HIS 120 Embryology of the Inner Ear

1. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearances/otocystsFirst Appearances/otocysts
The inner ear first appears as a thickening The inner ear first appears as a thickening
of ectoderm—the of ectoderm—the auditory placode.auditory placode.
In the twenty-fourth day of embryo In the twenty-fourth day of embryo
formation hollow cysts have formed-- formation hollow cysts have formed--
otocysts (auditory vesicles)otocysts (auditory vesicles)

2. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearances/otocystsFirst Appearances/otocysts
The otocysts soon become detached from The otocysts soon become detached from
the ectoderm from which they arose.the ectoderm from which they arose.
At that point where the otocyst has At that point where the otocyst has
detached from the ectoderm, the detached from the ectoderm, the
endolymphatic sac begins to extend in the endolymphatic sac begins to extend in the
medial direction. medial direction.

3. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearances/Cochlear DuctFirst Appearances/Cochlear Duct
It continues to dilate; and this slender It continues to dilate; and this slender
medial endolymphatic sac becomes the medial endolymphatic sac becomes the
cochlear duct. cochlear duct.
The dorsal portion begins to show The dorsal portion begins to show
indications of developing the semicircular indications of developing the semicircular
canals (for balance).canals (for balance).

4. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/CochleaFirst Appearance/Cochlea
By the end of the seventh week, the By the end of the seventh week, the
otocyst has been modeled roughly into the otocyst has been modeled roughly into the
membranous labyrinth with its semicircular membranous labyrinth with its semicircular
canals and a cochlea with one turn.canals and a cochlea with one turn.

5. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/Cochlear DuctFirst Appearance/Cochlear Duct
In the eighth week, the endolymphatic In the eighth week, the endolymphatic
duct with the three semicircular canal are duct with the three semicircular canal are
well defined and utricle and saccule have well defined and utricle and saccule have
been divided.been divided.
The cochlear duct has begun to coil into The cochlear duct has begun to coil into
its familiar snail shell shape.its familiar snail shell shape.

6. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/Cochlear DuctFirst Appearance/Cochlear Duct
By the third month, the adult form of the By the third month, the adult form of the
inner ear has been nearly completed. inner ear has been nearly completed.
Its further development results in complete Its further development results in complete
separation of the utricle and saccule which separation of the utricle and saccule which
remains attached to the endolymphatic remains attached to the endolymphatic
duct by a short slender canal. duct by a short slender canal.

7. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/Tectorial MembraneFirst Appearance/Tectorial Membrane
With the development of the membranous With the development of the membranous
labyrinth, the spiral organ divides into an labyrinth, the spiral organ divides into an
inner and outer ridge.inner and outer ridge.
Both ridges become covered with an Both ridges become covered with an
increasingly prominent tectorial increasingly prominent tectorial
membrane.membrane.

8. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/Hair CellsFirst Appearance/Hair Cells
In the area between these two ridges the In the area between these two ridges the
epithelial cells begin to form the sensory epithelial cells begin to form the sensory
hair cells.hair cells.

9. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/Bony LabyrinthFirst Appearance/Bony Labyrinth
The mesoderm surrounding the The mesoderm surrounding the
membranous (epithelial) labyrinth membranous (epithelial) labyrinth
becomes differentiated into a fibrous becomes differentiated into a fibrous
membrane and later into cartilage.membrane and later into cartilage.

10. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/PerilymphFirst Appearance/Perilymph
At about the tenth week, the cartilage At about the tenth week, the cartilage
immediately surrounding the membranous immediately surrounding the membranous
labyrinth undergoes a peculiar reversal of labyrinth undergoes a peculiar reversal of
development. development.
It returns to a precartilaginous condition in It returns to a precartilaginous condition in
which the cells lose their boundaries.which the cells lose their boundaries.

11. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/PerilymphFirst Appearance/Perilymph
This loose network of cells becomes the This loose network of cells becomes the
perilymphatic space surrounding the perilymphatic space surrounding the
membranous labyrinth.membranous labyrinth.
When this has taken place, the When this has taken place, the
membranous labyrinth becomes membranous labyrinth becomes
suspended in the fluid of the perilymphatic suspended in the fluid of the perilymphatic
spaces.spaces.

12. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/PerilymphFirst Appearance/Perilymph
The perilymphatic spaces continue to The perilymphatic spaces continue to
develop above and below the cochlear develop above and below the cochlear
duct creating the upper (scala vestibuli duct creating the upper (scala vestibuli
area) and the lower (scala tympani area). area) and the lower (scala tympani area).

13. Embryology of the Inner EarEmbryology of the Inner Ear
First Appearance/Bony LabyrinthFirst Appearance/Bony Labyrinth
By the fifth month, the cartilage By the fifth month, the cartilage
surrounding the membranous labyrinth surrounding the membranous labyrinth
has become the bony labyrinth (the has become the bony labyrinth (the
hardest bone in the human body).hardest bone in the human body).
Thus, by the middle of fetal life, the inner Thus, by the middle of fetal life, the inner
ear has attained its full adult size.ear has attained its full adult size.

14. Embryology of the Inner EarEmbryology of the Inner Ear
Now that we are aware of how the inner Now that we are aware of how the inner
ear is formed inside the womb, we can ear is formed inside the womb, we can
become more aware of how the mother’s become more aware of how the mother’s
health conditions may affect the health conditions may affect the
development of the fetus’s inner ear. development of the fetus’s inner ear.
This is an area where we will work This is an area where we will work
extensively when providing hearing extensively when providing hearing
“treatment” to an adult with cochlear “treatment” to an adult with cochlear
damage. damage.

15. Embryology of the Inner EarEmbryology of the Inner Ear
Whether it is a pre-natal malformation of Whether it is a pre-natal malformation of
the cochlea or damage in adulthood, the the cochlea or damage in adulthood, the
inner ear is an area that we must fully inner ear is an area that we must fully
understand when applying electroacoustic understand when applying electroacoustic
stimulation.stimulation.