What is the pharyngeal apparatus?
The pharyngeal (branchial) apparatus develops in the fourth week in the region between the head and the chest, and by about week 7 its structures have differentiated and migrated. 'Branchial' means gill-like; because humans never form gills, pharyngeal is the preferred term. It forms the lower face, neck, palate and part of the upper thorax.
| Part | Germ layer | What it forms |
|---|---|---|
| Arches (6 pairs; 5th disappears) | Ectoderm outside, endoderm inside, mesoderm + neural crest core | Mesoderm → muscles and arteries; neural crest → bones and cartilage |
| Pouches (between arches, inside) | Endoderm | Middle ear, tonsil, thymus, parathyroids, C cells |
| Clefts / grooves (between arches, outside) | Ectoderm | External auditory meatus (1st); others form the cervical sinus |
| Membranes | Between pouch and cleft | 1st → tympanic membrane; others obliterate |
Molecular control: Hox genes are expressed in arches 2 to 6, each arch with its own Hox group, and their transcription is strongly influenced by retinoic acid acting on response elements in their promoters. Dlx genes pattern the first arch and lower jaw, FGF drives endodermal outpocketing to form the pouches, and TBX1 is expressed in all four pouches.
What does each pharyngeal arch form?
| Arch | Nerve | Muscles | Skeletal (neural crest) | Artery |
|---|---|---|---|---|
| 1st (mandibular) — Meckel's cartilage | Mandibular nerve (V3) | Muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini | Malleus, incus, anterior ligament of malleus, sphenomandibular ligament, spine of sphenoid | Part of maxillary artery |
| 2nd (hyoid) — Reichert's cartilage | Facial nerve (VII) | Muscles of facial expression, stapedius, stylohyoid, posterior belly of digastric | Stapes, styloid process, stylohyoid ligament, lesser horn and upper body of hyoid | Hyoid and stapedial arteries |
| 3rd | Glossopharyngeal (IX) | Stylopharyngeus only | Greater horn and lower body of hyoid | Common carotid and first part of internal carotid |
| 4th | Vagus — superior laryngeal and pharyngeal branches | Pharyngeal constrictors, cricothyroid, levator veli palatini | Thyroid, corniculate and cuneiform cartilages | Left: arch of aorta; right: proximal right subclavian |
| 6th | Vagus — recurrent laryngeal | Intrinsic laryngeal muscles (except cricothyroid) | Arytenoid cartilages | Pulmonary arteries and ductus arteriosus (left) |
What do the aortic arch arteries become?
- 1st arch artery regresses early; a remnant forms part of the maxillary artery.
- 2nd arch artery regresses; remnants form the hyoid and stapedial arteries.
- 3rd arch artery forms both common carotid arteries and the proximal internal carotid arteries (including the carotid sinus).
- 4th arch artery — left forms the part of the aortic arch between the left common carotid and left subclavian arteries; right forms the proximal right subclavian artery.
- 5th arch artery never forms or regresses.
- 6th arch artery — ventral parts form the pulmonary arteries; the left dorsal part persists as the ductus arteriosus.
What do the pharyngeal pouches and clefts form?
| Pouch | Derivatives |
|---|---|
| 1st | Tubotympanic recess → epithelium of auditory (Eustachian) tube, middle ear cavity and mastoid air cells |
| 2nd | Epithelium of the palatine tonsil |
| 3rd | Thymus (ventral wing) and inferior parathyroid glands (dorsal wing) |
| 4th | Superior parathyroid glands |
| 4th/5th (ultimobranchial body) | Parafollicular C cells of the thyroid (calcitonin) |
| Structure | Derivative |
|---|---|
| 1st cleft | External auditory meatus and outer epithelium of the tympanic membrane |
| 2nd–4th clefts | Fuse into the cervical sinus (of His), which normally obliterates |
| 1st membrane | Tympanic membrane |
| 2nd–5th membranes | Obliterated |
Which ear structures come from the first and second arches?
- Malleus and incus — Meckel's cartilage of the 1st arch.
