The document summarizes the development of the ear from the early stages of formation through maturation. It describes: - The formation of the otic placode from surface ectoderm, which then invaginates to form the otic vesicle. - How the otic vesicle develops into the inner, middle, and outer ear structures. The inner ear forms from regionalization of the vesicle directed by homeobox genes. - The development of the middle ear bones and structures from the pharyngeal arches. The external ear develops from auricular hillocks that fuse to form the pinna. - Key processes like differentiation of hair cells and formation of the bony labyrinth that

1. ‫خدا‬‫نام‬ ‫به‬
‫زمستان‬96

2. Development of the Ears
By:R.E.Malekshah

3. EAR COMPONENTS
 The external ear:
 pinna (or auricle)
 external auditory canal.
 The middle ear:
 The auditory ossicles: the malleus,
incus, and stapes.
 Tympanic cavity.
 The inner ear
 Cochlea.
 vestibular apparatus: semicircular canals,
the utricle, and the saccule.

4. DEVELOPMENT OF INNER EAR
Late in the third week:
 a thickening of the
surface ectoderm
called the otic
placode or otic disc
appears next to the
hind brain

5. Continue
During the 3th and 4th
weeks:
 the otic placode
gradually invaginates to
form an otic pit
a closed, hollow otic vesicle or
otocyst which is connected briefly to
the surface by a stem of ectoderm
Statoacoustic
(vestibulocochlear)
ganglion

7. Continue
By day 28:
 the dorsomedial
region of the otic
vesicle begins to
elongate, forming an
endolymphatic
appendage
 an expanded pars
superior and an
initially tapered pars
inferior
the following week:
 Endolymphatic sac.
 is connected to the pars
superior by a slender
endolymphatic duct

8. Continue
During the fifth week:
 The ventral tip of the
pars inferior elongates
and coils, forming the
cochlear Duct
 The saccule,
 Ductus reuniens
During the
seventh week:
 Cells of the
cochlear duct
differentiate to
form the spiral
organ of Corti
the fifth week:
 Flattened bilayered discs
grow dorsally and laterally
from the pars superior.
 In the center of the discs,
the epithelial walls meet,
and in these regions the
epithelium regresses,
leaving the rudiments of
the semicircular canals
 The ampulla

9. INNER EAR HAIR CELLS
 Inner ear hair cells,
specialized
mechanotransducers,
arise in six
prosensory
regions within the
developing otic
vesicle.
Organ of Corti (1)
Maculae (2)
 Detecting
gravity and
linear
acceleration
Cristae (3)
 Detect
angular
acceleration
 Statoacoustic
ganglion, of the
vestibulocochlear
nerve (cranial nerve
VIII).
Paint-
filled
otocysts
shown in
lateral
view

10. Organ of Corti
 Outer hair cells
• One row
• About 95% of the
sensory nerve
fibers
• The primary
transducers of
signals.
 Inner hair cells
• Three rows
• About 80% of the motor
input
• Electromotility
(amplifies the sound
waves, increasing
sensitivity)
• Prestin
 Hair cells are
surroundedby
endolymph In the
cochlea,
 the endolymph has a
high K+ concentration
that is necessary for
hair cell function.

11.  The stereocilia of the
hair cells project into an
acellular gelatinous
matrix called the
tectorial membrane.
 Is necessary for hair
cell function.
 Consists of collagens
(types II, V, IX, and XI)
and ear-specific non-
collagenous proteins
such as α- and β-
tectorin
Tectorial membrane

12. Otoconial membranes &
Cupula
 In both the maculae
and the cristae, the
hair cells are also
overlain (capped) by
an acellular matrix;
Otoconial
membranes
Cupula

13. Notice
The vestibular sensory organs are functional at birth, but the
organ of Corti does not become fully differentiated and hence
fully functional until after birth.

14. Bony labyrinth
Beginning in the 9th week:
 the mesenchyme
surrounding the
membranous labyrinth
chondrifies to form a
cartilage called the otic
capsule
• Note
 The otic vesicle
induces
chondrogenesis
 The shape of the
vesicle controls
the
morphogenesis
of the capsule
During the 3th to 5th month:
 the layer
of cartilage immediately
surrounding the membranous
labyrinth undergoes vacuolization
to form a cavity called
the perilymphatic space.
Scala vestibuli
Scalatympaniscala media
 The otic capsule ossifies between sixteen and
twenty-three weeks (bony labyrinth)

15. DEVELOPMENT OF MIDDLE EAR
 Subsequently differentiates to
form most of the expanded
tympanic cavity of the middle
ear and all of the slender
auditory (Eustachian) tube
 Cartilaginous precursors of
the three auditory ossicles
condense in the mesenchyme
near the tympanic cavity.
 The first pharyngeal pouch
elongates to form the
tubotympanic recess,
 The developing ossicles remain
embedded in the mesenchyme
adjacent to the tympanic
cavity until the eighth month
of gestation.

