ENT Embryology

Question 1

Formation of pharyngeal arches?

Answer 1

Population of neural crest migrate into the arches, in waves, and this lays down a pathway for the cranial nerves to develop; arches develop in all vertebrates

Question 2

Function of the pharyngeal arches?

Answer 2

Each arch has derivatives and these contribute to the development of head and neck structures

Question 3

How many pharyngeal arches are there?

Answer 3

There are 5 in total and these are named arches 1, 2, 3, 4 and 6 (arch 5, if it appears, disappears quickly)

Question 4

Time period of pharyngeal arch development?

Answer 4

Arch 1 - day 22
Arches 2 and 3 - day 24
Arches 4 and 6 - day 29

Question 5

Constituents of each arch?

Answer 5

Core of mesenchyme (derived from paraxial and lateral plate mesoderm) - forms musculature of the face and neck

Neural crest cells - form skeletal components of the face

Each arch has an assoc. cranial nerve and artery

Question 6

How are the pharyngeal arches separated?

Answer 6

Externally separated by deep pharyngeal clefts (in the ectoderm lining)

Internally separated by pharyngeal pouches (in the endoderm lining)

Question 7

Position of cranial nerves in relation to the pharyngeal arches?

Answer 7

In arch:

1 - trigeminal nerve (maxillary and mandibular divisions - V2 and V3)

2 - facial nerve

3 - glossopharyngeal nerve

4-6 - vagus nerve (superior laryngeal nerve to arch 4 and recurrent laryngeal nerve to arch 6)

Question 8

Skeletal components of the 1st arch?

Answer 8

1. Maxillary process (dorsal portion) - contributes to the maxilla, zygomatic and part of the temporal bone
2. Mandibular process (ventral portion) - contains Merkel’s cartilage which contributes to the incus, malleus and mandible

Question 9

Muscles originating from arch 1?

Answer 9

1. Muscles of mastication
2. Anterior belly of digastric
3. Mylohyoid
4. Tensor tympani
5. Tensor palatini

Question 10

Nerve supply to the muscles from arch 1?

Answer 10

All supplies by V3 (motor and sensory function)

Question 11

Sensory supply to the skin?

Answer 11

V1 (opthalmic) and V2 (maxillary) only have a sensory function

V3 (mandibular) has a motor and sensory fucntion

Question 12

Skeletal components of the 2nd arch?

Answer 12

1. Stapes
2. Styloid process of temporal bone + stylohyoid ligament
3. Lessor horn and upper part of the body of the hyoid

Question 13

Muscles from the 2nd arch?

Answer 13

1. Muscles of facial expression
2. Stapedius
3. Stylohyoid
4. Posterior belly of the digastric
5. Auricular muscles (sub-set of the facial expression muscles that allow ear movements; minimal use)

Question 14

Nerve supply to the muscles from arch 2?

Answer 14

Facial nerve

Question 15

Skeletal components of the 3rd arch?

Answer 15

Greater horn and lower part of the body of the hyoid

Question 16

Muscles from the 3rd arch?

Answer 16

Stylopharyngeus

Question 17

Nerve supply to the muscles from the 3rd arch?

Answer 17

Glossopharyngeal nerve

Question 18

Skeletal components of the 4th and 6th arches?

Answer 18

Laryngeal cartilages

Question 19

Muscles from the 4th arch?

Answer 19

1. Cricothyroid
2. Levator palatini
3. Pharyngeal constrictors

Question 20

Nerve supply to the muscles from the 4th arch?

Answer 20

Vagus (superior laryngeal nerve)

Question 21

Muscles from the 6th arch?

Answer 21

Intrinsic laryngeal

Question 22

Nerve supply to the muscles from the 6th arch?

Answer 22

Vagus (recurrent laryngeal nerve)

Question 23

How many pharyngeal clefts are there?

Answer 23

4

Question 24

Functions of the pharyngeal clefts?

Answer 24

1st cleft - external auditory meatus

2nd-4th clefts - lose contact with outside and form the cervical sinus

Question 25

Fate of the cervical sinus?

Answer 25

Usually disappears but, if it remains, a cyst can occur

Question 26

Functions of the 1st pharyngeal pouch?

Answer 26

Contributes to the middle ear Communication with the eustachian tube Tympanic membrane

Question 27

Function of the 2nd pharyngeal pouch?

Answer 27

Palatine tonsil

Question 28

Functions of the 3rd pharyngeal pouch?

Answer 28

Inferior parathyroid gland Thymus

Question 29

Functions of the 4th pharyngeal pouch?

Answer 29

Superior parathyroid gland Ultimobranchial body (gives rise to C-cells/parafollicular cells of the thyroid gland)

Question 30

What are the otic placodes?

Answer 30

Thickenings of the ectoderm

Question 31

Location of the otic placodes?

Answer 31

One on each side, overlying the hindbrain

Question 32

Functions of the otic placodes?

Answer 32

Otic vesicles (derived from otic palcodes) give rise to the inner ear

Question 33

Development of the otic vesicles?

Answer 33

Utricular portion gives rise to the semi-circular canals Saccular portion gives rise to the cochlea

Question 34

Development of the semi-circular canals at week 6?

Answer 34

Flattened out-pocketings of utricular part of vesicles Central part of the walls appose each other and breakdown

Question 35

Regions of the semi-circular canals?

Answer 35

Dilated end is called the crus ampullare Non-dilated end is the crus nonampullare (shared by the superior and posterior semi-circular canals)

Question 36

Contents of the ampullae?

Answer 36

Contains the crista ampullaris, which has: • Sensory hair cells (for balance) • Vestibular fibres of CN VIII

Question 37

Development of the cochlear duct?

Answer 37

Grows in a spiral between weeks 6-8; the surrounding mesenchyme becomes cartilagenous

Question 38

Structures surrounding the cochlear duct?

Answer 38

Scala vestibuli Scala tympani

Question 39

What is the organ of Corti?

Answer 39

Located in the cochlea and is used to transduce auditory signals into nerve impulse action potentials

Question 40

Describe formation and disappearance of the Meatal plug

Answer 40

At month 3, the epithelial cells proliferate to form Meatal plug; this dissolves around month 7 and contributes to the eardrum

Question 41

What are the auricles of the external ear?

Answer 41

6 mesenchymal proliferations that surround the 1st pharyngeal cleft (3 of these arise from arch 1 and the other 3 from arch 2)

Question 42

Formation of the auricle of the ear?

Answer 42

Auricular Hillocks fuse to form the definitive auricle The external ears initially form in the lower neck but these ascend due to development of the mandible

Question 43

Abnormalities of the external ear?

Answer 43

Microtia (small ear) or anotia (no ear) Pre-auricular appendages (extra structures) Pre-auricular pit (skin indentation that may indicate a chromosomal abnormality)