2. Development of the H&N

Question 1

which embryological structures give rise to the components of the head and neck

Answer 1

1. frontonasal prominence
2. 5 pharyngeal arches
3. pharyngeal clefts
4. paryngeal pouches

Question 2

what type of tissue does each pharyngeal arch consist of

Answer 2

• mesenchyme tissue core (paraxial and lateral plate mesoderm)
• covered on outside by surface ectoderm and on inside by endodermal epithelium
• contains own artery, nerve and cartilage bar (derived from neural crest cells)

Question 3

Describe the muscular and bony/cartilaginous derivatives of Ph. Arch 1, as well as its innervation

Answer 3

Muscles: of mastication

Bones: Meckel’s cartilage (mandible, malleus + incus)

Nerve: CN V

Question 4

Describe the muscular and bony/cartilaginous derivatives of Ph. Arch 2, as well as its innervation

Answer 4

Muscles: of facial expression

Bones: Reichert’s cartilage (stapes, part of hyoid + styloid process)

Nerve: CN VII

Question 5

Describe the muscular and bony/cartilaginous derivatives of Ph. Arch 3, as well as its innervation

Answer 5

Muscles: stylopharyngeus

Bones: part of hyoid

Nerve: CN IX

Question 6

Describe the muscular and bony/cartilaginous derivatives of Ph. Arch 4, as well as its innervation

Answer 6

Muscles: cricothyroid + pharyngeal constrictors

Bones: thyroid cartilage + cricoid cartilage

Nerve: CN X (superior laryngeal br.)

Question 7

Describe the muscular and bony/cartilaginous derivatives of Ph. Arch 6, as well as its innervation

Answer 7

Muscles: intrinsic muscles of larynx

Bones: arytenoid cartilage

Nerve: CN X (recurrent laryngeal br.)

Question 8

what do the 4 pharyngeal pouches give rise to

Answer 8

1st - tympanic cavity + Eustachian tube
2nd - palatine tonsils
3rd - inferior parathyroid glands + thymus
4th - superior parathyroid glands

Question 9

what to the pharyngeal clefts give rise to

Answer 9

1st - external auditory meatus

2nd pharyngeal arch grows down and obliterates other clefts

Question 10

what is the result of failure of 2nd Ph. arch to grow caudally

Answer 10

EXTERNAL BRANCHIAL FISTULAS: remnants of 2nd, 3rd or 4th clefts in contact with neck surface by narrow canal, usually provide drainage for lateral cervical cyst

Fistulas are found on lateral aspect of neck, directly anterior to SCM.

Question 11

what is the stomatodeum

Answer 11

site of future mouth, covered by buccopharyngeal membrane

Question 12

name the 5 prominences from which the face develops and their mature derivatives

Answer 12

i. frontonasal prominence
- forehead and bridge of nose
- medial nasal processes: philtrum, primary palate, mid-upper jaw
- lateral nasal processes: sides of nose

ii. 2 maxillary prominences
- cheeks
- lateral upper lip and jaw
- secondary palate

iii. 2 mandibular prominences
- lower jaw and lip

Question 13

describe the formation of the nose and palate

Answer 13

1. horseshoe-shaped ridge forms around each nasal pit: medial and lateral nasal processes
2. maxillary prominences grow medially, pusing nasal processes closer together in midline
3. maxillary prominences fuse with medial nasal processes
4. midline fusion of medial nasal processes - formation of intermaxillary segment (philtrum and primary palate)
5. palatal shelves grow out from each maxillary prominence towards midline and fuse with each other and with primary palate - forms secondary palate that separates nasal cavity from oral cavity

Question 14

what causes cleft lip or cleft palate

Answer 14

Cleft lip = failure of fusion of medial nasal processes and maxillary prominences

Cleft palate = cleft lip combined with failure of palatal shelves fusion

Question 15

why does foetal alcohol syndrome present with abnormal facial characteristics

Answer 15

alcohol affects neural crest cell migration into paryngeal arches

Question 16

which structures give rise to the tongue

Answer 16

• 2 lateral lingual swellings (ph. arch 1)
• 3 median lingual swellings
  
  • Tuberculum impar (ph. arch 1) - is overgrown by lateral swellings
  • Cupola (ph. arches 2, 3 and 4) - 3rd arch component overgrows 2nd arch
  • Epiglottal swelling (ph. arch 4)

Question 17

describe the general sensory, special sensory and motor innervation of the tongue, with references to embryological origins

Answer 17

Anterior 2/3:

• general sensation: trigeminal n. (CN Vc) as derived from ph. arch 1
• special taste sensation: facial n. (chorda typmani br.) as innervates papillae (CN VII is arch 2 n. but travels via arch 1)

Posterior 1/3:
- general sensation + special taste sensation: glossopharyngeal n. as mainly derived from ph. arch 3

Tongue muscles:
- hypoglossal n. as most muscles originate from occipital somite myoblasts which then migrate

Question 18

describe the embryological origins of the pituitary gland

Answer 18

Stalk + posterior lobe: from INFUNDIBULUM, a neuroectoderm outpocketing of forebrain. Grows downwards towards pharynx roof.

Anterior lobe: from RATHKE’s POUCH, an ectoderm outpocketing of stomodeum (primitive oral cavity) roof. Grows dorsally towards forebrain, loses connection with oral cavity and extends to infundibulum stalk.