At CS12, the aortic sac was still situated in the pharyngeal mesenchyme [Figure 2]a.
Serial sections and 3D reconstruction revealed that the fourth and sixth aortic arch arteries were connected to the aortic sac [Figure 3]a, [Figure 3]c and [Figure 3]e.
From CS15, the aortic sac between the fourth and sixth aortic arch arteries was degenerated [Figure 4]a.
An a-SMA-positive protrusion formed in the dorsal wall of the aortic sac between the fourth and sixth aortic arch arteries and extended into the OFT.
It has been confirmed that the protrusion in the dorsal wall of the aortic sac and the OFT cushion play crucial roles in the aortopulmonary septation., Therefore, in this study, we observed the formation of the protrusion and the OFT cushion in human embryonic hearts.