within first few days( 2223 days)
Initially membranous labyrinth,
followed by encasement by
Ectodermal thickening in hind
(by 25th week of GA)
the otocyst becomes
chondrified to form otic
Ossification begins in
around 16th week .
Certain channels remain
within otic capsule like
oval window where part of
the otic capsule becomes
the stapes footplate and
the annular ligament.
inner ear is called as
labyrinth, from the
complexity of its shape.
consists of two parts:
The osseous labyrinth: A
series of cavities within
the petrous part of the
labyrinth: A series of
membranous sacs and
ducts, contained within
the bony cavities.
consists of three parts:
The vestibule, semicircular
canals, and cochlea.
hollowed out of the
substance of the
bone, and lined by
contain a clear
fluid, the perilymph, in
which the membranous
labyrinth is situated.
It is central part of the osseous labyrinth, and is situated medial
to the tympanic cavity, behind the cochlea, and in front of the
It is ovoid in shape, but flattened transversely.
Measures about 5 mm. from before backward, the same from
above downward, and about 3 mm. across.
In its lateral or tympanic wall is the fenestra vestibuli, closed by
the base of the stapes and annular ligament.
On its medial wall, at the forepart, is a small circular depression,
the recessus sphaericus, which is perforated, at its anterior and
inferior part, by several minute holes (macula cribrosa media) for
the passage of filaments of the acoustic nerve to the saccule.
Behind this depression is an oblique ridge, the crista vestibuli, the
anterior end of which is named the pyramid of the vestibule.
This ridge bifurcates below to enclose a small depression, the fossa
cochlearis, which is perforated by a number of holes for the
passage of filaments of the acoustic nerve which supply the
vestibular end of the ductus cochlearis.
Hinder part of the medial wall is the orifice of the aqueductus
vestibuli, which extends to the posterior surface of the petrous
portion of the temporal bone.
On the upper wall or roof is a transversely oval depression,
the recessus ellipticus, separated from the recessus sphaericus by
the crista vestibuli.
The pyramid and adjoining part of the recessus ellipticus are
perforated by a number of holes (macula cribrosa superior).
The apertures in the pyramid transmit the nerves to the utricle.
The bony semicircular canals are three in
number, superior, posterior, and lateral,
These are situated above and behind the
They are unequal in length, compressed
from side to side, and each describes the
greater part of a circle.
Each measures about 0.8 mm. in
diameter, and presents a dilatation at
one end, called the ampulla, which
measures more than twice the diameter
of the tube.
They open into the vestibule by five
orifices, one of the apertures being
common to two of the canals
15 to 20 mm. in length.
vertical in direction, and is
placed transversely to the long
axis of the petrous portion of
the temporal bone.
Its lateral extremity is
ampullated, and opens into
the upper part of the vestibule;
the opposite end joins with the
upper part of the posterior
canal to form the crus
It is also vertical, is directed
backward, nearly parallel to
the posterior surface of the
It is the longest of the three,
measuring from 18 to 22 mm.
It’s lower or ampullated end
opens into the lower and back
part of the vestibule, its upper
into the crus commune.
Shortest of the three.
Measures from 12 to 15 mm.
Its ampullated end corresponds to
the upper and lateral angle of the
vestibule, just above the fenestra
vestibuli, where it opens close to
the ampullated end of the
It’s opposite end opens at the
upper and back part of the
It bears some resemblance to a common snail-shell; it forms the
anterior part of the labyrinth.
It is conical in form, and placed almost horizontally in front of the
apex (cupula) is directed forwards and lateral, with a slight inclination
downward, toward the upper and front part of the labyrinthic wall of
the tympanic cavity.
base corresponds with the bottom of the internal acoustic meatus,
and is perforated by numerous apertures for the passage of the
cochlear division of the acoustic nerve. It measures about 5 mm. from
base to apex, and its breadth across the base is about 9 mm
The modiolus is the conical central axis
or pillar of the cochlea.
Its base is broad, and appears at the
bottom of the internal acoustic
It is perforated by numerous
orifices, which transmit filaments of the
cochlear division of the acoustic
The nerves for the first turn and a
half, pass through the foramina of the
tractus spiralis foraminosus.
Those for the apical turn,pass through
the foramen centrale.
The canals of the tractus spiralis
foraminosus pass up through the
modiolus and successively bend
outward to reach the attached
margin of the lamina spiralis ossea.
The bony canal of the cochlea
takes two turns and threequarters around the modiolus.
It is about 30 mm. in length, and
diminishes gradually in diameter
from the base to the summit,
where it terminates in
the cupula, which forms the apex
of the cochlea.
The beginning of this canal is
about 3 mm. in diameter; it
diverges from the modiolus
toward the tympanic cavity and
vestibule, and presents three
1st the fenestra
cochlea, communicates with the
tympanic cavity.In the fresh state
this aperture is closed by
the secondary tympanic
2nd of an elliptical form, opens into
3rd is the aperture of the
aquaeductus cochleae, leading to
a minute funnel-shaped canal,
which opens on the inferior surface
of the petrous part of the temporal
bone and forms a communication
between the subarachnoid cavity
and the scala tympani.
