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. Author manuscript; available in PMC: 2013 Jul 1.
Published in final edited form as: Differentiation. 2012 Jun 17;84(1):117–130. doi: 10.1016/j.diff.2012.05.006

Figure 2. Interaction of the mesenchymal cap of the septum primum and the endocardial cushions.

Figure 2

At 10.5ED, the AV canal is positioned to the left of midline (2A). The septum primum is capped by mesenchyme and protrudes toward the major endocardial cushions (2B). At this stage, the ostium primum is patent while the ostium secundum has yet to develop (2B). By 11.5ED, the mesenchymal cap of the septum primum has fused with the sAVC (*2C′). The ostium primum has yet to fully close while the ostium secundum is already forming, as demonstrated by the fenestrations in the septum primum (2D′). Figures 2E and 2E′ depict a specimen at 15ED; all components of the AV septal complex have fused. The septum secundum has formed, the ostium primum is fully closed and right to left shunting is permitted through the ostium secundum only. The septum primum acts as the flap valve of the foramen ovale and shortly after birth, will fuse with the septum secundum to fully partition the atria. MC, mesenchymal cap; SP, septum primum; SS, septum secundum; OP, ostium primum; OS, ostium secundum; DMP, dorsal mesenchymal protrusion

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