Lecture - Embryology of the Head & Neck

Question 1

Describe the structure of the head & neck in the embryo after the 4th week of development

Answer 1

• No discernible face yet
• H&N take up almost half the body
• Pharyngeal arches 1,2,3,4 & 6 (swellings) are present

Question 2

Describe the structure of a pharyngeal arch from in to out

Answer 2

• Inside lined by endoderm, outside lined by ectoderm
• Cartilage bars within these layers provide support for the arches
• Each arch has different cranial nerve associated with cartilage, and supplies motor function to different muscles

Question 3

Which nerve is each pharyngeal arch associated with & what muscles do they supply?

Answer 3

1) Carries trigeminal (V) nerve, supplies muscles of mastication
2) Carries facial (Vll) nerve, supplies muscles of facial expression
3) Carries glossopharyngeal (lX) nerve, supplies stylopharyngeus (single muscle)
4 + 6) Carries vagus (X) nerve, supplies pharyngeal & laryngeal muscles

Question 4

What are pharyngeal clefts & pouches?

What does the 1st - 4th pharyngeal clefts & pouches give rise to?

Answer 4

• Between adjacent swellings there are depressions, on the outside it is a pharyngeal cleft (evagination of ectoderm) on the inside it is a pharyngeal pouch (evagination of endoderm).
  1) 1st cleft & pouch met to form the tympanic membrane, 1st cleft becomes external acoustic meatus, 1st pouch becomes eustation tube. 2nd, 3rd + 4th clefts disappear
  2) 2nd pouch give rise to palatine tonsil
  3) 3rd + 4th pouches give rise to thymus and parotid glands

Question 5

What can arise in the neck if obliteration of the 2nd - 4th pharyngeal clefts is disordered?

Answer 5

Formation of branchial/pharyngeal cysts (enclosed with fluid), sinuses or fistulas

Question 6

How is the primitive mouth, nostrils & nose/upper lip formed during embryological development

Answer 6

Mouth = buccopharyngeal membrane disappears via apoptosis to form stomodeum, which is primitive mouth.

Nostrils = nasal placodes evaginate to form nasal pits, medial & lateral processes of nasal pits move medially towards midline and fuse to form nostrils

Nose/upper lip = maxillary processes start laterally and move medially to fuse with medial nasal processes to form nose and filtrum of upper lip

Question 7

How is the mandible and external ear formed during development?

Answer 7

Mandible = mandibular processes fuse in midline to form mandible

External ear = develops from contributions from 1st & 2nd pharyngeal cleft

Question 8

How is the palate of the mouth formed during development?

What happens if this process does not occur?

Answer 8

Palatial shelves grow medially and fuse at midline, tongue has to drop down for this to happen

If palatial shelfs dont fuse you get cleft palate, if there’s no fusions of frontonasal process with maxillary process you get cleft lip (see slide 11)

Question 9

How is ascent of the external ears achieved in development?

How can this ascent become disordered?

Answer 9

• Auricular hillocks ascend in order to form external ear

- If toxins such as alcohol ingested during early development, ascent is disordered leading to fetal alcohol syndrome

Question 10

Which areas of the tongue are formed from which pharyngeal arches?

From what nerves do the ant 2/3 and posterior 1/3 receive their general sensation & taste sensation from?

Answer 10

• Ant 2/3 develops from region of 1st arch, post 1/3 develops from 2nd and 3rd arches. 4th & 6th arches develop area around epiglottis.

Ant 2/3 = sensation from trigeminal, taste from facial
Post 1/3 = sensation & taste from glossopharyngeal

Question 11

How is the thyroid gland formed during development?

Answer 11

• Thyroid diverticulum grows down and becomes thyroglossal duct, this duct breaks down and completely disappears leaving thyroid gland.
• Failure of this duct to break down can result in cysts and fistulae forming, opening are foramen caecum