Embryology of Pharyngeal Arches

Question 1

• Pharyngeal/branchial arch

Answer 1

o Each has same basic structure
o Artery, nerve and cartilage
o Originally ectomesenchyme
o Formed from 1st 7 pairs of somitomeres
o Form everything in the head and neck
o Give rise to striated muscles of face, jaws and throat
o Send neural crest cells into the arches during 4th week
o Major source of cartilage, bone, ligament in facial and oral regions
o Ganglia of CN 5, 7, 9, and 10 partly derived from neural crest cells

Question 2

• Pharyngeal/branchial cleft/groove

Answer 2

o No mesoderm, only endo and ectoderm

1st groove only one that forms adult derivative: external acoustic meatus

Question 3

• Pharyngeal branchial membrane

Answer 3

o Formed by union of cleft and pouch

1st groove only one that forms adult derivative: tympanic membrane

Question 4

• Pharyngeal/branchial pouch

Answer 4

o Internal of the cleft
1st, 2nd, 3rd, and 4th & 6th form adult derivatives
-pharyngotympanic tube
-tonsillar sinus/fossa crypts
-inferior parathyroid & thymus
- superior parathyroid & parafollicular cells

Question 5

• Mandibular arch

Answer 5

o	1st arch (M arch)
o	Meckel’s cartilage
o	CN 5 (V2 & V3)- maxillary and mandibular division of trigeminal
o	Muscles of mastication
	Anterior belly of digastric
	Mylohyoid
o	Maxillary artery-arch 1

Question 6

Meckel’s cartilage

Answer 6

• 1st arch
• Maxilla & zygomatic bone, mandible (template) & squamous part of temporal
• Malleus, incus, ½ of hyoid
• Sphenomandibular ligament
• Anterior ligament of Malleus

Question 7

Muscles of mastication:

Answer 7

• tensor tympani,
• tensor veli palatine,
• masseter,
• lateral medial pterygoids
• Anterior belly of digastric
• Mylohyoid

Question 8

1st Arch Pouch:

Answer 8

 Pharyngo-tympanic tube (eustacian or auditory tube)

• Opens up unto pharynx

Question 9

o Ist arch Groove:

Answer 9

 External Acoustic Meatus

Question 10

1st Arch Membrane

Answer 10

typmanic membrane

Question 11

• Hyoid arch

Answer 11

o	2nd arch (S arch)
o	Overgrows clefts of 2nd, 3rd, and 4th arches to form cervical sinus
o	Reichert’s cartilage
o	CN 7- facial nerve
o	Stapedial artery-arch 2

Question 12

o Reichert’s cartilage

Answer 12

- cartilage of 2nd arch
	Stapes, 
	styloid process
	upper body of Hyoid bone
	lesser cornu bone
	Stylohoid ligament

Question 13

o Muscles of 2nd arch

Answer 13

 Facial expression muscles
 Posterior belly of digastric
 Stylohyoid
 Stapedius muscle

Question 14

o Pouch:of 2nd arch

Answer 14

 Tonsillar sinus/fossa
• This is where palatine tonsil sits
 Surface epithelium of Tonsillar Crypts from endoderm

Question 15

• 3rd Arch

Answer 15

o	CN IX- Glossopharyngeal
o	Muscles
	Stylopharngeus
o	ligaments
	Lower hyoid
	Greater cornu (horn)
o	Pouch:
	Inferior parathyroid glands from Dorsal 
	Thymus 
•	Ventral: bilateral elongate and  hollow primordial come together and thymus descends

Question 16

Muscles and ligaments of 3rd arch

Answer 16

Muscles
	Stylopharngeus
o	ligaments
	Lower hyoid
	Greater cornu (horn)

Question 17

Pouch of 3rd arch

Answer 17

 Inferior parathyroid glands from Dorsal
 Thymus
• Ventral: bilateral elongate and hollow primordial come together and thymus descends

Question 18

• 4th & 6th Arches: Larynx Pharynx

Answer 18

o	Muscles of larynx and pharynx
	
o	Cartilage
	Laryngeal cartilages
•	Thyroid
•	Cricoid
•	Corniculate
•	Cuneiform
o	CN X – Vagus
	Superior Laryngeal to 4th arch
	Recurrent laryngeal to 6th arch
o	Pouch:
	Dorsal: Superior parathyroid glands
	Ventral: ultimobranchial body
•	Parafollicular (C) cells of the thyroid