16. CONTINUE
 The malleus and
incus arise from the
first pharyngeal arch,
 The stapes arises
from the second
pharyngeal arch
 During the ninth
month :
 The mesenchyme
surrounding the auditory
ossicles is removed, and
the tympanic cavity
expands to enclose them
 Tensor tympani and the
stapedius both of which form in
the ninth week from first and
second pharyngeal arch
mesoderm, respectively.
 The pharyngeal membrane
separating the tympanic cavity
from the external auditory
meatus(derived from the first
pharyngeal cleft) develops into
the tympanic membrane or
eardrum
• The Eardrum is
composed of :
 an outer lining of
ectoderm
 an inner lining of
endoderm,
 an intervening layer
(fibrous stratum) is
derived from
infiltrating neural
crest cells

17.  During the ninth month:
• The suspended auditory ossicles assume their functional relationships with each other and with associated
structures of the external, middle, and inner ears.
 The ossicles are not totally free to vibrate/ move in response to sound until two months after
birth.
 During the ninth month, the tympanic cavity expands into the mastoid part of the temporal bone to form the
mastoid antrum.
 The mastoid air cells do not form until about two years of age, when the action of the SCM muscle on the
mastoid part of the temporal bone induces the mastoid process to form.

18. DEVELOPMENT OF EXTERNAL EAR
 The ectodermal lining of
the deep portion of this
tube later proliferates,
producing a solid core of
tissue called the meatal
plug by week twenty-six.
 The precursor of the
external auditory
meatus: develops by an
invagination of the first
pharyngeal cleft during
the sixth week and
requires the formation of
the tympanic ring.

19. Continue
 Canalization of
this plug begins
almost
immediately
and produces
the medial two
thirds of the
definitive
meatus
 The definitive
tympanic
membrane is
formed during
recanalization of
the external
auditory meatus.

20. Continue
 The auricle develops from six
auricular hillocks that arise
during the fifth week on the
first and second pharyngeal
arches
From ventral to dorsal:
 The hillocks on the first pharyngeal arch are called
the tragus, helix, and cymba concha (or one to
three, respectively),
 The hillocks on the second arch are called the
antitragus, antihelix, and concha (or four to six,
respectively)
During the seventh week:
 The auricular hillocks begin to enlarge, differentiate,
and fuse to produce the definitive form of the auricle.
 Gradually translocated from its original location low
on the side of the neck to amore lateral and cranial
site

21. Development of Placodes
The placodes:
 Otic, epibranchial,
Trigeminal, Olfactory,
Adenohypophyseal and
lens
 Arise from a horseshoe-
shaped domain surrounding
the anterior neural plate,
called the preplacodal
region
The preplacodal region :
 Is initially multipotent,
 Competent to form all of the
placodal derivatives.
 Is characterized by the
expression of the Six and Eya
families of transcription factors
(specifically, Six1, Six4, Eya1,
and Eya2)
 Together with neural crest
progenitors, is established by
Fgf signaling and intermediate
levels of Bmp signaling (low
levels of Bmps specify the
neural plate, and high levels the
ectoderm)

23. Regionalization of preplacodal region:
 The presumptive olfactory and lens placodes are initially
characterized by the expression of Pax6.
 The presumptive otic and epibranchial placodes express Pax2.
 Pax6 is specifically expressed in the developing lens
 The developing olfactory placode switches off Pax6 expression
and is now distinguished by Dlx5 expression
 The otic placode is induced by Fgf signaling from the
mesoderm, together with signals such as Wnts and
Fgfs from the hindbrain.
 Fgf8 (expressed by the endoderm in chick and by all
three germ layers in mouse) induces the expression of
other Fgfs in the mesoderm.
 Fgf signaling from the mesoderm, endoderm,and
anterior neural ridge (the cranial U-shaped junction
betweenthe ectoderm of the neural plate and the
non-neural, or surface ectoderm) is initially required
for olfactory, trigeminal,otic, and epibranchial
placode development but is inhibitory for lens
induction.
 Signals (Tgfβ) from neural crest cells also inhibit lens
development;

24.  Specification of different
regions of otic vesicle is
achieved by the differential
expression of homeobox
genes
 Pax2 is expressed in the ventral otocyst
and is essential for development of the
cochlea.
 The homeobox genes Dlx5 and 6 are
expressed in the dorsal otocyst and are
required for development of the
vestibular apparatus.
 signals from the hindbrain
and notochord control
homeobox gene
expression.
 Shh signaling from the notochord and
floor plate controls Pax2 expression,
whereas Wnt signaling from the dorsal
neural tube controls expression of Dlx5
and 6.
 In the absence of Shh, the cochlea duct
and saccule do not form.
 Again emphasizing the importance of hindbrain
signals, double knockout of Hoxa1 and Hoxb1,
which results in the loss of rhombomere 5, affects
the development and morphogenesis of the
entire inner ear.
OTIC PIACODE FORMATION

25.  Development of
the vestibular
structures and
cochlea is
differentially
controlled by
homeobox
genes, as
revealed by
using knockout
mice.

26. ‫کند‬ ‫طلوع‬ ‫افق‬‫از‬ ‫اد‬‫ر‬‫م‬‫ماه‬ ‫که‬ ‫شبی‬
‫افتد‬ ‫ما‬ ‫بام‬ ‫به‬ ‫نوری‬‫پرتو‬‫که‬ ‫بود‬
‫فالی‬ ‫ن‬‫بز‬ ‫مرو‬‫در‬ ‫این‬‫از‬ ‫امیدی‬ ‫نا‬ ‫به‬
‫به‬ ‫دولت‬ ‫قرعه‬‫که‬ ‫بود‬‫ما‬ ‫نام‬‫افتد‬
«‫حافظ‬»