- Stapes — partly Reichert's cartilage of the 2nd arch and partly neural crest.
- Tensor tympani — 1st arch mesoderm, supplied by the mandibular nerve; stapedius — 2nd arch, supplied by the facial nerve.
- Auricle — six auricular hillocks: hillocks 1–3 from the 1st arch (tragus, helix, cymba concha) and 4–6 from the 2nd arch (concha, antihelix, antitragus).
- External auditory meatus — 1st cleft; middle ear and Eustachian tube — 1st pouch.
What are branchial cleft cysts, sinuses and fistulas?
Remnants of the clefts and pouches may persist as a cyst (no opening), a sinus (one opening) or a fistula (two openings). Most branchial cleft cysts come from the second cleft and are usually found in children under 10 years. About 10% are bilateral.
| Cleft | Typical location / course | Clinical point |
|---|---|---|
| 1st | Near the ear, parotid gland and external auditory canal | Closely related to the facial nerve — expose it (superficial parotidectomy approach) during excision; Work classification |
| 2nd (commonest) | Cyst at the anterior border of the sternocleidomastoid; a pit low on the anterior border if a sinus reaches the skin | Fistula tract runs from the skin to the palatine tonsil, between the internal and external carotid arteries, superficial to CN IX and XII; Bailey types I–IV (type II commonest) |
| 3rd / 4th | Lower neck, near thyroid; open into the piriform sinus | 4th-cleft anomalies are more often left-sided; risk to the recurrent laryngeal nerve during surgery |
Why does DiGeorge syndrome affect the thymus, parathyroids and heart?
DiGeorge syndrome (22q11.2 deletion syndrome) is the commonest microdeletion syndrome, seen in about 1 in 2,000–4,000 births. Loss of genes in the deleted region — especially TBX1, which regulates development of structures from the third and fourth pharyngeal pouches and arches — disturbs pouch development and the migration of cardiac neural crest cells into the outflow tract.
| Developmental defect | Clinical feature |
|---|---|
| Thymus (3rd pouch) hypoplasia | T-cell immunodeficiency — viral and fungal infections |
| Parathyroid (3rd and 4th pouch) hypoplasia | Hypocalcaemia — cramps, spasms, seizures, Chvostek and Trousseau signs |
| Cardiac neural crest / arch arteries | Conotruncal heart defects, septal defects, interrupted aortic arch |
| Arch mesenchyme | Abnormal facies, cleft palate |
| Later life | Developmental delay; psychosis and schizophrenia may emerge in adolescence or adulthood |
- Diagnosis: chromosomal microarray is the first-tier, gold-standard test for the 22q11.2 deletion; TREC-based newborn screening picks up severe T-cell deficiency early.
- Complete DiGeorge (no thymus) is fatal by 12–24 months without thymus transplantation, now FDA-approved using cultured thymus tissue.
- Hypocalcaemia is treated with calcium supplements and calcitriol; infants needing heart surgery receive irradiated, CMV-negative blood products.
What are the common exam traps on pharyngeal arches?
- Stylopharyngeus is the only muscle of the 3rd arch — supplied by the glossopharyngeal nerve.
- Cricothyroid belongs to the 4th arch (superior laryngeal nerve); all other intrinsic laryngeal muscles belong to the 6th (recurrent laryngeal).
- Tensor veli palatini (V3, 1st arch) is the exception among palatal muscles; levator veli palatini is 4th arch (vagus).
- Lesser horn of hyoid = 2nd arch; greater horn = 3rd arch.
- Thymus and inferior parathyroids share the 3rd pouch; C cells come from the ultimobranchial body (4th/5th pouch).
- The 5th arch does not develop in humans; the 6th keeps its number for comparative anatomy.
- Ductus arteriosus = left 6th arch artery; right subclavian (proximal) = right 4th.
Related reading: cranial nerves for the adult course of V, VII, IX and X, triangles of the neck for where branchial cysts present, and fetal circulation for the ductus arteriosus.