Development of the arterial pole of the heart is a critical step in cardiogenesis, yet its embryological origin remains obscure. We have analyzed a transgenic mouse line in which beta-galactosidase activity is observed in the embryonic right ventricle and outflow tract of the heart and in contiguous splanchnic and pharyngeal mesoderm. The nlacZ transgene has integrated upstream of the fibroblast growth factor 10 (Fgf10) gene and comparison with the expression pattern of Fgf10 in pharyngeal mesoderm indicates transgene control by Fgf10 regulatory sequences. Dil labeling shows a progressive movement of cells from the pharyngeal arch region into the growing heart tube between embryonic days 8.25 and 10.5. These data suggest that arterial pole myocardium originates outside the classical heart field.

Abstract-The sinoatrial node, which resides at the junction of the right atrium and the superior caval vein, contains specialized myocardial cells that initiate the heart beat. Despite this fundamental role in heart function, the embryonic origin and mechanisms of localized formation of the sinoatrial node have not been defined. Here we show that subsequent to the formation of the Nkx2-5-positive heart tube, cells bordering the inflow tract of the heart tube give rise to the Nkx2-5-negative myocardial cells of the sinoatrial node and the sinus horns. Using genetic models, we show that as the myocardium of the heart tube matures, Nkx2-5 suppresses pacemaker channel gene Hcn4 and T-box transcription factor gene Tbx3, thereby enforcing a progressive confinement of their expression to the forming Nkx2-5-negative sinoatrial node and sinus horns. Thus, Nkx2-5 is essential for establishing a gene expression border between the atrium and sinoatrial node. Tbx3 was found to suppress chamber differentiation, providing an additional mechanism by which the Tbx3-positive sinoatrial node is shielded from differentiating into atrial myocardium. Pitx2c-deficient fetuses form sinoatrial nodes with indistinguishable molecular signatures at both the right and left sinuatrial junction, indicating that Pitx2c functions within the left/right pathway to suppress a default program for sinuatrial node formation on the left. Our molecular pathway provides a mechanism for how pacemaker activity becomes progressively relegated to the most recently added components of the venous pole of the heart and, ultimately, to the junction of the right atrium and superior caval vein. Key Words: heart development Ⅲ sinus node Ⅲ sinoatrial node Ⅲ Nkx2-5 Ⅲ Pitx2c Ⅲ Tbx3 Ⅲ Cx40 Ⅲ transgenic mice T he sinoatrial node (SAN) is the pacemaker of the heart that initiates the heartbeat and controls its rate of contraction. It is a specialized myocardial structure positioned at the junction of the right atrium and the superior caval vein. Disease, ageing, or gene defects may cause SAN dysfunction (sick sinus syndrome), a common cardiac disorder that currently requires the implantation of a pacemaker. SAN function depends on a complex tissue architecture and the expression and function of a set of ion-channel, sarcomeric and gap junction proteins expressed at levels distinct from the atrial working myocardium. 1,2 Hyperpolarization-activated pacemaker channel Hcn4 is expressed in the SAN and plays an important role in pacemaker activity in vivo. 2,3 Furthermore, the SAN expresses very low levels of connexin 40 (Cx40) and Cx43 gap-junctional subunits that are essential for the fast conduction of the electrical impulse in the atrial and ventricular working myocardium and ventricular conduction system. 4,5 The T-box transcription factor Tbx3, which, when mutated, causes ulnar-mammary syndrome of congenital malformation in human, 6 is expressed in the developing nodal tissues of the heart of mammals and chicken and is able to suppress promoter activity of atrial chamb...

Chronic venous disease and venous hypertension are common consequences of valve insufficiency, yet the molecular mechanisms regulating the formation and maintenance of venous valves have not been studied. Here, we provide what we believe to be the first description of venous valve morphogenesis and identify signaling pathways required for the process. The initial stages of valve development were found to involve induction of ephrin-B2, a key marker of arterial identity, by venous endothelial cells. Intriguingly, developing and mature venous valves also expressed a repertoire of proteins, including prospero-related homeobox 1 (Prox1), Vegfr3, and integrin-α9, previously characterized as specific and critical regulators of lymphangiogenesis. Using global and venous valve-selective knockout mice, we further demonstrate the requirement of ephrin-B2 and integrin-α9 signaling for the development and maintenance of venous valves. Our findings therefore identified molecular regulators of venous valve development and maintenance and highlighted the involvement of common morphogenetic processes and signaling pathways in controlling valve formation in veins and lymphatic vessels. Unexpectedly, we found that venous valve endothelial cells closely resemble lymphatic (valve) endothelia at the molecular level, suggesting plasticity in the ability of a terminally differentiated endothelial cell to take on a different phenotypic identity.