Does the Dorsal Mesenchymal Protrusion Act as a Temporary Pacemaker during Heart Development?

Sun et al. (1) report that the dorsal mesenchymal protrusion (DMP) acts as a temporary pacemaker during early development, prior to formation of the atrioventricular node (AVN). We would like to question whether the anatomical designation of the DMP used in this paper is correct.

The DMP was originally described as a mesenchymal protrusion from the dorsal mesocardium toward the atrial cavity contributing to the base of the atrial septum along with the mesenchymal cap and AV cushions (see Fig. 1) (2). To differentiate between the DMP prior to muscularization (which occurs from approximately embryonic day (E)13 in mouse) and other structures in the AV junction (AVJ) during early development, stainings of myocardial markers are essential, which are lacking in the paper by Sun et al. (1). In our opinion, the Shox2⁺ region described by Sun et al. (1) as DMP at E11.5 and E12.5 corresponds to CTNI⁺/HCN4⁺/ISL1⁺ cells of the putative AVN region and not the DMP (CTNI⁻/HCN4⁻/ISL1⁺) (see Fig. 1 and Refs. 3 and 4). HCN4 expression to date has not been described in mesenchyme (3–5). Co-expression of HCN4/TBX3 in cells in the AVJ, with nodal-like electrophysiological characteristics (1) corresponds to the (putative) AVN region instead of the DMP, which is in accordance with our own observations (4). Misnaming the DMP during early development leads to the erroneous conclusion that this structure shows pacemaking properties.

FIGURE 1.

Description of structures in the atrioventricular junction at E11.5. Panels A–E show transverse sections just below the entrance of the pulmonary veins. A, overview of CTNI/HCN4/ISL1 staining. Boxed area is shown in B–E. B, merge of different channels showing the HCN4⁻/CTNI⁻/ISL1⁺ DMP (arrowhead) protruding from the dorsal mesocardium (DM) toward the atrial cavity. The close relation with the mesenchymal cap (MC) is shown. C–E, separate gray values of CTNI (C), HCN4 (D), and ISL1 (E). Panels F–J show transverse sections at the level of the base of the atrial septum, more caudal to the level shown in panels A–E. F, overview, with boxed region shown in G–J. G, the HCN4⁻/CTNI⁻/ISL1⁺ (low expression of ISL1 at this level) DMP (arrowhead) is continuous with the mesenchymal cap, which in turn is continuous with the inferior atrioventricular cushion (iAVC). The arrow indicates the HCN4⁺/CTNI⁺/ISL1⁺ region designated by Sun et al. (1) as DMP. However, co-expression of markers and the location directly above the inferior atrioventricular cushion at the base of the right atrium shows that this region corresponds to the region of the future AVN. H–J, gray values of CTNI (H), HCN4 (I), and ISL1 (J). Scale bar in A and F is 100 μm; in other panels it is 50 μm. AS, atrial septum; CTNI, cardiac troponin-I; HCN4, hyperpolarization-activated cyclic nucleotide-gated channel 4; ISL1, islet1; L/RCV, left/right cardinal vein; L/RV, left/right ventricle; L/RVV, left/right venous valve; RA, right atrium; sAVC, superior atrioventricular cushion.

Although we do not support the conclusion that the DMP acts as a temporary pacemaker, the results described by Sun et al. (1) elegantly show an important role for the Shox2-BMP pathway during AVN development.