BRANCHIAL APPARATUS - Systematic Anatomy

1. INTRODUCTION AND TERMINOLOGY

2. EMBRYOLOGICAL DEVELOPMENT - TIMELINE

• 4th week of gestation: Six branchial arches begin to emerge as neural crest cells migrate cranially into the mesenchyme surrounding the primitive foregut and pharynx
• 5th week: All six arches are apparent; the 5th arch obliterates soon after becoming apparent (thus functionally 5 arches: 1, 2, 3, 4, 6)
• 3rd to 8th week: Development of the branchial apparatus extends through this period

3. COMPONENTS OF THE BRANCHIAL APPARATUS

A. Branchial Arches (Mesoderm + Neural Crest)

• Five to six parallel thickenings of mesenchyme that surround the pharynx
• Numbered 1 to 5 (or 6) from head to tail (cranial to caudal)
• Formed anterior to the 40-43 paired somites
• Each arch contains:
  1. A bar of cartilage (skeletal element)
  2. A group of muscles
  3. An associated artery (aortic arch artery)
  4. A cranial nerve (post-trematic nerve) - supplying structures of that arch and its derivatives
  5. A pretrematic nerve - from the arch below, supplying the endodermal (inner) surface of the arch above

B. Branchial Clefts / Grooves (Ectoderm - External)

• Ectodermal grooves on the external surface between adjacent arches
• Also called pharyngeal grooves
• In fish these break down to form gill clefts; in humans they normally regress

C. Pharyngeal Pouches (Endoderm - Internal)

• Endodermal outpouchings on the inner pharyngeal surface
• Each pouch lies opposite its corresponding cleft
• In early development, the endoderm and ectoderm come into contact momentarily; mesoderm then rapidly intervenes

4. NERVE SUPPLY TERMINOLOGY

5. ARCH-BY-ARCH DERIVATIVES (Comprehensive Table)

6. IMPORTANT ARCH-SPECIFIC NOTES

First Arch (Mandibular)

• Meckel's cartilage: the ventral portion forms the ossicles (malleus and incus), while the dorsal portion contributes to the mandible
• The tensor tympani attaches to the malleus (1st arch origin), hence innervated by V (mandibular)
• The tensor veli palatini is the only soft palate muscle from arch 1 (all others are from arch 2/vagus)
• First pouch expands to form the Eustachian tube, middle ear, and mastoid antrum - this space is fully formed by 8 months' gestation

Second Arch (Hyoid / Reichert's)

• Stapedius muscle - attached to stapes (2nd arch), hence innervated by VII (facial)
• The 2nd arch grows caudally and comes to cover the 3rd and 4th clefts - forming the cervical sinus of His
• The chorda tympani (pretrematic nerve of arch 2) supplies taste to anterior 2/3 of tongue and secretomotor to submandibular gland - it runs through the middle layer of the tympanic membrane (the mesodermal layer between arch 1 and arch 2 ectoderm/endoderm)
• Second arch mesoderm almost covers the neck entirely and forms the platysma muscle

Pinna / Auricle

• Develops from cartilaginous hillocks (tubercles) surrounding the 1st pharyngeal groove
• Majority of auricle is derived from 2nd arch cartilages
• Tragus alone is the only contribution from the 1st arch
• Rudimentary pinna is formed by ~60 days

Tympanic Membrane (3 layers)

• Outer layer (ectoderm): 1st branchial cleft
• Inner layer (endoderm): 1st pharyngeal pouch
• Middle layer (mesoderm): intervening mesenchyme from 1st and 2nd arches

7. CERVICAL SINUS OF HIS

• Formed when the 2nd branchial arch grows caudally to meet the 5th arch
• The 2nd arch overgrows the 3rd and 4th clefts, burying them
• This creates a transient ectodermal-lined space called the cervical sinus
• Normally obliterates completely; failure to do so leads to branchial remnants

8. PHARYNGEAL POUCH DERIVATIVES (Summary)

9. AORTIC ARCH ARTERIES - SUMMARY

10. CONGENITAL BRANCHIAL ANOMALIES

• Cyst - epithelium-lined cavity with no opening
• Sinus - blind-ended tract opening onto one epithelial surface (external or internal)
• Fistula - a complete tract running from one epithelial surface to another (ecto-endodermal junction breakdown)

Branchial Anomalies by Cleft Origin

Work Classification (1st Cleft Anomalies - 1972)

• Type I: Duplication of membranous external auditory canal (ectoderm only) - superficial to facial nerve
• Type II: Contains both ectoderm and mesoderm (skin and cartilage) - inconstant relationship with facial nerve

Aetiology of Branchial Cysts - 4 Theories

1. Branchial apparatus theory - remnants of pharyngeal pouches or branchial clefts; cysts should be present at birth (but peak is 2nd-3rd decade - contradicts this)
2. Cervical sinus theory - remnants of the cervical sinus of His (2nd arch growing down to meet 5th arch)
3. Thymopharyngeal duct theory - remnants of the connection between thymus and 3rd branchial pouch
4. Inclusion theory (most widely accepted) - epithelial inclusions within lymph nodes; supported by lymphoid tissue in cyst walls and late peak presentation

Histology of Branchial Cysts

• Lined by stratified squamous epithelium (90%)
• 10% may have respiratory epithelium
• 80% have lymphoid tissue in the outer wall
• Contents: straw-coloured fluid containing cholesterol crystals

11. KEY CLINICAL POINTS

• Structures innervated by cranial nerves in the neck are derived from the branchial apparatus
• The 5th arch has no clinical significance as it obliterates early
• Branchial anomalies account for up to 19% of all paediatric cervical masses
• Peak incidence of branchial cysts is in the 3rd decade (2nd-3rd decade)
• 2nd branchial cleft anomalies are the most common (~90-95% of all branchial cleft anomalies)
• Treatment of 1st and 2nd branchial fistulae: complete surgical excision of tract with CN VII monitoring
• 3rd and 4th branchial cleft anomalies may be managed with endoscopic ablation of the internal (pyriform fossa) opening

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery (Vol. 1 and Vol. 2) - Developmental Anatomy, Branchial Cysts, Development of the Human Ear
• K.J. Lee's Essential Otolaryngology - Branchial Arch Syndromes