Describe the pharyngeal arches with their nerves, muscles and skeletal derivatives. Describe the derivatives of the pharyngeal pouches.

General Plan

• Six arches form, but the fifth disappears and the sixth is rudimentary externally — so four are visible
• Each arch has a core of mesoderm covered by ectoderm outside and endoderm inside, and contains a cartilage, a muscle component, an artery and a nerve
• The skeletal and connective tissue of the arches is derived from neural crest, and the muscle from mesoderm

Derivatives of the Arches

Derivatives of the Pouches (endoderm)

• The inferior parathyroid comes from the third pouch and the superior from the fourth — the reverse of what the names suggest
• The explanation is the descent of the thymus, which drags the third-pouch parathyroid down past the fourth-pouch gland
• This is why an ectopic inferior parathyroid may lie in the mediastinum, and why the search at surgery goes there

Clefts and Membranes

• Only the first cleft (ectoderm) persists, forming the external acoustic meatus
• The second arch overgrows the second, third and fourth clefts, burying them as the cervical sinus, which normally disappears
• The first membrane becomes the tympanic membrane — ectoderm outside, endoderm inside, with mesoderm between; the only three-layered membrane to persist

The Arteries of the Arches and the Development of the Tongue

• The recurrent laryngeal nerve of each side hooks round the artery of the sixth arch. On the right the sixth arch artery disappears, so the nerve rises to hook round the fourth — the subclavian artery. On the left the sixth persists as the ductus arteriosus, so the nerve stays low, round the arch of the aorta
• The tongue mucosa is arch-derived, and its nerve supply records this — first arch (V) in front, third arch (IX) behind, with taste in front carried by the second arch nerve (VII, chorda tympani) because the second arch contribution was overgrown
• The muscles of the tongue come from occipital myotomes, not from the arches, and are therefore supplied by the hypoglossal nerve

Applied Aspects

• Branchial cyst — a remnant of the cervical sinus; a painless swelling at the anterior border of sternocleidomastoid at the junction of its upper and middle thirds, usually presenting in young adults
• Branchial fistula — opens at the same site externally and into the tonsillar fossa internally, following the second pouch and cleft
• Digeorge syndrome (22q11 deletion) — failure of the third and fourth pouches: absent thymus (T cell deficiency, infection) and absent parathyroids (hypocalcaemic tetany), with cardiac outflow anomalies and facial dysmorphism — all neural crest and pouch derivatives together
• Treacher Collins syndrome — first arch hypoplasia with malar and mandibular underdevelopment and ear anomalies
• Pierre Robin sequence — mandibular hypoplasia causes the tongue to obstruct palatal closure, giving cleft palate and airway obstruction
• Referred otalgia is explained by arch nerves — the ear receives V, VII, IX and X, so disease anywhere in their territories can present as earache with a normal ear
• Ectopic thymus or parathyroid anywhere along the path of descent, from the angle of the mandible to the mediastinum
• Preauricular sinus and skin tags — from faulty fusion of the auricular hillocks of the first and second arches around the first cleft
• Congenital deafness may follow anomalies of the ossicles, which are first and second arch derivatives
• Ankyloglossia (tongue tie) — the frenulum fails to regress, restricting protrusion and interfering with feeding and speech
• Hemifacial microsomia — underdevelopment of first and second arch derivatives on one side, the second commonest facial anomaly after cleft lip and palate
• Retinoic acid and isotretinoin are potent disruptors of neural crest migration, producing craniofacial, cardiac outflow and thymic defects together — a pattern that follows directly from the arch anatomy
• The arch origin explains referred pain and the grouping of anomalies: structures of one arch tend to be affected together because they share an origin, a nerve and a blood supply
• Thyroglossal cyst arises from the median pharyngeal floor rather than from an arch, and lies in the midline — which is how it is distinguished from a branchial cyst at the bedside
• Cystic hygroma — a lymphatic malformation of the posterior triangle from failure of the jugular lymph sacs to connect; it is brilliantly transilluminant and associated with Turner syndrome
• First arch syndromes arise from insufficient neural crest migration, and give the underdeveloped mandible and malar bones of Treacher Collins syndrome
• The middle ear ossicles derive from the first and second arches, so external ear anomalies are frequently accompanied by conductive deafness — hearing must always be tested
• Palatal muscles come from the fourth arch except tensor veli palatini, which is first arch; this is why the tensor is supplied by V and everything else by X
• Pharyngeal pouch derivatives are all epithelial, which is why their failure produces glandular rather than skeletal defects — thymus and parathyroid in DiGeorge syndrome