Pharyngeal arch arteries: third, fourth and sixth arch derivatives
The third, fourth and sixth pharyngeal arch arteries (arteries of paired ridges of tissue in the neck of an early embryo) become different adult vessels.
Remember
The third arch artery forms the common carotid artery and the proximal internal carotid artery (the vessels that supply the head). The fourth forms part of the arch of the aorta (the curve of the aorta) on the left and only the proximal right subclavian artery (the large artery that runs to the arm) on the right, and the sixth forms the proximal pulmonary arteries.
Memory aid
the vessels run down the body as the arches go down: the third arch supplies the head (the carotids), the fourth the aorta and the arm (the right subclavian), and the sixth the lungs (the pulmonary arteries)
The vessels follow the arches down the body, from the head to the aorta and arm and then the lungs.
The pharyngeal arches are paired ridges of tissue in the neck of an early embryo, and each has its own artery. Early on, these arch arteries form a symmetrical set (a pattern shared by both sides). They are then remodelled, which means some segments are kept and others disappear, so the two sides can end up with different adult vessels.
The third arch artery becomes, on both sides, the common carotid artery (the main trunk in the neck that later divides into two branches) and the proximal internal carotid artery (the branch that runs up to supply the brain). Proximal simply means the part nearest the origin of the vessel, so only the first part of the internal carotid comes from the third arch. The external carotid artery is not listed as a third arch derivative, meaning a vessel that comes from that arch.
The fourth arch artery makes different vessels on each side. On the left it forms part of the arch of the aorta, and on the right it forms only the proximal part of the right subclavian artery. If the left fourth arch artery fails to develop or breaks down, part of the aortic arch is missing.
The sixth arch artery forms the proximal pulmonary arteries, the vessels that carry blood from the right side of the heart to the lungs. On the left it also forms the ductus arteriosus, the foetal channel that normally joins the pulmonary artery to the aorta.
The fifth arch disappears within days, which is why the numbering skips from four to six.
Key facts
- Third arch
- forms the common carotid artery and the proximal internal carotid artery, on both sides.
- Left fourth arch
- forms part of the arch of the aorta.
- Right fourth arch
- forms only the proximal part of the right subclavian artery.
- Sixth arch
- forms the proximal pulmonary arteries, and the ductus arteriosus on the left.
- Fifth arch
- disappears within days, so the numbering skips from four to six.
- External carotid artery
- is not listed as a third arch derivative (a vessel that comes from that arch).
Exam traps
- The external carotid artery is not listed as a third arch derivative, because the list of third arch derivatives names only the common carotid and the proximal internal carotid.
- The right subclavian artery does not come from the left fourth arch artery, because the left fourth arch forms part of the arch of the aorta and the right fourth arch forms the proximal part of the right subclavian artery.
- The arch of the aorta does not come from the fourth arch artery on the right, because, of the two fourth arch arteries, only the left one forms part of it.
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