UC San Diego

Physical inputs orchestrate cellular dynamics to sculpt organs of various morphologies. In the embryonic heart, for example, functional cues engendered by blood flow and contractility regulate cell size, shape and number to create the curvatures of the ventricular chamber. However, we have limited understanding of the many ways by which forces associated with function integrate diverse cellular mechanisms in multiple tissues to build a composed organ. Here, we leverage the junctional location and precise dimensions of the cardiac outflow tract (OFT), a cylindrical carrier of blood between the heart and the vasculature, to address this question. Since the OFT is formed by late-differentiating second heart field cells annexing to the arterial pole of a contractile heart tube, we hypothesize that function could influence the earliest steps of OFT development. Using high-resolution morphometrics in the optically accessible zebrafish embryo, we show that in the context of normal cardiac function, the OFT expands via concerted accumulation of inner endocardial and outer myocardial cells. However, in mutants with disrupted cardiac function, OFT endocardial growth ceases and the arrangement of the overlying myocardial cells is altered. By evaluating patterns of cell behavior in both wild-type and mutant embryos, we identify essential roles for cardiac function in promoting both the proliferation of the OFT endocardium and the addition of endothelial cells to the OFT from the adjacent aortic arches. Intriguingly, loss-of-function of the TGFβ receptor Alk1 leads to reduced addition of endothelial cells to the OFT endocardium without inhibiting OFT endocardial proliferation, suggesting that these two endocardial cell behaviors can be uncoupled through molecular, and possibly physical, mechanisms. Overall, our studies illuminate the cellular basis for essential early phases of OFT morphogenesis, while also pointing toward molecular mechanisms by which cardiac function shapes OFT development.