The Tbx2 ⁺ Primary Myocardium of the Atrioventricular Canal Forms the Atrioventricular Node and the Base of the Left Ventricle

Abstract

Abstract-The primary myocardium of the embryonic heart, including the atrioventricular canal and outflow tract, is essential for septation and valve formation. In the chamber-forming heart, the expression of the T-box transcription factor Tbx2 is restricted to the primary myocardium. To gain insight into the cellular contributions of the Tbx2 ϩ primary myocardium to the components of the definitive heart, genetic lineage tracing was performed using a novel Tbx2 Cre allele. These analyses revealed that progeny of Tbx2 ϩ cells provide an unexpectedly large contribution to the Tbx2-negative ventricles. Contrary to common assumption, we found that the embryonic left ventricle only forms the left part of the definitive ventricular septum and the apex. The atrioventricular node, but not the atrioventricular bundle, was found to derive from Tbx2 ϩ cells. The Tbx2 ϩ outflow tract formed the right ventricle and right part of the ventricular septum. In Tbx2-deficient embryos, the left-sided atrioventricular canal was found to prematurely differentiate to chamber myocardium and to proliferate at increased rates similar to those of chamber myocardium. As a result, the atrioventricular junction and base of the left ventricle were malformed. Together, these observations indicate that Tbx2 temporally suppresses differentiation and proliferation of primary myocardial cells. A subset of these Tbx2Cre -marked cells switch off expression of Tbx2, which allows them to differentiate into chamber myocardium, to initiate proliferation, and to provide a large contribution to the ventricles. These findings imply that errors in the development of the early atrioventricular canal may affect a much larger region than previously anticipated, including the ventricular base. Key Words: atrioventricular canal Ⅲ lineage Ⅲ fate Ⅲ patterning Ⅲ transgenic Ⅲ Cre T he embryonic heart tube is composed of "primary" myocardium and rapidly elongates by addition of progenitor cells to its poles. During looping, specific regions in the embryonic tubular heart differentiate to chamber myocardium and expand to form the future working myocardium of the ventricles and atria. In contrast, the region in between these expanding chambers does not differentiate or expand and becomes visible as an atrioventricular constriction. 1 During prenatal life, the atrioventricular canal (AVC) myocardium conducts the electric impulse between the atrial and ventricular chambers in a slow manner, reminiscent of the function of the mature atrioventricular (AV) node. This slow conducting feature allows the AVC to act as a sphincter preventing backflow from the ventricles to the atria, analogous to the AV valves. Furthermore, the primary myocardium provides the signals that initiate formation of the cushions, which subsequently will form the valves and partake in septation. 2,3 The primary myocardial AVC is extensively remodeled to properly connect and align both atria and ventricles and to coordinate the formation of the fibrous insulation. 4 Given all these roles of the AV...

Biphasic Development of the Mammalian Ventricular Conduction System

“…The existence of such a population has been proposed by others using a Tbx2-Cre lineage analysis in which a population of progenitors in the AVC giving rise to the AVN is distinguishable at early stages of development. 30 Similarly, a slow proliferative population of Tbx3-expressing cells has been identified at the crest of the septum and correlated with the development of the His bundle. 12 Inductive signals coming from endothelial cells such as endothelin or neuregulin in the chick and mouse, respectively, or from epicardially derived cells, have been shown to play a role in Purkinje fiber differentiation.…”

The formation and function of the cardiac conduction system

“…A domain in the AVC positive for G1N2 and Tbx3 expression extends dorsally and ventrally into the ventricular compartment and runs through the ventricular septum crest, forming the interventricular ring from which the AVB is formed (Wessels et al, 1992;Hoogaars et al, 2004). It is directly connected to the AVC-derived AVN, but the respective progenitors for these two tissues segregate before the onset of Tbx2 expression in AVC precursors and before the activation of a Mef2c enhancer (as marked by a Mef2c-AHF-enhancer-Cre; Verzi et al, 2005) in septum precursors, at approximately E8-9 (Aanhaanen et al, 2010). The location of AVB (septum crest) progenitors in the embryo prior to ventricle formation has not been assessed.…”

Tbx2 Controls Lung Growth by Direct Repression of the Cell Cycle Inhibitor Genes Cdkn1a and Cdkn1b

“…Mice homozygous for a null allele of Tbx2 (Tbx2 cre ) that is maintained on an NMRI outbred background survive embryogenesis but die shortly after birth due to a cleft palate [9,21]. Morphological and histological examination of mutant embryos at E18.5 revealed hypoplastic lungs that frequently manifested with alveolar haemorrhages.…”