The aortic sac or aortic bulb[1] is a dilated structure in mammalian embryos, lined by endothelial cells and is the most distal part of the truncus arteriosus.

[2] It is the primordial vascular channel from which the aortic arches arise (and eventually the dorsal aortae) and is homologous to the ventral aorta of gill-bearing vertebrates.

The aortic sac eventually forms right and left horns, which subsequently give rise to the brachiocephalic trunk and the proximal segment of the arch of aorta, respectively.

[2] Genes HAND2 (dHAND) and HAND1 (eHAND) are expressed during the development of the aortic bulb and the arteries which arise from it.

[3] The protein encoded by these genes belong to the basic helix-loop-helix family of transcription factors.