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.CartilageMusclesNervesArteriesSyndromes
1Meckel’s cartilagemandiblemalleusincussphenomandibular ligamentMuscles of masticationtemporalismasseterlateral and medial pterygoidsMylohyoidAnterior belly of digastricTensor tympaniTensor veli palatiniAnterior 2/3 of tongueCN V2 and V3Maxillary artery, external carotid arteryTreacher 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
2Reichert’s cartilagestapesstyloid processlesser horn of hyoidstylohyoid ligamentMuscles of facial expressionstapediusstylohyoidposterior belly of digastricCN VIIStapedial artery, hyoid arteryCongenital pharyngocutaneous fistula persistence of cleft and pouchfistula between internal tonsillar area and external neckfound along anterior border of sternocleidomastoid
3Cartilagegreater horn of hyoidStylopharyngeusCN IXCommon carotid, internal carotid arteries 
4-6Cartilagesthyroidcricoidarytenoidscorniculatecuneiform4th arch pharyngeal constrictors except stylopharyngeuscricothyroidmuscles of soft palate except tensor veli palatini 6th arch intrinsic muscles of larynx except cricothyroidupper muscles of esophagus4th 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