Development of the Auditory System Flashcards
At what weeks do major features of the ears, face, & palate emerge during the embryonic period?
4th - 8th weeks
Pharyngeal or Branchial Arches
what develops into major features of the ears, face and palate emerge during the embryonic period
- Arches form at 22 days post conception
how do the arches pair?
- Initially 1 to 6
- Then Arch 5 disappears
- & 4 & 6 fuse forming 1 arch
Which arches are critical to the development of the face, external & middle ear?
Arch 1 & 2
What part of the arches develop into facial & auditory muscles
Mesoderm of the arches
How are the arches seperated?
Separated externally by clefts & internally by pouches
- each arch, cleft, & pouch gives rise to different structures
what is a main reason why HL is a part of many disorders?
they develop from the 1st and 2nd arch which also develops many features of the face
how does the external auditory canal form
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
cervical sinus
2nd, 3rd, and 4th pharyngeal clefts/grooves create the cervical sinus that later disappears.
- if it persists it forms branchial cysts or fistulas
Formation of the Tympanic Membrane/ Ear Drum (Branchial Memebranes)
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.
Formation of the Tympanic Cavity & Eustachian Tube (Pharyngeal Pouches)
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)
Formation of the Ossicles (Pharyngeal Arches)
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
How does the pinna form
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
Boney derivative of Arches
1st arch: malleus & incus
2nd arch: stapes
Muscle derivative of Arches
1st arch: tensor tympani
2nd arch: stapedius
Cranial Nerve derivative of Arches
1st arch: CN 5- Trigeminal nerve
2nd arch: CN 7 - Facial nerve (innervated by stapedius muscle)
1st & 2nd Pharyngeal Arch Syndromes
- Treacher Collins syndrome:
- Pierre Robins syndrome/sequence:
- Stickler syndrome
Characteristics of 1st & 2nd arch syndromes
- cleft lip
- small lower jaw
- EAC abnormalities
i.e atresia
Development of inner ear
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
Development of the Cochlea
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
Why don’t we regenerate hair cells?
during weeks 7 to 8 the cochlea exits the cell cycle.
Development of the Vestibular System
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)
SCC utricle, & saccule are filled with ____
endolymph
Fluid between the outer bony covering & the SCC canal is _____
perilymph
3 SCC are sensitive to
angular motion or rotation
i.e. turning head
macculae in the urticle & saccule are sensitive to
linear acceleration
i.e. body or head tilted
Where are otoliths located?
in the saccule & utricle of the inner ear, specifically the vestibular labyrinth
