Branchial Apparatus

Overview

Branchial apparatus (pharyngeal apparatus)
- branchial clefts (branchial grooves)
  - derived from ectoderm located between the arches
- branchial arches
  - derived from mesoderm (muscles, arteries) and neural crest cells (bones, cartilage)
  - each arch is associated with a cranial nerve
- branchial pouches
derived from endoderm which line the foregut

Branchial arch innervation

1st arch derivatives
- CN V₂ and V₃: muscles for “chewing”
2nd arch derivatives
- CN VII: muscles for “facial expression”
3rd arch derivatives
- CN IX
4th and 6th arch derivatives
- supplied by CN X: muscles for “swallowing” and “speaking”
CN V₃, VII, IX, and X are both sensory and motor.
CN V₂ is only sensory

Branchial arch derivatives


Deriv.	Cartilage	Muscles	Nerves	Arteries	Syndromes
1	Meckel’s cartilagemandiblemalleusincussphenomandibular ligament	Muscles of masticationtemporalismasseterlateral and medial pterygoidsMylohyoidAnterior belly of digastricTensor tympaniTensor veli palatiniAnterior 2/3 of tongue	CN V2 and V3	Maxillary artery, external carotid artery	Treacher Collins syndrome neural crest cells fail to migrate into arch 1results in underdeveloped zygomatic bones, mandibular hypoplasia, lower eyelid colobomas, and malformed earsPierre Robin syndromesmall jaw, tongue falls back into throat causing choking and difficulty breathing
2	Reichert’s cartilagestapesstyloid processlesser horn of hyoidstylohyoid ligament	Muscles of facial expressionstapediusstylohyoidposterior belly of digastric	CN VII	Stapedial artery, hyoid artery	Congenital pharyngocutaneous fistula persistence of cleft and pouchfistula between internal tonsillar area and external neckfound along anterior border of sternocleidomastoid
3	Cartilagegreater horn of hyoid	Stylopharyngeus	CN IX	Common carotid, internal carotid arteries
4-6	Cartilagesthyroidcricoidarytenoidscorniculatecuneiform	4th arch pharyngeal constrictors except stylopharyngeuscricothyroidmuscles of soft palate except tensor veli palatini 6th arch intrinsic muscles of larynx except cricothyroidupper muscles of esophagus	4th arch CN X: superior laryngeal branch (swallowing) 6th arch CN X: recurrent laryngeal branch (speaking)	4 – Right: subclavian artery; Left: aortic arch 6 – Right: pulmonary artery; Left: pulomonary artery & ductus arteriosus

Branchial pouch derivatives

1st pouch
- forms
  - epithelial (endoderm derived) lining of middle ear cavity and eustachian tube
  - mastoid air cells
2nd pouch
- forms epithelial lining of palatine tonsil
3rd pouch
- dorsal wings
  - forms inferior parathyroids
- ventral wings
  - forms thymus
- 3rd pouch derivatives end up below the 4th derivatives
4th pouch dorsal wings
- forms superior parathyroids
DiGeorge syndrome (22q11 deletion)
- 3rd and 4th pouches fail to differentiate into thymus and parathyroid glands
- Symptoms
  - ↓ PTH due to parathyroid aplasia
    - tetany due to hypocalcemia
  - T cell deficiency due to thymic aplasia
    - recurrent viral/fungal infections
    - cell mediated immunity still functional and can fight bacterial infections
  - congenital heart and great vessel defects
  - absent thymic shadow on CXR
MEN 2A
- mutation of germline RET (neural crest cells)
  - pheochromocytoma
  - parathyroid tumor
    - derived from 3rd/4th pharyngeal pouch
  - medullary thyroid cancer
    - derived from parafollicular cells from the 4th/5th pharyngeal pouch
NOTE: 5th pouch degenerates during development

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