Figure 4. Relationship of the DMP to other Septal Components and Potential Mechanisms Leading to its Maldevelopment.

Figure 4A depicts the relationship of the DMP to the other mesenchymal components of the AV septal complex at 11.5ED. The DMP (blue) is continuous with the mesenchymal cap (MC, red) of the septum primum cranially as well as the inferior atrioventricular cushion (iAVC, green) ventrally. The cap of the septum primum is also in continuity with the superior atrioventricular cushion (sAVC, yellow). Thus, the sAVC and iAVC, which have yet to fuse, are continuous through the mesenchymal cap of the septum primum and the DMP. At 13ED (4B), all components of the septal complex have fused and the DMP is wedged between the sAVC (yellow) and iAVC (green) dorsally. At this point, the mesenchyme derived from the cap of the septum primum cannot be distinguished from that of the sAVC. Figure 4C depicts the proliferation of SHF cells that ultimately give rise to the DMP at 9.5ED. The DMP may fail to properly form due to decreased proliferation of these cells (4D), increased apoptosis (4E), or premature mesenchymal-to-myocardialization (4F). Figures 4G– H demonstrate that the Mef2c-AHF-cre transgene is expressed within the developing DMP. DMP, dorsal mesenchymal protrusion; iAVC, inferior atrioventricular cushion; sAVC, superior atrioventricular cushion; rlAVC, right lateral atrioventricular cushion; RV, right ventricle; LV, left ventricle; FG, foregut