1284 Electrical Activation of Sinus Venosus Myocardium and Expression Patterns of RhoA and Isl-1 in the Chick Embryo REBECCA VICENTE-STEIJN, M.Sc., ∗ ,† DENISE P. KOLDITZ, M.D., Ph.D., ∗ ,† EDRIS A.F. MAHTAB, M.D., Ph.D., ∗ SA ¨ ID F.A. ASKAR, M.Sc.,† NOORTJE A.M. BAX, M.Sc., ∗ LINDA M. VAN DER GRAAF, B.Sc., ∗ LAMBERTUS J. WISSE, B.Sc., ∗ ROBERT PASSIER, Ph.D., ∗ DANI ¨ EL A. PIJNAPPELS, Ph.D.,† MARTIN J. SCHALIJ, M.D., Ph.D.,† ROBERT E. POELMANN, Ph.D.,† ADRIANA C. GITTENBERGER-DE GROOT, Ph.D., ∗ and MONIQUE R.M. JONGBLOED, M.D., Ph.D. ∗ From the ∗ Department of Anatomy and Embryology; and †Department of Cardiology, Leiden University Medical Center, Leiden, The Netherlands Electrical Activity and RhoA in the Embryo. Introduction: Myocardium at the venous pole (sinus venosus) of the heart has gained clinical interest as arrhythmias can be initiated from this area. During development, sinus venosus myocardium is incorporated to the primary heart tube and expresses different markers than primary myocardium. We aimed to elucidate the development of sinus venosus myocardium, including the sinoatrial node (SAN), by studying expression patterns of RhoA in relation to other markers, and by studying electrical activation patterns of the developing sinus venosus myocardium. Methods and Results: Expression of RhoA, myocardial markers cTnI and Nkx2.5, transcription factors Isl-1 and Tbx18, and cation channel HCN4 were examined in sequential stages in chick embryos. Electrical activation patterns were studied using microelectrodes and optical mapping. Embryonic sinus venosus myocardium is cTnI and HCN4 positive, Nkx2.5 negative, complemented by distinct patterns of Isl-1 and Tbx18. During development, initial myocardium-wide expression of RhoA becomes restricted to right- sided sinus venosus myocardium, comprising the SAN. Electrophysiological measurements revealed initial capacity of both atria to show electrical activity that in time shifts to a right-sided dominance, coinciding with persistence of RhoA, Tbx18, and HCN4 and absence of Nkx2.5 expression in the definitive SAN. Conclusion: Results show an initially bilateral electrical potential of sinus venosus myocardium evolving into a right-sided activation pattern during development, and suggest a role for RhoA in conduction system development. We hypothesize an initial sinus venosus-wide capacity to generate pacemaker signals, becoming confined to the definitive SAN. Lack of differentiation toward a chamber phenotype would explain ectopic pacemaker foci. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1284-1292, November 2010) cardiac conduction system, cardiac development, posterior heart field, second heart field, sinoatrial node Introduction The venous pole or sinus venosus of the heart comprises the myocardium that surrounds the caval veins and the pul- monary veins, and also includes the sinoatrial node (SAN). The myocardium in this area has attracted the interest of those working in the field of clinical electrophysiology by the dis- covery that this myocardium can be the source of ectopic pacemaker foci initiating clinical arrhythmias. Examples are atrial fibrillation originating from the myocardium surround- ing the pulmonary veins and atrial arrhythmias originating This work was supported partly by DiMI LSHB-CT-2005-512146. No disclosures. Address for correspondence: M.R.M. Jongbloed, M.D., Department of Anatomy and Embryology, Leiden University Medical Center, P.O. Box 9600, Postzone S-1-P, 2300 RC Leiden, The Netherlands. Fax: +31-71- 5268289; E-mail: m.r.m.jongbloed@lumc.nl Manuscript received 22 January 2010; Revised manuscript received 8 March 2010; Accepted for publication 16 March 2010. doi: 10.1111/j.1540-8167.2010.01790.x from the myocardium of the vein of Marshall, a remnant of the embryonic left cardinal vein. 1,2 The fact that the adult sinus venosus-derived myocardium is able to show ectopic activity, whereas in the embryo this area contributes to the regular primary pacemaker of the heart, the SAN, prompted us to study the embryonic development and electrical activa- tion patterns of this myocardium. During embryonic development, the heart tube is formed at the midline of the embryo by the fusion of the two bi- lateral cardiogenic plates. 3,4 The primary heart tube initially consists of a primitive left ventricle, atrioventricular (AV) canal, and part of the atria. The myocardium of the right ventricle and outflow tract at the arterial pole, and the sinus venosus myocardium at the venous pole, are progressively being incorporated during further development via a process of epithelial to mesenchymal transformation from the meso- dermal cells from the second heart field. 5-7 This implies that major parts of myocardium, including the sinus venosus my- ocardium, are being incorporated to the primary heart tube later during development from a mesenchymal progenitor population. In mice, cells from the second heart field, including the putative sinus venosus cells, express the transcription factor Islet-1 (Isl-1), whereas the myocardium of the primary heart