Embryologic Development of the Auditory System

Question 1

Embryologic Development of the Auditory System: General terminology

Answer 1

• Gestation: 40 wks
• Premature birth: before 37th week
  1. Pre-embryonic stage: fert. until 3 wks
  2. Embryonic stage: 4-8 weeks ( brachial arches emerge and 3 germ layers differentiate → major features of ear arises)
  3. Fetal stage: 9 wks-term
• After fertiliz. → Zygotes (Mitosis → cells divide) → Blastocyst: very early mass of cells implanted in the uterus then dev. into embryo
• Placenta

Question 2

Three germ layers

Answer 2

1. Ectoderm: cochlea
2. Mesoderm: outer and middle ear
3. Endoderm: epithelial lining of Eustachian tube, middle ear cavity

Question 3

Pharyngeal arches (AKA branchial arches)?

Answer 3

• 4 pairs of branchial arches made of all 3 germ layers
• Pharyngeal groove (ecto. - separate arches externally)
• Pharyngeal pouch (endo - separate arches internally)
• 1st/2nd arch becomes outer/middle ear
• 1st branchial groove forms permanent structure (ear canal)

Question 4

Embryologic origins of outer ear?

Answer 4

• Pinna: 1st and 2nd branchial arc; auricular hillocks form and fuse into external ear
• EAM: 1st branchial groove (ectoderm) deepens into ear canal
• TM: 1st branchial groove (ectoderm) and pouch (endoderm)

Question 5

Embryologic origins of middle ear?

Answer 5

Mandibular arch/1st branchial arch
- malleus and incus

1st pharyngeal pouch (endo)

 - mid ear cavity
 - eustachian tube 

Hyoid arch/2nd branchial arch

 - stapes
 - stapedius stylohyoid

Question 6

Embryologic origins of inner ear?

Answer 6

Otic Placodes (sink into mesoderm) → Otic Pits → otocyst (AKA otic vesicle) → inner ear:

 - Cochlea
 - Semicircular canals
 - Saccule and utricle * ***inner ear sensory system  is from ectoderm**** - Rapid dev btwn 4th - 12th wk

Question 7

Embryologic origins of CN VIII?

Answer 7

4th wk: CN VIII originates from otic placode and neural crest
5th wk: Nerve fibers contact wall of otocyst from vestibular ganglion
7th wk: vest. ganglion forms nerve trunks (innervate semi canals and utricule/saccule)
11/12th wk: efferent fibers contact IHC

Question 8

Define teratogen

Answer 8

Drugs, infections, or toxins that cause birth defects

Question 9

Teratogens: effect on aud dev.

Answer 9

During periods of rapid tissue differentiation (3-8 wks), exposure to teratogens = severe congenital anomalies

• concentration and maternal interaction also affect teratogenicity
• genetics of embryo influence if teratogen affects its dev.