Pharyngeal arches

The pharyngeal apparatus

Pharyngeal arches appear in week 4 as bars on the side of the head and neck. Each arch has an outer ectodermal cleft, an inner endodermal pouch, and a core of mesoderm and neural crest containing its own nerve, artery, cartilage and muscles. Muscles keep the nerve of their arch wherever they migrate, which is why the arch table predicts nerve supply. (The 5th arch is rudimentary.)

Arches

• The left recurrent laryngeal nerve hooks under the ligamentum arteriosum because the left 6th arch artery persists as the ductus; on the right the 5th and 6th arch arteries vanish, so it hooks under the subclavian (4th arch).
• The auricle forms from six hillocks, three on the 1st arch (tragus, crus of helix) and three on the 2nd arch (rest of the helix, antihelix, antitragus). Preauricular sinuses come from imperfect hillock fusion.
• First arch syndromes: Treacher Collins (mandibular and zygomatic hypoplasia, ear defects, down-slanting eyes) and Pierre Robin sequence (micrognathia, glossoptosis, U-shaped cleft palate, airway obstruction).

Pouches (endoderm)

• The inferior parathyroids (3rd pouch) are dragged down with the descending thymus, so they finish below the 4th pouch glands and have a more variable position (sometimes in the mediastinum).
• DiGeorge syndrome (22q11.2 deletion): 3rd and 4th pouches and the conotruncal neural crest fail. Features: absent thymus with T-cell deficiency (recurrent viral and fungal infections), absent parathyroids with hypocalcaemia and tetany, conotruncal heart defects (truncus arteriosus, tetralogy, interrupted arch), cleft palate and facial anomalies.

Clefts (ectoderm)

• 1st cleft: the only one that persists, as the external acoustic meatus; its outer layer forms the tympanic membrane epithelium.
• The 2nd arch grows down over the 2nd–4th clefts, enclosing them in the cervical sinus, which normally disappears.
• Branchial cyst: remnant of the cervical sinus (or 2nd cleft); smooth swelling at the anterior border of sternocleidomastoid at the junction of its upper and middle thirds, in young adults; aspirate contains cholesterol crystals.
• Branchial fistula: opening at the lower anterior border of sternocleidomastoid, tracking up between the internal and external carotid arteries, over IX and XII, to the tonsillar fossa (2nd pouch).

Tongue

• Anterior two-thirds: two lateral lingual swellings and the tuberculum impar of the 1st arch; general sensation V3 (lingual nerve), taste VII (chorda tympani).
• Posterior third: hypobranchial eminence from the 3rd (and 4th) arches; sensation and taste IX; the 3rd arch overgrows the 2nd, so VII has no general sensory share here.
• Muscles: occipital myotomes, supplied by XII (palatoglossus by X).

Thyroid

• Begins as an endodermal diverticulum at the foramen caecum between the anterior two-thirds and posterior third of the tongue, and descends in front of the hyoid and laryngeal cartilages through the thyroglossal duct, reaching its final place by week 7.
• Thyroglossal cyst: midline, commonest just below the hyoid (subhyoid), moves up on swallowing and on tongue protrusion because the duct is attached to the tongue base; treated by Sistrunk operation, which removes the cyst, tract and middle of the hyoid body.
• Lingual thyroid: failure to descend; may be the only thyroid tissue, so check before excising.
• Pyramidal lobe: remnant of the lower duct.

Face and palate

• The face forms from five swellings around the stomodeum in weeks 4–8: the frontonasal prominence, paired maxillary and mandibular prominences (1st arch); the frontonasal prominence gives medial and lateral nasal prominences.
• The two medial nasal prominences merge to form the intermaxillary segment: philtrum, the four incisor-bearing part of the maxilla, and the primary palate.
• Cleft lip: failure of the maxillary prominence to fuse with the medial nasal prominence on that side; lies lateral to the philtrum and may extend into the primary palate as far as the incisive foramen.
• Secondary palate: the lateral palatine shelves of the maxillary prominences rise above the tongue and fuse with each other, the primary palate and the nasal septum (weeks 7–10). Failure gives cleft palate behind the incisive foramen.

• Cleft lip and palate together form the commonest craniofacial birth defect. An isolated, non-syndromic cleft follows multifactorial inheritance: several genes plus environment, with recurrence risk rising with the number of affected relatives and severity.
• Environmental contributors: maternal smoking, alcohol, anticonvulsants (phenytoin, valproate), isotretinoin (retinoids for acne), folate deficiency.
• Apert syndrome (FGFR2): craniosynostosis with a flat midface, prominent eyes, syndactyly and often cleft palate.

Remember Arch muscles keep their arch nerve: 1st V3, 2nd VII, 3rd IX, 4th and 6th X.

Exam trap The 3rd pouch gives the inferior parathyroids and thymus; the 4th gives the superior parathyroids.

Exam trap Cleft lip is maxillary with medial nasal prominence; cleft palate is the palatine shelves.

How FCPS asks about this

• Bone from the 2nd arch: stapes (also styloid and lesser horn of hyoid); malleus and incus are 1st arch.
• Only muscle of the 3rd arch: stylopharyngeus, supplied by IX; 3rd arch artery is the common carotid.
• Derivative of the 3rd pouch: inferior parathyroids and thymus; absence gives DiGeorge syndrome.
• Unilateral cleft lip with or without the primary palate: failure of maxillary and medial nasal prominences to fuse.
• Inheritance of isolated cleft lip and palate: multifactorial.
• Midline neck lump moving on tongue protrusion: thyroglossal cyst, commonest just below the hyoid.
• Lateral neck cyst at the anterior border of sternocleidomastoid: branchial cyst; fistula opens into the tonsillar fossa.