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of early endocardial development and used this line to examine endocardial differentiation. Our results show that the endocardium emerges from the anterior lateral plate mesoderm at the 8-somite stage (13 hpf). Analysis in a number of loss-of-function models showed that whilst nkx2.5, hand2 and tal1 loss-of-function have no effect on the endocardial progenitor domain, both etv2 loss-of-function and inhibition of BMP signalling reduce the endocardial domain. Furthermore, manipulating BMP signalling alters etv2 expression. Together, these results describe the onset of endocardial molecular identity and suggest a signalling cascade whereby BMP signalling acts upstream of etv2 to direct differentiation of endocardial progenitors.