Embryology, Aortic Arch

The thoracic aorta is subdivided into three sections: the ascending aorta, the aortic arch, and the descending aorta (see . Thoracic Aorta and the Aortic Arch, Computed Tomography Angiography). The ascending thoracic aorta arises from the left ventricle of the heart, anterior to the pulmonary artery, and rises to approximately the level of the fourth thoracic vertebra. The aorta then begins to travel posteriorly and to the left, where it is known as the arch of the aorta. The normal arch of the aorta gives off three vessels. The brachiocephalic trunk, also known as the innominate artery, is the first branch, bifurcating into the right subclavian and right common carotid artery. The brachiocephalic trunk is then followed by the left common carotid and subclavian arteries. The 'typical' pattern of aortic arch vessels occurs in approximately 70% of the population.  Around the vertebral level of T4, the aorta continues as the descending thoracic aorta until it reaches the diaphragm. The ascending aorta develops as a component of the primitive heart tube.  The primitive heart develops from five dilations: the truncus arteriosus, conus cordis, primitive ventricle, primitive atrium, and the sinus venosus.  The truncus arteriosus forms the basis for developing the ascending aorta and pulmonary trunk, beginning during the fifth week of development. The truncus starts as a single outflow tract from the right and left ventricles but is eventually divided by the aorticopulmonary septum into separate vascular outflow channels. The truncal and conal ridges are invaded by neural crest cells, leading to spiraling that forms the aorticopulmonary septum. The arch of the aorta develops from multiple structures. The portion of the arch proximal to the brachiocephalic trunk arises directly from the aortic sac. The medial area of the arch, between the brachiocephalic trunk and the left common carotid artery, arises from the left fourth aortic arch. The portion of the arch distal to the left common carotid artery arises from the dorsal aorta. The descending aorta arises from the dorsal aortae. Early in development, paired right and left dorsal aortae are confluent with the aortic sac.  The right and left dorsal aortae later fuse along vertebral levels T4 to L4, forming a single, continuous dorsal aorta. The dorsal aorta ultimately gives off many vital branches, including intersegmental, splanchnic or visceral, and umbilical arteries. The dorsal aorta in this region is later referred to as the descending thoracic and abdominal aorta.