10 The Pharyngeal Apparatus

1. Pharyngeal Apparatus Formation (4th week embryo)

  • Pharyngeal arches are bulges in the head/neck region.

  • Each arch contains:

    • Mesoderm core (cartilage, artery, nerve)

    • Ectoderm (outer layer)

    • Endoderm (inner lining)

Mnemonic: CAP – Covers from C (clefts/ectoderm), A (arches/mesoderm), to P (pouches/endoderm)

  • There are arches 1–6 (but arch 5 disappears early).

2. Arch Innervation and Derivatives

Each arch is innervated by a cranial nerve. The arch-derived muscles stay innervated by the same nerve.

1st Arch (CN V - Trigeminal)

  • Muscles: mastication

  • Cartilage: malleus, incus

  • Linked to Treacher-Collins and Pierre-Robin syndromes

2nd Arch (CN VII - Facial)

  • Muscles: facial expression

  • Cartilage: stapes, styloid process, part of hyoid

3rd Arch (CN IX - Glossopharyngeal)

  • Muscle: stylopharyngeus

  • Cartilage: part of hyoid

4th Arch (CN X - Vagus, superior laryngeal branch)

  • Muscles: pharynx, cricothyroid

  • Cartilage: thyroid, cricoid

5th Arch – disappears
6th Arch (CN X - recurrent laryngeal branch)

  • Muscles: larynx, esophagus (upper)

Vestigial cartilage remnants occasionally seen

Nerve mnemonic: 5791010

3. Derivatives of Clefts and Pouches

Pharyngeal Clefts (outside)

  • 1st cleft forms external auditory meatus

  • Others usually disappear due to growth of 2nd arch covering them.

    • If persistent and does not degenerate → branchial cyst, cervical sinus, or fistula (see photos below)

  • Branchial cyst: 2nd pharyngeal arch failed to cover 3rd and 4th, leaving a remnant that forms a fluid-filled cyst along the anterior sternocleidomastoid.

  • Cervical sinus: Persistent space from incomplete fusion of pharyngeal arches; should regress during development.

  • Branchial fistula: Abnormal tract from persistent cervical sinus, opening to skin or pharynx.

Branchial Cyst - Occurs when 2nd arch does not obliterate the rest of the pharygneal clefts

Pharyngeal Pouches (inside)

Mnemonic: 1A, 2P, 3TIP, 4ParaSP

  • 1st pouch = auditory tube, middle ear

  • 2nd = palatine & pharyngeal tonsils

  • 3rd = thymus + inferior parathyroids (migrate down)

  • 4th = superior parathyroids + parafollicular (C) cells of thyroid

Accessory thymic tissue and ectopic parathyroid glands fairly common

4. Tongue Development

  • Comes from 3 swellings that merge (Lateral lingual)

Anterior 2/3 (oral part) — Lateral Lingual Swellings

  • From 1st arch

  • Sensation: CN V

  • Taste: CN VII

Posterior 1/3 (pharyngeal part) — Hypobranchial eminience

  • From 3rd arch

  • Sensation + taste: CN IX

  • Muscle: CN XII (Hypoglossal), except 1 exception

5. Thyroid Development

  • Begins at foramen cecum on tongue.

  • Then, migrates downward through thyroglossal duct, eventually settling in the neck.

    • Picks up cells from 4th pouch on the way.

Issues with Thyroid development:

  • Thyroglossal duct cysts: If the thyroglossal duct doesn’t disappear, it can form a cyst in the tongue or front of the neck. If infected, it might drain through the skin called a thyroglossal duct sinus.

  • Ectopic thyroid: If the thyroid doesn’t migrate properly, it might end up in the wrong place or leave behind extra thyroid tissue (accessory thyroid) along the path it took.

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