Branchial Apparatus - Embryology

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 cartilage mandible malleus incus sphenomandibular ligament	Muscles of mastication temporalis masseter lateral and medial pterygoids Mylohyoid Anterior belly of digastric Tensor tympani Tensor veli palatini Anterior 2/3 of tongue	CN V2 and V3	Maxillary artery, external carotid artery	Treacher Collins syndrome neural crest cells fail to migrate into arch 1 results 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 Pierre Robin syndrome small jaw,tongue falls back into throat causing choking and difficulty breathing
2	Reichert's cartilage stapes styloid process lesser horn of hyoid stylohyoid ligament	Muscles of facial expression stapedius stylohyoid posterior belly of digastric	CN VII	Stapedial artery, hyoid artery	Congenital pharyngocutaneous fistula persistence of cleft and pouch fistula between internal tonsillar area and external neck found alonganterior border of sternocleidomastoid
3	Cartilage greater horn of hyoid	Stylopharyngeus	CN IX	Common carotid, internal carotid arteries
4-6	Cartilages thyroid cricoid arytenoids corniculate cuneiform	4th arch pharyngeal constrictors except stylopharyngeus cricothyroid muscles of soft palate except tensor veli palatini 6th arch intrinsic muscles of larynx except cricothyroid upper 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 cleft derivatives
- 1st cleft
  - develops into epithelial lining of external auditory meatus
- 2nd - 4th cleft
  - forms temporary cervical sinuses
  - later obliterated by proliferation of 2nd arch
- Persistent cervical sinus
  - branchial cleft cyst within lateral neck/angle of mandible

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

Action	Numeric Key	Letter Key	Function Key
Show Bullets		S	Enter (frontside only)
20%	1	N
40%	2	H
60%	3	F	Enter (backside only)
80%	4	E
100%	5	M
Toss	0	T