BRANCHIAL APPARATUS - SYSTEMATIC ANATOMY

1. INTRODUCTION AND ETYMOLOGY

2. DEVELOPMENT - TIMELINE

3. STRUCTURAL COMPONENTS OF EACH ARCH

1. A bar of cartilage (skeletal element) - forms bone, cartilage, or connective tissue
2. Musculature (derived from mesoderm of the arch)
3. An associated aortic arch artery
4. A cranial nerve - the "post-trematic nerve" of that arch; additionally, a "pre-trematic nerve" from the arch below supplies the inner endodermal surface of the arch above

• External surface covered by ectoderm
• Internal surface lined by endoderm
• Core is mesodermal (pharyngeal arch musculature)

4. SCHEMATIC DIAGRAM OF THE BRANCHIAL APPARATUS

5. THE BRANCHIAL ARCHES - INDIVIDUAL DERIVATIVES

Arch I - Mandibular Arch (First Pharyngeal Arch)

• Proximal portion - forms the ramus of mandible (via remodelling)
• Distal portion - withers; body of mandible forms by intramembranous ossification
• Also gives: Malleus (except manubrium), Incus (except long process), Sphenomandibular ligament, Anterior malleolar ligament

• Muscles of mastication: temporalis, masseter, medial pterygoid, lateral pterygoid
• Mylohyoid
• Anterior belly of digastric
• Tensor tympani
• Tensor veli palatini

Arch II - Hyoid Arch (Second Pharyngeal Arch)

• Pre-trematic nerve: Chorda tympani (from CN VII) runs to the first arch territory

• Proximal portion forms: Stapes suprastructure, Styloid process of temporal bone, Manubrium of malleus, Long process of incus
• Central portion withers leaving: Stylohyoid ligament (fibrous remnant)
• Distal portion forms: Lesser cornu and superior body of hyoid bone

• Muscles of facial expression (all)
• Stylohyoid
• Posterior belly of digastric
• Stapedius
• Auricular muscles, occipitalis

Arch III (Third Pharyngeal Arch)

• Pre-trematic nerve: Jacobson's nerve (tympanic branch of CN IX) runs to the second arch territory (CN VII)

Arch IV and Arch VI (Fourth, Fifth, and Sixth Arches)

• Fourth arch: superior laryngeal nerve
• Sixth arch: recurrent laryngeal nerve

• Thyroid cartilage
• Cricoid cartilage
• Arytenoid cartilage
• Corniculate cartilages
• Cuneiform cartilages

• Fourth arch artery (right) - forms right subclavian artery
• Fourth arch artery (left) - forms the arch of the aorta (between left common carotid and left subclavian)
• Sixth arch artery - forms pulmonary arteries; the left sixth arch persists as the ductus arteriosus until birth

Summary Table: Branchial Arch Derivatives

6. BRANCHIAL CLEFTS (ECTODERM - EXTERNAL)

Fate of the Clefts

• Forms the external auditory canal (EAC)
• Together with the closing plate of the first pouch, forms the outer epithelial layer of the tympanic membrane
• The ectodermal lining of the EAC is derived from this cleft

• Normally obliterated as the second arch (hyoid arch) grows caudally to overgrow and submerge the third and fourth arches
• This creates a potential space called the cervical sinus of His, which normally disappears completely
• Failure of obliteration leads to branchial cleft cysts, sinuses, or fistulae

7. BRANCHIAL POUCHES (ENDODERM - INTERNAL)

Pouch I (First Pharyngeal Pouch)

• Elongates laterally to form the tubotympanic recess
• Gives rise to: Eustachian tube (proximal portion) and middle ear cavity (distal/lateral portion)
• The lateral end, together with the closing plate of the first cleft, forms the inner layer of the tympanic membrane

Pouch II (Second Pharyngeal Pouch)

• Endodermal layer forms the epithelial lining of the tonsillar fossa
• Contributes to the palatine tonsil (epithelium; mesenchymal elements incorporate the lymphoid tissue of the tonsil)
• Residual contact between second pouch endoderm and second cleft ectoderm = closing plate that normally disappears

Pouch III (Third Pharyngeal Pouch)

• Divided into superior and inferior portions:
  • Superior portion → inferior parathyroid glands (parathyroid III)
  • Inferior portion → thymic tissue (migrates caudally and medially into retrosternal mediastinum to form the thymus)
• Because the third pouch derivatives start superiorly and descend, the inferior parathyroid glands end up inferior to the superior parathyroid glands despite their name

Pouch IV (Fourth Pharyngeal Pouch)

• Endoderm forms the superior parathyroid glands (parathyroid IV)
• These descend only slightly, ending up in the superior poles of the thyroid gland

Pouch V (Fifth Pharyngeal Pouch / Ultimobranchial Body)

• Sometimes called the "fifth pouch," "sixth pouch," or ultimobranchial body
• Infiltrated by neural crest cells that migrate to this region
• Incorporated into the thyroid gland as parafollicular C cells (calcitonin-secreting)

Summary Table: Pharyngeal Pouch Derivatives

8. CERVICAL SINUS OF HIS

• Formed when the second arch grows caudally to overgrow the third and fourth arches
• Creates a transient ectodermal-lined space lateral to the neck
• Normally obliterates completely by week 7
• Failure of obliteration results in second branchial cleft cysts (most common branchial anomaly)
• Forms the basis of the cervical sinus theory of branchial cyst formation

