Branchial Arches

Question 1

What are the 4 fundamental aspects of developmental biology?

Answer 1

• proliferation/growth
• pattern formation: process by which cells in a developing embryo acquire identities that lead to a well ordered spatial pattern of cell activities
• morphogenesis - shape
• differentiation - type

Question 2

Give an overview of pharyngeal arches:

Answer 2

• appear in 4th and 5th week
• bars of mesenchymal tissue mugrate in
• arches separated by: clefts/grooves (externally), puches (internally)
• contribute exclusively to the formation of the face, nasal cavities, mouth, larynx, pharynx and neck

Question 3

What does each pharyngeal arch consist of?

Answer 3

• ectoderm - outside
• endoderm - inside (except 1st arch)
• cartilage
• blood vessels and nerve
• muscle

Question 4

Give an overview of the first arch:

Answer 4

1st arch: Mandibular arch - mandibular and maxillary processes

• meckels cartilage: primitive support, largely disappears and replaced by bone (incus, malleus) and ligaments (sphenomandibular, sphenomalleolar)
• gives rise to: body of tongue muscles of mastication
• nerve: CN V
• maxillary and carotid arteries
• Abnormalities: treacher collins syndrome

Question 5

Give an overview of the 2nd arch:

Answer 5

2nd arch: Hyoid arch

• cartilage: Reichert’s cartilage - forms staples, lesser horn, superior hyoid body, styloid process, stylohyoid ligament
• muscles: facial expression and PBD
• thyroid gland and tonsil
• CN VII

Question 6

Give an overview of the 3rd arch:

4th arch:

What happens to the 5th arch?

Answer 6

3rd arch:

• forms greater horn of hyoid, inferior hyoid and root of tongue
• stylopharyngeus muscle
• CN IX - glossopharyngeal nerve

4th arch:

• forms thyroid cartilage, pharynx and epiglottis
• muscles: pharyngeal constrictor, soft palate
• CN X - vagus

5th arch: disappears rapidly

Question 7

Give an overview of 6th arch:

Answer 7

6th arch:

• cricoid and arytenoid cartilages
• larynx
• muscles of larynx
• CN X - inferior laryngeal nerve

Question 8

What does the first pouch form?

2nd?

3rd?

4th?

Answer 8

1st pouch:

• eustacian tube
• middle ear cavity
• tympanic membrane
• ventral part obliterated by the tongue

2nd pouch: tonsillar fossa

• ventral part also oblitrated by developing tongue

3rd pouch: inferior parathyroid gland and thymus

4th pouch: superior parathyroid gland

Question 9

Give an overview of pharyngeal membranes:

Answer 9

• appear in floor of the pharyngeal grooves
• form where epithelia of the grooves and pouches approach each other
• the endoderm of the pouches and ectoderm of the grooves are soon separated by mesenchyme
• only first pharyngeal membrane becomes tympanic membrane, others are obliterated

Question 10

What are some clinical aspects of arches/pouches/grooves?

Answer 10

Branchial fistula/cyst

• 2nd arch fails to grow sufficiently over 2nd-4th clefts
• along anterior border of sternocleidomastoid muscle

Ectopic Thymic or Parathyroid Tissue:

• failure to migrate completely, so parathyroid gland closer to mouth