Question 19

o Muscles of larynx and pharynx

Answer 19

• 4th and 6th arch derivatives
  -Cricothyroid
   Levator veli palatine
   Constrictors of pharynx
   Intrinsic muscles of larynx

Question 20

laryngeal cartilage

Answer 20

- 4th and 6th arch derivatives
o     Thyroid
•	Cricoid
•	Corniculate
•	Cuneiform

Question 21

4th and 6th arch pouch

Answer 21

 Dorsal: Superior parathyroid glands
 Ventral: ultimobranchial body
• Parafollicular (C) cells of the thyroid

Question 22

• Aortic Arches

Answer 22

o As pharyngeal arches develop, aortic arches supply them
o 6 aortic arches develop
o 1st Aortic arch – maxillary arteries
o 2nd Aortic Arch –stapedial arteries
o 3rd aortic arch–carotid
 Proximal: common carotid
 Distal: internal carotid with dorsal aorta

Question 23

• Branchial abnormalities:

Answer 23

o	Usually results from persistence of 2nd groove and pouche
o	Opens into side of neck
o	10% of cases ar bilateral
o	Cyst- 
o	Sinus
o	Fistula-

Question 24

branchial cyst

Answer 24

• mucosal lined, fluid filled cavity
• Usually at angle of mandible where 2nd and 3rd come together or at the anterior border of the SCM muscle
• Accumulate fluid and cellular debris

Question 25

branchial sinus

Answer 25

• blind-ending pit with one opening

Question 26

branchial fistula

Answer 26

2 openings
 Often drain cervical cysts
 Open along the anterior border of SCM muscle
 ascends to pass between internal and external carotid arteries and opens into tonsillar fossa
 Treatment is excision

Question 27

• First Arch syndromes

Answer 27

o	Insufficient migration of neural crest cells
o	Deformed auricle
o	Perauricular appendage
o	Defect in cheek
o	Hypoplasia of mandible
o	macrostomia
o	Treacher Collins syndrome/mandibulofacial dysostosis
o	Pierre Robin Syndrome

Question 28

o Treacher Collins syndrome/mandibulofacial dysostosis

Answer 28

-1st arch syndrome
	TCOF1 gene on 5tq31-q34
•	Autosomal dominant
•	60% new mutation
•	> paternal age
•	Uses OMENS to differentiate diseases
	Malar hypoplasia
	Downslanting palpebral fissures
	Deformed external ears
	Defects of lower eyelids

Question 29

o Pierre Robin Syndrome

Answer 29

-1st arch syndrom
	Hypoplasia of mandible
	Celft palate
	Defects of eye, ear
- clenched fists
- SOX9, 50% cases (a sequence)

Question 30

• 3rd and 4th pouch syndromes

Answer 30

o DiGeorge syndrome

o Goldenhar syndrome


Question 31

o DiGeorge syndrome

Answer 31

-3rd and 4th arch syndrome
- Microdeletion of Chrom 22
 Absence of thymus
• Lots of infections
 Malformation of mouth (fish mouth)
 Nasal clefts
 Cardiac abnormalities
 hypoparathyroidism
• 1st and 2nd arch syndromes

Question 32

o Goldenhar syndrome

Answer 32

-3rd and 4th arch syndrome
-Hemifacial microsomia
• Half of the face usually around the eyes, ears, and the mouth (60%)
• Bilateral in the rest with 10-33% of those more severe on one side
• Majority of cases, right more affected than the left
 Oculo-auricular dysplasia
 Oculoauricular-vertebral spectrum
 Incomplete development of the ear, nose, soft palate and mandible and spinal column

Question 33

• Van Der Woude syndrome

Answer 33

o Autosomal dominant
o Most common clefting syndrome
o Cleft lip/palate with/without distinctive lip pits
o Tooth agenesis
o No other apparent structural abnormalities
o Pits in 88% and 64% they are the only manifestation
o 25% have no symptoms or minimal like missing teeth or lip pits
o Most common syndrome assoc. with cleft lip and palate
o IRF6 gene
- rare (1/30,000)
- (70% )VWS exonic mutation
- problems with wound healing

Question 34

• Dumbo rats

Answer 34

o 1st arch syndrome?
o MSX and Dlx homeobox genes expressed in the arches
o big ears, small lower mandible