9. TONGUE DEVELOPMENT FROM BRANCHIAL ARCHES

• Anterior 2/3 of tongue: lateral lingual swellings + tuberculum impar from arch I
  • Sensory: lingual nerve (CN V3)
  • Taste: chorda tympani (CN VII branch)
• Posterior 1/3 of tongue: copula/hypobranchial eminence from arches II, III, and IV
  • Predominant innervation: CN IX (from arch III)
  • Posterior tongue (vallecula) and epiglottis: superior laryngeal nerve (CN X, arch IV)
• Junction: V-shaped terminal sulcus (sulcus terminalis); the foramen caecum marks the apex
• Intrinsic tongue muscles: derived from occipital somite myoblasts that migrate into the tongue, innervated by CN XII (hypoglossal)

10. EAR DEVELOPMENT FROM THE BRANCHIAL APPARATUS

External Ear

• External auditory canal (EAC): from first branchial cleft ectoderm
• Pinna/auricle: from six auricular hillocks (three from arch I, three from arch II) surrounding the first cleft; fully formed by day 60 of gestation
• The first branchial arch artery has a developmental relationship to the pinna (juvenile angiofibroma considerations)

Middle Ear

• Middle ear cavity and Eustachian tube: from first pharyngeal pouch endoderm (tubotympanic recess)
• Tympanic membrane: inner layer from first pouch endoderm, outer layer from first cleft ectoderm, middle fibrous layer from mesoderm
• Ossicular chain:
  • Malleus and Incus: first arch (Meckel's cartilage) - except manubrium of malleus and long process of incus which are from second arch
  • Stapes suprastructure: second arch (Reichert's cartilage)
  • Stapes footplate: largely from the otic capsule (NOT branchial arch)

11. ANOMALIES OF THE BRANCHIAL APPARATUS

Terminology

• Cyst: enclosed epithelial-lined cavity with no opening to skin or pharynx
• Sinus: blind-ended tract opening onto one epithelial surface (external or internal)
• Fistula: tract running from one epithelial surface to another (both external opening on neck skin AND internal opening in pharynx/ear)

First Branchial Cleft Anomalies (rare, ~5% of all branchial anomalies)

• Work Type I: Duplication of the external auditory canal - track in pre/post-auricular region running parallel to EAC, lined by squamous epithelium only; external opening anterior and inferior to tragus
• Work Type II: Duplication of the first arch AND cleft - track runs from skin near angle of mandible to EAC or conchal bowl; lined by squamous epithelium + skin appendages; inconstant relationship to the facial nerve (may run deep or superficial to it)
• Surgery: Complete excision; must identify and preserve facial nerve

Second Branchial Cleft Anomalies (most common, ~95% of all branchial cysts)

• Result from failure of obliteration of the cervical sinus or remnant of second cleft
• External opening: along anterior border of SCM in its lower third
• Internal opening: tonsillar fossa (second pharyngeal pouch territory)
• Track passes between external and internal carotid arteries, lateral to CN IX and XII, medial to CN XI, crossing the hypoglossal nerve
• Lined by stratified squamous epithelium (90%); 10% by respiratory epithelium; 80% have lymphoid tissue in wall with cholesterol crystals in straw-colored fluid

Third Branchial Cleft Anomalies

• External opening: lower anterior border of SCM
• Internal opening: pyriform fossa/posterior larynx (third pouch territory)
• Track passes posterior to internal carotid artery, lateral to CN X, below CN IX

Fourth Branchial Cleft Anomalies

• Extremely rare
• Internal opening: apex of pyriform fossa
• Track descends into the chest (looping around the aortic arch on the left or subclavian artery on the right) then ascends

12. BRANCHIAL CYST - AETIOLOGY THEORIES

1. Branchial apparatus theory: Cysts represent remnants of pharyngeal pouches or branchial clefts, or their fusion. Development extends from week 3-8 of gestation. Limitation: if congenital, cysts should be present at birth; peak incidence is 2nd-3rd decade.
2. Cervical sinus (of His) theory: Cysts represent remnants of the cervical sinus, formed as the second arch mesoderm grows down to meet the fifth arch. Second arch mesoderm almost covers the entire neck and forms the platysma muscle.
3. Thymopharyngeal duct theory: Cysts are remnants of the original thymus-third pouch connection. Largely discredited as no persistent thymic duct has been found, and no branchial cyst has been found deep to the thyroid gland.
4. Inclusion theory (currently most favored for classical branchial cysts): Cysts represent epithelial inclusions within cervical lymph nodes. Supported by: (a) presence of lymphoid tissue in 80% of cyst walls; (b) occurrence in the parotid gland and pharynx; (c) absence in neonates; (d) peak onset in second-third decade of life after URTI.

13. CLINICAL RELEVANCE SUMMARY

• Knowledge of branchial anatomy explains the course of fistula tracts (critical for complete surgical excision)
• The facial nerve's relationship to first branchial anomalies requires identification during surgery
• Second branchial cysts can be confused with lymphadenopathy; in adults >40 years, must rule out cystic nodal metastasis (especially from HPV+ oropharyngeal SCC)
• Third and fourth arch anomalies can be managed with endoscopic ablation of the internal opening (pyriform fossa opening) rather than open excision
• Aberrant migration of third pouch derivatives explains ectopic parathyroid and ectopic thymus
• C-cell (parafollicular) tumors (medullary thyroid carcinoma) arise from neural-crest-derived ultimobranchial body remnants incorporated into the thyroid