Microstructures regulate distant cellular interactions during heart formation

Communication between endocardium and myocardium, separated by cardiac jelly, is essential for cardiac morphogenesis. How membrane-localized receptors and ligands achieve interaction across the cardiac jelly is not understood. Utilizing a number of approaches, including genetic labeling of cell-cell contacts, we found that myocardium and endocardium interact directly via microstructures, termed Tunneling Nanotube-Like microstructures (TNTLs). TNTLs extend from cardiomyocytes to directly contact endocardial cells (ECs). TNTLs transport cytoplasmic proteins, transduce signals between cardiomyocytes and ECs, and initiate myocardial growth toward the lumen to form ventricular trabeculae-like structures. Loss of TNTLs disturbs signaling interactions and, subsequently, ventricular patterning. Whole hearts from E9.5 control and myocardial Cdc42 knockout (MCKO) are analyzed, each set contains three hearts.