Fate of the aortic arches and asymmetry of the recurrent laryngeal nerves 

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Fate of the aortic arches (arteries)

Figure 32.28. MOORE ET AL., THE DEVELOPING HUMAN, FIGURE 13-39.

Five sets of aortic arches (1, 2, 3, 4, and 6) connect the aortic sac to the dorsal aortae. The aortic arches course into the neck and pass through swellings of mesenchyme called pharyngeal arches. The 5th pharyngeal arch disappears, so it has no aortic arch. The arteries of the 1st and 2nd arches are rudimentary, so not worthy of discussion. The 3rd, 4th, and 6th aortic arches remain and are remodeled:

The third aortic arches are large vessels. The segments of the left and right dorsal aortae between the third and fourth arches disappear. Because of this, blood destined for the head must first pass through the third aortic arches. Thus, the third aortic arches give rise to the definitive right and left common carotid arteries and the proximal portions of the internal carotid arteries.

The fourth aortic arches undergo asymmetric remodeling.

    • Right: The right dorsal aorta and right fourth aortic arch are relocated and become associated with the right upper limb. The fourthaortic arch on the right gives rise to the proximal part of the right subclavian artery. The distal right subclavian artery is formed partially by the right dorsal aorta. The aortic sac develops into the brachiocephalic artery (trunk).
    • Left: The left dorsal aorta remains in the thorax, where it forms the descending thoracic aorta. The left 4th aortic arch contributes tothe central part of the arch of the aorta.

The sixth aortic arches branch from the caudal-most part of the aortic sac. The proximal ends of the 6th aortic arches attach to the pulmonary trunk after it splits from the truncus arteriosus.

    • The right sixth aortic arch gives rise to the proximal part of the right pulmonary artery.
    • The left sixth arch remains connected to the left dorsal aorta, thus linking the pulmonary and aortic vessels. It gives rise to the proximal part of the left pulmonary artery and to the ductus arteriosus (described earlier).
The curious asymmetry of the recurrent laryngeal nerves

Originally, the recurrent laryngeal nerves (branches of the vagus nerves) looped under the sixth aortic arches to supply the muscles andmucosa of the larynx (voice box). As the embryo grows and the larynx is translocated to the neck, the nerves are lengthened.

    • The origin of the left recurrent laryngeal nerve remains in the thorax, looped under the vestige of the left sixth aortic arch = the ligamentum arteriosum.
    • No ligamentum arteriosum develops on the right and very little of the right sixth aortic arch is preserved. Since there is no fifth aortic arch,the origin of the right recurrent laryngeal nerve moves superiorly and loops under the derivative of the fourth aortic arch = the proximal right subclavian artery. Thus, the origin of the recurrent laryngeal nerve from the vagus is much higher on the right (in the base of the neck), than on the left (in the mediastinum).

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