Pharyngeal arches

Pharyngeal arches, historically called branchial arches, appear in the fourth week of development as bars of tissue flanking the primitive pharynx. Each arch contains a core of mesoderm and neural crest cells and is supplied by its own cranial nerve and aortic arch artery. Humans form arches 1, 2, 3, 4, and 6; the fifth arch regresses without contributing structures. Between the arches lie pharyngeal clefts externally and pharyngeal pouches internally, which give rise to their own derivatives.

Each arch produces a predictable set of tissues. The first arch is innervated by the trigeminal nerve (V) and forms the muscles of mastication along with the maxilla, mandible, malleus, and incus. The second arch is innervated by the facial nerve (VII) and forms the muscles of facial expression, the stapes, the styloid process, and the lesser horn of the hyoid. The third arch is innervated by the glossopharyngeal nerve (IX) and forms the stylopharyngeus muscle and the greater horn and lower body of the hyoid. The fourth and sixth arches are innervated by branches of the vagus nerve (X) — the superior laryngeal nerve for the fourth and the recurrent laryngeal nerve for the sixth — and together form the laryngeal cartilages and the pharyngeal and laryngeal musculature.

The arterial derivatives follow the same numbering. The third aortic arch becomes the common carotid arteries, the fourth becomes the aortic arch on the left and the proximal right subclavian artery on the right, and the sixth becomes the pulmonary arteries and the ductus arteriosus. That asymmetry explains why the left recurrent laryngeal nerve loops under the aortic arch while the right loops under the right subclavian artery.

Pharyngeal arch derivatives are high-yield USMLE Step 1 embryology material. Questions typically give a defective structure or a cranial nerve palsy and ask which arch is implicated, so memorize the nerve, muscle, cartilage, and artery of each arch as a single set.