Development of Head and Neck

Question 1

How many pharyngeal arches are there

Answer 1

• 5 in total, numbered 1 to 6
  
  • 5th does not form in humans
  • 1st arch is the largest, with size decreasing the further down

Question 2

State the muscle derivates of each pharyngeal arch

Answer 2

• Muscles of mastication are Ph arch 1 derivatives
• Muscles of facial expression are Ph arch 2 derivatives
• 3rd arch - stylopharyngeus
• 4th arch - cricothyroid, constrictors of the pharynx
• 6th arch - intrinsic muscles of the larynx

Question 3

State the nerves associated with each pharyngeal arch

Answer 3

Ph 1 - CN V
Ph 2 - CN VII
Ph 3 - CN IX
Ph 4 - CN X - superior laryngeal branch 
Ph 6 - CN X - recurrent laryngeal branch

Question 4

State the cartilage bar associated with each pharyngeal arch

Answer 4

• Ph arch 1 - Meckel’s cartilage - mandible, malleus, incus
• Ph arch 2 - Reichert’s cartilage - stapes, hyoid bone
• Ph arch 3 - hyoid bone
• Ph arch 4 & 6 - cartilages of the larynx

Question 5

State the blood supply associated with each pharyngeal arch

Answer 5

• Ph 1,2,3 - carotid arteries
• Ph 4 - arch of aorta (L) and brachiocephalic artery (R)
• Ph 6 - pulmonary arteries

Question 6

Distinguish between pharyngeal pouches and clefts

Answer 6

• Pouch - endoderm lined pockets in the pharynx
• Pharyngeal clefts
  
  • Lined with ectoderm
  • 1st cleft is all that remains - becomes the external acoustic meatus
  • 2nd arch grows down to cover others - obliterating all the other clefts

Question 7

Describe brachial cyst and fistula

Answer 7

• Branchial cysts or fistulae can occur if there are remnants of pharyngeal clefts
• Branchial cyst - tissue fluid collects
  
  • Closed to outside
• Branchial fistula - remnants of cleft
  
  • Opening to surface of neck
    
    • Occur at anterior border of SCM

Question 8

How does the nose develop embryologically

Answer 8

Onenote

Question 9

How does the ear develop

Answer 9

• External auditory meatus develops from the 1st pharyngeal cleft
• Middle ear cavity and ossicles develop from 1st pharyngeal pouch and cartilages of 1st and 2nd arches respectively
• Auricles develop from proliferation within the 1st and 2nd pharyngeal arches surrounding the meatus

Question 10

How are the ears positioned

Answer 10

• External ears develop initially from the neck
• As mandible grows, the ears ascend to the side of the head to lie in line with the eyes
• All common chromosomal abnormalities have associated external ear anomalies

Question 11

Explain fetal alcohol syndrome

Answer 11

• Facial skeleton derives from neural crest cells populating the pharyngeal arches
• Neural crest migration as well as development of the brain are known to be extremely sensitive to alcohol
• Abnormal facial features, mental retardation

Question 12

State the derivatives for each pharyngeal pouch

Answer 12

• Pouch 1 - Eustachian tube and middle ear cavity
• Pouch 2 - palatine tonsils
• Pouch 3, 4 - parathyroid gland and thymus

Question 13

Describe the embryological development of the pituitary gland

Answer 13

• Rathke’s pouch develops into anterior pituitary
  
  • Has ectoderm origin
  • Evagination of the roof of the oropharynx
  • Grows upwards towards the developing forebrain from the oral cavity
  • Sphenoid bone ossifies and pinches off anterior pituitary from pharynx
• Infundibulum develops into posterior pituitary and pituitary stalk
  
  • Has neurectoderm origin (from development of CNS)
  • Downward out-growth of the forebrain towards the roof of the pharynx

Question 14

Describe the embryological development of the tongue

Answer 14

• Tongue receives a component from each of the pharyngeal arches
• Forms in the floor of the pharynx
• 2 lateral lingual swellings from Ph arch 1
• 3 median lingual swellings
  
  • Tuberculum impar from Ph arch 1
  • Cupola from Ph arch 2, 3, 4
  • Epiglottal swellings from Ph arch 4
• Lateral lingual swellings overgrow the tuberculum impar
• 3rd arch component of cupola overgrows 2nd arch component
• Extensive degeneration occurs, freeing the tongue from the floor of the oral cavity through apoptosis
• Connected to the oral cavity by the lingual frenulum

Question 15

Describe the embryological development of the thyroid gland

Answer 15

• Primordium of the thyroid gland appears in the floor of the pharynx between the tuberculum impar and the cupola
• Descends and bifurcates as a bi-lobed diverticulum connected by the isthmus
• Thyroglossal duct - during its descent, the thyroid gland remains connected to the tongue by the thyroglossal duct

Question 16

Describe disorders due to inappropriate development of thyroid gland

Answer 16

• Thyroglossal cysts and fistula - remnant of thyroglossal duct which can enlarge and collect fluid
  
  • Found along the pathway of thyroid descent
• Ectopic thyroid tissue found anywhere along thyroid descent pathway (thyroglossal duct)