Nkx2.5‐negative myocardium of the posterior heart field and its correlation with podoplanin expression in cells from the developing cardiac pacemaking and conduction system

Abstract

Recent advances in the study of cardiac development have shown the relevance of addition of myocardium to the primary myocardial heart tube. In wild-type mouse embryos (E9.5-15.5), we have studied the myocardium at the venous pole of the heart using immunohistochemistry and 3D reconstructions of expression patterns of MLC-2a, Nkx2.5, and podoplanin, a novel coelomic and myocardial marker. Podoplanin-positive coelomic epithelium was continuous with adjacent podoplanin-and MLC-2a-positive myocardium that formed a conspicuous band along the left cardinal vein extending through the base of the atrial septum to the posterior myocardium of the atrioventricular canal, the atrioventricular nodal region, and the His-Purkinje system. Later on, podoplanin expression was also found in the myocardium surrounding the pulmonary vein. On the right side, podoplanin-positive cells were seen along the right cardinal vein, which during development persisted in the sinoatrial node and part of the venous valves. In the MLC2a-and podoplanin-positive myocardium, Nkx2.5 expression was absent in the sinoatrial node and the wall of the cardinal veins. There was a mosaic positivity in the wall of the common pulmonary vein and the atrioventricular conduction system as opposed to the overall Nkx2.5 expression seen in the chamber myocardium. We conclude that we have found podoplanin as a marker that links a novel Nkx2.5-negative sinus venosus myocardial area, which we refer to as the posterior heart field, with the cardiac conduction system.

Podoplanin deficient mice show a rhoa‐related hypoplasia of the sinus venosus myocardium including the sinoatrial node

“…These early observations on the addition of the secondary myocardium have recently been supported by several studies describing the addition of myocardium at the arterial pole, being secondary (Waldo et al, 2001) and anterior heart fields (Kelly et al, 2001;Mjaatvedt et al, 2001;Meilhac et al, 2004;Kelly, 2005) and the posterior heart field (PHF) at the venous pole (Cai et al, 2003;Meilhac et al, 2004;Christoffels et al, 2006;Gittenberger-de Groot et al, 2007) of the developing heart. The complete length of the splanchnic mesoderm contributing to the addition of myocardium at both poles of the heart was called second heart field (Cai et al, 2003) or second lineage (Cai et al, 2003;Meilhac et al, 2004;Kelly, 2005).…”

“…Several genes and proteins, considered as early markers of the second heart field at the outflow and inflow tract, have been reported, such as MesP1 (Saga et al, 1999), fibroblast growth factor (Fgf) 8 and 10 (Kelly et al, 2001), BMP-2 and Nkx2.5 (Waldo et al, 2001;Christoffels et al, 2006;Gittenberger-de Groot et al, 2007), Isl1 (Cai et al, 2003), inhibitor of differentiation Id2 (Martinsen et al, 2004), GATA factors targeting Mef2c (Dodou et al, 2004;Verzi et al, 2005), Tbx1, Tbx3, and Tbx18 Christoffels et al, 2006) and Shox2 (Blaschke et al, 2007). Recently, we have added podoplanin to this list as a novel gene in cardiac development.…”

Platelet‐derived growth factor is involved in the differentiation of second heart field‐derived cardiac structures in chicken embryos

“…The establishment of a fully septated heart is dependent on the addition of cells from a second heart cell lineage to the primary heart tube (Cai et al, 2003;Kelly, 2005;Gittenberger-de Groot et al, 2007). The second heart field or second lineage is located in the splanchnic mesoderm and can be divided into two fields related to the craniocaudal axis of the primary heart tube.…”

Expression of Id2 in the second heart field and cardiac defects in Id2 knock‐out mice

“…The SHF is also involved in incorporation of myocardium at the venous pole of the heart, and cells from this population have been identified by the expression of several markers (Cai et al, 2003;Christoffels et al, 2006;Gittenberger-de Groot et al, 2007;Kelly and Buckingham 2002;Kelly 2005;Snarr et al, 2007a). We have referred to this contribution as being derived from the posterior heart field (PHF) , as opposed to the anterior heart field recognized at the arterial pole.…”

“…Myocardium derived from the PHF is characterized by the expression of several genes, including Podoplanin Mahtab et al, 2009), its downstream factor RhoA (VicenteSteijn et al, 2010), HCN4 (Garcia-Frigola et al, 2003), Shox2 (Blaschke et al, 2007), Tbx5 (Puskaric et al, 2010), and Tbx18 (Christoffels et al, 2006). PHF-derived sinus venosus myocardium is furthermore characterized by the lack of expression of the transcription factor Nkx2.5 (Christoffels et al, 2006;Gittenberger-de Groot et al, 2007). Assessing the expression pattern of the coelomic and myocardial marker Podoplanin, we postulated the addition of cells to the venous pole from two populations: a mesenchymal population, which gives an early contribution including the pro-epicardial organ and its derivatives (Mahtab et al, 2008), and a myocardial population that will form the myocardium surrounding the cardinal veins, the pulmonary vein myocardium, the interatrial septum, and parts of the cardiac conduction system (Gittenberger-de Mahtab et al, 2009).…”