Cardiac contractions shape endocardial identity by repressing veno-lymphatic fate.

Multiple cardiac features are shaped by these mechanical forces, including epicardial coverage, trabeculation, and valves. To further investigate the role of cardiac contractions during cardiac development, we developed a specific marker for endocardial cells that also labels the sinus venosus in zebrafish.

We show that cardiac contractions are necessary to maintain endocardial identity during development by restricting venous fate to the sinus venosus and the lower atrium. Furthermore, we found that the ventricular endocardium, which expresses arterial markers, is also defined by mechanical forces. We identify Notch signaling as a key regulator of venous fate in the endocardium, via the repression of the transcription factors mafb and nr2f1a.