Development of the Auditory System

Question 1

At what weeks do major features of the ears, face, & palate emerge during the embryonic period?

Answer 1

4th - 8th weeks

Question 2

Pharyngeal or Branchial Arches

Answer 2

what develops into major features of the ears, face and palate emerge during the embryonic period
- Arches form at 22 days post conception

Question 3

how do the arches pair?

Answer 3

• Initially 1 to 6
• Then Arch 5 disappears
• & 4 & 6 fuse forming 1 arch

Question 4

Which arches are critical to the development of the face, external & middle ear?

Answer 4

Arch 1 & 2

Question 5

What part of the arches develop into facial & auditory muscles

Answer 5

Mesoderm of the arches

Question 6

How are the arches seperated?

Answer 6

Separated externally by clefts & internally by pouches
- each arch, cleft, & pouch gives rise to different structures

Question 7

what is a main reason why HL is a part of many disorders?

Answer 7

they develop from the 1st and 2nd arch which also develops many features of the face

Question 8

how does the external auditory canal form

Answer 8

Formed from the 1st & 2nd pharyngeal clefts
- Initially lined w/ a meatal plug
- Formation begins during week 6 w/ invagination of the 1st cleft
- Formation ends during week 26 with the opening of the meatal plug
- Canal is now formed BUT continues to mature until 6-7 years of age

Question 9

cervical sinus

Answer 9

2nd, 3rd, and 4th pharyngeal clefts/grooves create the cervical sinus that later disappears.
- if it persists it forms branchial cysts or fistulas

Question 10

Formation of the Tympanic Membrane/ Ear Drum (Branchial Memebranes)

Answer 10

It’s created where the meatal plug (from ectoderm) of the external ear meets the tympanic cavity (from endoderm).
- Outer layer of the TM is formed by ectoderm, specifically the meatal plug of the EAC
- Middle layer is mesoderm derived from neural crest cells.
- inner layer is made of endoderm, the lining of the tympanic cavity.

Question 11

Formation of the Tympanic Cavity & Eustachian Tube (Pharyngeal Pouches)

Answer 11

the middle ear
- begins around 8 weeks & ends at abt 21 weeks
- formed by the tubotympanic recess of the 1st pouch (derived from endoderm)
- cilia within middle ear clears mucus and pathogens through the tube (broken = ME infection)

Question 12

Formation of the Ossicles (Pharyngeal Arches)

Answer 12

From 1st & 2nd arches
ME ossicles develop separately within the tubotympanic recess with contributions from neural cells
- are made of of cartilage & embeded in mesenchyme
- During 8th month tympanic cavity envelops ossicles forming a lever system

Question 13

How does the pinna form

Answer 13

develops from 6 mesenchymal projections (auricular hillocks) from migration of neural crest cells
- hillocks between 1st & 2nd arch ( 3 on each side of canal)
- 5th week hillocks arise on branchial clefts
- 7th week enlarge & fuse forming pina
- 30 weeks = complete

Question 14

Boney derivative of Arches

Answer 14

1st arch: malleus & incus
2nd arch: stapes

Question 15

Muscle derivative of Arches

Answer 15

1st arch: tensor tympani
2nd arch: stapedius

Question 16

Cranial Nerve derivative of Arches

Answer 16

1st arch: CN 5- Trigeminal nerve
2nd arch: CN 7 - Facial nerve (innervated by stapedius muscle)

Question 17

1st & 2nd Pharyngeal Arch Syndromes

Answer 17

• Treacher Collins syndrome:
• Pierre Robins syndrome/sequence:
• Stickler syndrome

Question 18

Characteristics of 1st & 2nd arch syndromes

Answer 18

• cleft lip
• small lower jaw
• EAC abnormalities
  i.e atresia

Question 19

Development of inner ear

Answer 19

22 days : neural ectodermal thickening will occur (placode), placode invaginates, moving inward to form the otic pit, which later becomes the primitive auditory or otic vesicle (otocyst).
28 days: otic vesicle splits into ventral and dorsal portions. The ventral portion forms the cochlear duct, while the dorsal portion forms the vestibular system.
6th week: saccule forms a tubular shape out-pocket in the low pole, which is the cochlea and after 8 week it will have 2 3/4 turns
9th week: mesenchyme surrounding inner ear labyrinth forms otic capsule which becomes the petrous portion of the temporal bone knowns as the bony labrynith
- Then the auditory nerve pierces otic capsule in two divisions one for vestibular system and the other for the cochlea

Question 20

Development of the Cochlea

Answer 20

begins around the 10th week of development
- cartilage surrounding the cochlear duct starts to undergo vacuolization resulting int two perilymphatic spaces; scala vestibule & scala tympani
- epithelial cells of cochlear duct form 2 ridges
- inner ridge forms future spiral limbus which forms tectorial membrane that lies over the OHC’s
- outer ridge gives rise to 3 rows of OHC & 1 row of IHC
- fully developed & functional at fifth month of gestation

Question 21

Why don’t we regenerate hair cells?

Answer 21

during weeks 7 to 8 the cochlea exits the cell cycle.

Question 22

Development of the Vestibular System

Answer 22

around 6th week the SCC appear
as pockets of the utricular portion of otic vesicle
-central portion will give rise to the 3 SCC
-one end of each dilates to form the crus ampullare (contains cells for equilibrium & balance)

Question 23

SCC utricle, & saccule are filled with ____

Answer 23

endolymph

Question 24

Fluid between the outer bony covering & the SCC canal is _____

Answer 24

perilymph

Question 25

3 SCC are sensitive to

Answer 25

angular motion or rotation
i.e. turning head

Question 26

macculae in the urticle & saccule are sensitive to

Answer 26

linear acceleration
i.e. body or head tilted

Question 27

Where are otoliths located?

Answer 27

in the saccule & utricle of the inner ear, specifically the vestibular labyrinth