Pharyngeal (Branchial) Apparatus
Arches, pouches, and clefts - and the syndromes when they fail
Overview: arches, pouches and clefts
Three components. The pharyngeal apparatus of the embryonic neck has three parts, each named by its germ layer.
- Arches (mesoderm plus neural crest) form muscles, cartilage, arteries and nerves.
- Pouches (endoderm, on the inside) form glands and the middle ear.
- Clefts (grooves of ectoderm, on the outside) form the external ear canal and, if they persist, cysts.
There are six arches, numbered 1, 2, 3, 4 and 6 in practice, because the 5th regresses.
CAP runs from outside to inside: Clefts = ectoderm, Arches = mesoderm and neural crest, Pouches = endoderm.
Pharyngeal arches: nerves, muscles and cartilage
The nerve rule. Each arch brings its own cranial nerve. Arch 1 = CN V3 (with V2), arch 2 = CN VII, arch 3 = CN IX, and arches 4 and 6 = CN X. A muscle keeps the nerve of the arch it came from, so motor innervation follows the arch of origin.
| Arch | Nerve | Muscles | Cartilage / skeleton |
|---|---|---|---|
| 1 (mandibular) | CN V2 and V3 | Muscles of mastication (temporalis, masseter, pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini | Meckel cartilage: malleus, incus, mandible |
| 2 (hyoid) | CN VII | Muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric, platysma | Reichert cartilage: stapes, styloid process, lesser horn of hyoid |
| 3 | CN IX | Stylopharyngeus only | Greater horn and lower body of hyoid |
| 4 | CN X (superior laryngeal nerve) | Most pharyngeal constrictors, cricothyroid, levator veli palatini | Thyroid cartilage |
| 6 | CN X (recurrent laryngeal nerve) | All intrinsic laryngeal muscles except cricothyroid | Cricoid and arytenoid cartilages |
Laryngeal nerve injuries. The recurrent laryngeal nerve (arch 6) supplies every intrinsic laryngeal muscle except cricothyroid. Its injury during thyroid surgery causes hoarseness. The superior laryngeal nerve (arch 4) supplies cricothyroid, which tenses the vocal cords. Its injury mainly weakens pitch and projection.
First-arch syndromes. Treacher Collins syndrome is a neural crest defect of the 1st arch, causing mandibular and zygomatic hypoplasia, ear malformations and conductive hearing loss. Pierre Robin sequence is micrognathia with a posteriorly displaced tongue and cleft palate.
Aortic arch arteries
- 1st and 2nd: largely regress. Small remnants contribute to the maxillary artery (1st) and the stapedial and hyoid arteries (2nd).
- 3rd: common carotid artery and proximal internal carotid artery.
- 4th: part of the aortic arch on the left; the proximal right subclavian artery on the right.
- 6th: proximal pulmonary arteries on both sides and, on the left only, the ductus arteriosus.
This asymmetry explains the recurrent laryngeal nerves. The left nerve hooks under the ductus (ligamentum) arteriosus, a 6th-arch structure, beside the aortic arch, which comes from the 4th arch. The right nerve hooks under the right subclavian artery, a 4th-arch structure, because the distal right 6th arch disappears.
Pouches, clefts and DiGeorge syndrome
| Pouch | Derivatives |
|---|---|
| 1st | Middle ear cavity, eustachian tube, mastoid air cells |
| 2nd | Epithelial lining of the palatine tonsil |
| 3rd | Inferior parathyroids (dorsal wing) and thymus (ventral wing) |
| 4th | Superior parathyroids (dorsal wing) and the ultimobranchial body, which gives rise to parafollicular C cells (classically taught as neural crest cells that migrate into it) |
"Three below four." The 3rd-pouch parathyroids end up as the inferior glands, below the 4th-pouch superior glands, because the descending thymus drags them down.
Clefts. The 1st cleft forms the external auditory meatus. Clefts 2–4 are normally obliterated by the overgrowing cervical sinus. If they persist, they form a branchial (lateral neck) cleft cyst, most often from the 2nd cleft.
DiGeorge syndrome. A 22q11.2 microdeletion disrupts development of the 3rd and 4th pouches and the neural crest.
- Thymic aplasia: absent thymic shadow, T-cell deficiency and recurrent viral and fungal infections.
- Parathyroid aplasia: hypocalcemia with tetany or neonatal seizures.
- Conotruncal heart defects: tetralogy of Fallot and truncus arteriosus.
CATCH-22: Cardiac defects, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia, chromosome 22.
Neck cysts and facial clefts
| Feature | Branchial cleft cyst | Thyroglossal duct cyst |
|---|---|---|
| Origin | Persistent 2nd pharyngeal cleft | Remnant of the thyroid's descent from the foramen cecum |
| Location | Lateral neck, anterior to the sternocleidomastoid | Midline neck |
| With swallowing or tongue protrusion | Does not move | Moves upward |
Ectopic thyroid tissue along the thyroglossal tract, including a lingual thyroid at the tongue base, may be the patient's only functioning thyroid. Confirm a normal gland with imaging before excision.
Cleft lip vs cleft palate. The two defects arise by different mechanisms and can occur separately.
- Cleft lip: the maxillary prominence fails to fuse with the medial nasal prominence.
- Cleft palate: the lateral palatine shelves fail to fuse with each other, with the nasal septum or with the median palatine process (primary palate).