Embryology

Question 1

How many pharyngeal arches are there and how are they named?

Which is the first to develop?

Answer 1

Five
Numbered 1, 2, 3, 4, 6
Arch 1 is most cranial and is the first to develop

Question 2

What does each pharyngeal arch consist of?

Answer 2

Four things

• Central core of mesenchyme -> musculature of face and neck
• Neural crest cells -> skeletal components of face
• Cranial nerve component
• Artery (aortic arch)

Question 3

Where are the parathyroid glands and thymus derived from?

Answer 3

Pharyngeal pouches

Question 4

Do all five of the pharyngeal arches exist together?

Answer 4

No, you don’t ever have all five at the same time - as the later ones develop the first few will have regressed

Question 5

What are the pharyngeal pouches separated by
- Externally
- Internally
What type of lining does each have?

Answer 5

Externally separated by deep pharyngeal clefts
- Ectoderm lining

Internally separated by pharyngeal pouches
- Endoderm lining

Question 6

What do the neural crest cells lay down a path for in terms of pharyngeal arches?

Answer 6

Cranial nerve (s)

Question 7

Which cranial nerves are associated with which pharyngeal arches?

Answer 7

Arch 1 – CN V (trigeminal)
Arch 2 – CN XII (facial)
Arch 3 – CN IX (glossopharyngeal)
Arch 4+6 – CN X (vagus, through different branches)

Question 8

Pharyngeal arch 1

- Which skeletal components does this form?

Answer 8

First arch is special - it divides itself into two:
- Maxillary process: maxilla, zygomatic bone and part of temporal bone
- Mandibular process: incusm malleus and mandible
Mandibular process is also associated with Meckel’s cartilage - mandible is layed down alongside it

Question 9

Which muscles and nerves are associated with pharyngeal arch 1?

Answer 9

Muscles of mastication, anterior belly of digastric, mylohyoid, tensor tympani
Sensory supply to skin of face - CN V1, V2, V3

Question 10

Which skeletal components does the 2nd pharyngeal arch give rise to?

Answer 10

Stapes
Styloid process of temporal bone
Stylohyoid ligament
Lesser horn and upper part of hyoid

Question 11

Which muscles and nerves area associated with pharyngeal arch 2?

Answer 11

Muscles of facial expression, stapedius, stylohyoid, auricular muscles
All supplied by CN VII - facial nerve

Question 12

Which bones, muscles and nerves are associated with pharyngeal arch 3?

Answer 12

Greater horn and lower part of body of hyoid
Stylopharyngeus mm.
CN IX (glossopharyngeal)

Question 13

How many pharyngeal clefts are there?

Which is the most important and what does it give rise to?

Answer 13

4

1 gives rise to external auditory meatus

Question 14

What do the 1st and 2nd pharyngeal pouches give rise to?

Answer 14

1st Pouch
- Middle ear
- Eustachian tube
- Tympanic membrane
2nd pouch 
- Palatine tonsil

Question 15

What do the 3rd and 4th pharyngeal pouches give rise to?

Answer 15

3rd Pouch
Inferior parathyroid gland
Thymus

4th Pouch
Superior parathyroid gland
Ultimobranchial body (C cells of thyroid gland)

Question 16

What is an otic placode?

Answer 16

A thickening of the ectoderm - folds in on itself to form a vesicle (Otic vesicle) which gives rise to the inner ear

Question 17

What are the names of the straight and dilated ends of the semicircular canals and how many are there of each?

Answer 17

Dilated end = Crus Ampullare (3)

Non-dilated end = Crus Nonampullare (2)