Early in the 4th week, a thickening of the surface ectoderm
on each side of the hindbrain -> otic placode -> otic pit -> form
the otic (auditory) vesicle à membranous labyrinth. Each vesicle
The ventral component -> saccule and the cochlear
duct (scala media)
The dorsal portion -> utricle, semicircular canals and
endolymphatic duct and sac (Figures 13 & 14).
In the 6th week of development: saccular lower pole ->
the cochlear duct -> penetrates surrounding mesenchyme
in a spiral fashion -> completed 2.5 turns at the end of the
8th week. And the ductus reuniens connects the saccule with
Mesenchyme surrounding the cochlear duct -> forming
In the 10th week, this cartilaginous shell undergoes
vacuolization, and two perilymphatic spaces, the scala
vestibuli and scala tympani.
The vestibular membrane separates cochlear duct from
the scala vestibuli.
The basilar membrane separates cochlear duct from
the scala tympani.
The spiral ligament attaches lateral wall of the cochlear
duct to the surrounding cartilage.
The median angle of the cochlea duct is connected to
and partly supported by a long cartilaginous process, the
modiolus -> the future axis of the bony cochlea (Figure15).
The epithelial cells of the cochlear duct form two
The inner ridge, the future spiral limbus,
The outer ridge which forms the sensory hair cells
*One inner row,
*Three or four outer rows.
They are covered by the tectorial membrane.
The sensory cells + tectorial membrane ->constitute
the organ of Corti.
Impulses received by this organ -> the spiral ganglion
to -> the nervous system by the auditory fibers of 8th cranial
The capsular cartilage serves as a template for the later
formation of the true bony labyrinth. The conversion from
the cartilaginous to the bony labyrinth occurs between 16
and 23 weeks’ gestation (Figure16).
During the 6th week of development, semicircular
canals appear as flattened outpocketings of the utricular
part of the otic vesicle -> Central portions of the walls of
them -> appose each other and disappear à giving rise to 3
SCC. ( Programmed cell death)
One end of each canal dilates -> form the crus ampullare,
the other, the crus nonampullare, does not widen.
Five crura enter the utricle, three with an ampulla and
Cells in the ampullae form a crest -> the crista
ampullaris, containing sensory cells.
Similar sensory areas develop in the walls of the utricle
Impulses generated in sensory cells of the cristae and
maculae as a result of a change in body position à the brain
by vestibular fibers of VIII cr n.
The statoacoustic ganglion forms during formation
of the otic vesicle à The ganglion splits into cochlear and
vestibular portions (Figure17).
The auditory vesicle elongates and develops an indenting
groove, which demarcates a tubular diverticulum on its medial
side -> which will be modified into the endolymphatic sac and
duct, -> and continues to grow postnatally until the age of three
or four years (Figure18).
They are 5 arches developed by 23 days of gestation called
mandibular arches in proximal portion of embryo, separated by
pharyngeal grooves/clefts. They are 5 arches 1st, 2nd, 3rd, 4th and
6th. The 5th arch did not develop in human but in other species.
Each arch is formed from:
Ectoderm: -> skin of head and neck region.
Endoderm: -> lining for oral, nasal, pharyngeal and
The middle ear cavity and the auditory tube arise from
-> first pharyngeal pouch called the tubotympanic sulcus, so
-> lined with an endoderm.
This pouch expands in a lateral direction and comes in
contact with the floor of the first pharyngeal cleft.
The distal part -> forming the tubotympanic recess.
The proximal part -> the auditory tube (Eustachian
tube) (Figure 22).
Later, the endodermal epithelium of the tubotympanic
sulcus becomes close to the ectoderm lining the first
pharyngeal cleft, separated by mesoderm. This complex
forms the tympanic membrane (eardrum). During fetal life,
a prominent ring-shaped, called the tympanic ring, supports
the tympanic membrane. Later, the tympanic ring becomes
absorbed into the temporal bone.
Just dorsal to the end of the tubotympanic sulcus,
a condensation of mesenchyme -> appears at 6th weeks ->
form the middle ear ossicles. The malleus and incus arise
from mesenchyme of the 1st pharyngeal arch, whereas the
stapes -> from 2nd arch. The tensor tympani muscle, which
is attached to the malleus -> from first-arch mesoderm, so
->innervated by trigeminal nerve (cr n V). The stapedius
muscle -> is of second-arch origin, and is innervated by the
facial nerve (cr n VII).
Ossicles -> lie in a bed of very loose embryonic
connective tissue; extend from the inner layer of the tympanic
membrane to the oval window of the inner ear. The future
middle ear cavity remains filled with loose mesenchyme
until late in pregnancy.
During the 8th and 9th months, programmed cell death
and other resorptive processes -> clear the middle ear
cavity and leave the auditory ossicles suspended within it.
Free movement of the auditory ossicles is acquired within 2
months after birth.
During late fetal life, the tympanic cavity expandsdorsally by vacuolization of surrounding tissue to form
the tympanic antrum. And After birth, epithelium of the
tympanic cavity invades bone of the developing mastoid
process. Later, most of the mastoid air sacs come in contact
with the antrum and tympanic cavity.
By the end of 4th week, the auricle develops from
6 mesenchymal proliferations (hillocks) of 1st and 2nd
pharyngeal arches, surrounding the first pharyngeal cleft.
These swellings (auricular hillocks), three on each side
of the external meatus, later fuse and form the definitive
As fusion of the auricular hillocks is complicated developmental abnormalities of the auricle are common
Initially, the external ears are in the lower neck region,
but with development of the mandible, they ascend to the
side of the head at the level of the eyes.
Because of its association with the pharyngeal
arches, the external ear is a sensitive indicator of abnormal
development in the pharyngeal region. Other anomalies of
the first and second arches are often attended by abnormally
located external ears (Figures 24 & 25).
For example rubella can lead to maldevelopment of the
organ of Corti.
Abnormalities of the middle ear ossicles or ligaments,
which can be associated with anomalies of the 1st and 2nd
Mutants, such as those of Pax-3 causing variants of
Waardenburg’s syndrome, can affect development at levels
ranging from gross morphogenesis of the ear to specific
cellular defects in the cochleosaccular complex.
Because of the multiple origins of its components ->
pinna malformations such as
Auricular appendages, or sinuses, low set ear, Bat ear,
May be associated with other developmental anomalies,
such as malformations of the kidneys and pharyngeal arches.
Exposure to excess retinoic acid or derivatives commonly
results in anomalies of the external ear (Figure 26).