Pharyngeal Arches and Their Derivatives
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📚 Pharyngeal Arches and Their Derivatives
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Around the 3rd week of intrauterine life, the embryo undergoes cranio-caudal and lateral folding.
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By the 4th week, the region between the stomodeum and the cardiac bulge elongates.
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This area forms the neck region, due to bilateral mesodermal thickenings → Pharyngeal arches.
🔬 Origin of Pharyngeal Arches
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Neural crest cells interact with pharyngeal endoderm and migrate into each arch.
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Each pharyngeal arch consists of:
- Mesoderm (muscle myoblasts)
- Neural crest cells (skeletal & connective tissue)
- Endodermal lining (internally)
- Ectodermal lining (externally)
🧱 Pharyngeal Apparatus Components
| Component | Germ Layer | Location | Description |
|---|---|---|---|
| Pharyngeal Cleft | Ectoderm | Outer surface | Forms grooves between arches externally |
| Pharyngeal Arch | Mesoderm | Middle | Core containing artery, nerve, cartilage |
| Pharyngeal Pouch | Endoderm | Inner surface | Forms evaginations between arches inside |
Mnemonic: From outside inwards (CAP)
Cleft → Arch → Pouch (Ectoderm → Mesoderm → Endoderm)
🧩 Structure of a Single Pharyngeal Arch
Each arch contains:
- Cartilage (skeletal derivative)
- Aortic arch artery
- Striated muscle (supplied by that arch's nerve)
- Cranial nerve (motor + sensory)
Muscle derivatives may:
- Migrate away from pharyngeal region
- Retain innervation from the original arch
🧠 Nerve Supply: Post- and Pre-trematic Nerves
Morphologically, each arch is supplied by two nerves:
- Post-trematic Nerve: Runs along the cranial (anterior) border of the arch. This is typically the main nerve referred to.
- Pre-trematic Nerve: Runs along the caudal (posterior) border of the arch. Present in the first and second arches.
Can you name Pre Trematic and Post Trematic nerves of each arch?
📋 Derivatives of the Pharyngeal Arches
| Pharyngeal Arch | Nerve | Artery | Cartilage/Skeletal Elements | Muscles |
|---|---|---|---|---|
| 1st (Mandibular) | Mandibular (CN V3) | Maxillary | Maxilla, Malleus, Incus, Anterior ligament of malleus, Sphenomandibular ligament | Mylohyoid, Anterior belly of digastric, Tensor tympani, Tensor veli palatini, Muscles of mastication (Masseter, Temporalis, Medial & Lateral Pterygoid) |
| 2nd (Hyoid) | Facial (CN VII) | Stapedial, Hyoid | Lesser cornu of hyoid, Upper part of body of hyoid, Stapes superstructure, Styloid process, Stylohyoid ligament | Muscles of facial expression, Stapedius, Stylohyoid, Posterior belly of digastric |
| 3rd | Glossopharyngeal (CN IX) | Common Carotid, Proximal Internal Carotid | Greater cornu of hyoid, Lower part of body of hyoid | Stylopharyngeus |
| 4th | Superior Laryngeal (CN X) | Arch of aorta (left), Subclavian (right), Part of Pulmonary arteries | Thyroid, Cuneiform (parts of cricoid, arytenoid) | Pharyngeal constrictors, Cricothyroid, Levator veli palatini, Palatoglossus, Palatopharyngeus |
| 6th | Recurrent Laryngeal (CN X) | Ductus arteriosus (left), Proximal Pulmonary arteries | Cricoid, Arytenoid, Corniculate (parts of thyroid) | Intrinsic muscles of larynx (except Cricothyroid), Parts of pharyngeal constrictors |
(Note: The mnemonic "MADLY STUDYING PLAN VACATION" is sometimes used, linking M-1, S-2, P-3, V-4/6, but is not a perfect fit for all derivatives).
🦠 Fate of the Pharyngeal Pouches (Endoderm)
There are four pharyngeal pouches. Their endodermal linings give rise to:
- 1st Pouch:
Elongates to form the Tubotympanic Recess.
Proximal part remains tubular → Eustachian Tube (Auditory Tube).
Distal part expands → Middle Ear Cavity, Mastoid Antrum, and the inner layer of the Tympanic Membrane.
- 2nd Pouch:
Ventral part forms the Palatine Tonsil. Endoderm forms the epithelial lining of the tonsillar fossa and crypts. Lymphoid tissue infiltrates later.
Dorsal part contributes to the Tubotympanic Recess.
- 3rd Pouch:
Ventral tubular part → Thymus.
Dorsal bulbar part → Inferior Parathyroid Gland (Parathyroid III - located inferiorly despite arising from the 3rd pouch, due to migration with the thymus).
- 4th Pouch:
Ventral part forms the Ultimobranchial Body → merges with the thyroid to form the Parafollicular C cells of the thyroid (produce calcitonin).
Dorsal bulbar part → Superior Parathyroid Gland (Parathyroid IV - located superiorly despite arising from the 4th pouch).
💧 Fate of the Pharyngeal Clefts (Ectoderm)
There are initially four pharyngeal clefts on the external surface.
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1st Cleft: Persists and gives rise to the Epithelial lining of the External Auditory Canal (EAC) and the outer layer of the Tympanic Membrane.
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2nd, 3rd, and 4th Clefts: The second arch grows rapidly and overgrows the succeeding arches. This fuses the second arch with the caudal arches and obliterates the 2nd, 3rd, and 4th clefts. The space created by this overgrowth is the Cervical Sinus.
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Normally, the Cervical Sinus obliterates.
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Failure of the cervical sinus to obliterate can result in Branchial Cysts, Sinuses, or Fistulae (often opening along the anterior border of the sternocleidomastoid muscle).
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