Bony shelf or ledge which projects
from the modiolus into the interior of
the canal, and, like the canal, takes
two-and three-quarter turns around
It reaches about half-way toward
the outer wall of the tube, and
partially divides its cavity into two
passages or scalae, of which the
upper is named the scala
vestibuli, while the lower is termed
the scala tympani.
Helicotrema:through which the two
scalae communicate with each
The osseous labyrinth is lined by
an exceedingly thin fibro-serous
It’s attached surface is rough
and fibrous, and closely
adherent to the bone.
It’s free surface is smooth and
pale, covered with a layer of
epithelium, and secretes a
thin, limpid fluid, the perilymph.
The membranous labyrinth is
lodged within the bony cavity,
separated from the bony walls by
a quantity of fluid, the perilymph.
In certain places it is fixed to the
walls of the cavity.
The membranous labyrinth
the endolymph, and on it’s walls
the ramifications of the acoustic
nerve are distributed.
Rich in Na+ ions.
1)filtrate of blood
serum from capillaries
of spiral ligament.
labyrinth via aqueduct
Rich in K+ ions.
1) Stria vascularis
2) Dark cells of utricle
& ampullated ends of
Dark cells are specialized non
sensory epithelial cells found on
either side of the vestibular
organs, and lining the
These dark-cell areas in the
vestibular organ are structures
involved in the production of
potassium towards the
Dark cells take part in fluid
homeostasis to preserve the
unique high-potassium and lowsodium content of the endolymph
and also maintain the calcium
homeostasis of the inner ear.
The membranous labyrinth consists of two
membranous sacs, the utricle, and
The utricle, the larger of the two, is of an
oblong form, and occupies the upper and
back part of the vestibule.
The portion which is lodged in the recess
forms a sort of pouch or cul-de-sac, the
floor and anterior wall of which are
thickened, and form the macula, which
receives the utricular filaments of the
The cavity of the utricle
communicates behind with the
semicircular ducts by five orifices.
It is globular in form and lies in the recessus sphaericus near the opening of
the scala vestibuli of the cochlea.
Its anterior part exhibits an oval thickening, the macula, to which are
distributed the saccular filaments of the acoustic nerve.
Its cavity does not directly communicate with that of the utricle.
From the posterior wall a canal, the ductus endolymphaticus, is given off;
this duct is joined by the ductus utriculosaccularis, and then passes along
the aquaeductus vestibuli and ends in a blind pouch (saccus
endolymphaticus) on the posterior surface of the petrous portion of the
temporal bone, where it is in contact with the dura mater
From the lower part of the saccule a short tube, the canalis reuniens of
Hensen, passes downward and opens into the ductus cochlearis near its
The walls of the utricle, saccule, and semicircular ducts consist of
The outer layer is a loose and flocculent structure, apparently
composed of ordinary fibrous tissue containing blood vessels and
The middle layer, thicker and more transparent, forms a
homogeneous membrana propria, and presents on its internal
surface, especially in the semicircular ducts, numerous papilliform
The inner layer is formed of polygonal nucleated epithelial cells.
In the maculae of the utricle and saccule, and in the transverse
septa of the ampullae of the semicircular ducts, the middle coat is
thickened and the epithelium is columnar, and consists of supporting
cells and hair cells.
Utricle and the saccule, respond
to gravity and linear acceleration
in horizontal (utricle) and vertical
The sense organs within the utricle
and saccule are the maculae.
They occupy the concave
spaces at the bottom of the
utricle and the saccule and
contain tiny pieces of calcium
carbonate, called otoliths (ear
stones) or otoconia (ear dust),
which are embedded into a
gelatinous membrane (otolithic
membrane) into which the
stereocilia of the maculae
Since the otoliths are quite
numerous in the otolithic
membrane and they are
heavier than the
surrounding fluid, the
membrane gets displaced
towards the Earth during
head tilting (due to gravity)
and away from the source
of motion during linear
acceleration (due to
The hair cells are aligned over the
surface of the maculae in an
orderly fanlike pattern so that the
optimal direction response for
each hair cell is related to its
position on the macular surface.
Striola divides the utricular
macula into the pars medialis
(also known as pars interna) and
pars lateralis (also know as pars
There are two types of hair cells
in the semicircular canals and
Type I hair cells are flaskshaped cells while type II hair
cells are cylinder-shaped cells.
Type I and type II hair cells are
very similar in their structure and
innervation to the inner hair
cells and the outer hair cells of
the organ of Corti, respectively.
Each hair cell in the semicircular
canals has 50 to 100 small
stereocilia and a single larger
cilium called a Kino cilium, which
only exists in rudimentary form in
the hair cells of the cochlea.
The stereocilia are arranged by
length, with the longest
stereocilia located close to the
Kino cilium, and are all
connected by tip links.
Movement of the stereocilia
hair bundle toward the
kinocilium causes a
sensory response whereas
movement away from the
kinocilium causes a
The organ of Corti converts the mechanical vibrations of the basilar
membrane into neural impulses that then travel through the
auditory nerve and brainstem to the brain.
The fibers of the auditory nerves travel from the organ of Corti
through a system of small perforations in the spiral lamina
collectively called habenula perforata.
From habenula perforata, nerve fibers travel through a channel in
the center of the modiolus (Rosenthal's canal), exit the base of the
cochlea, and join vestibular nerve fibers to form the
There are two types of hair cells
in the organ of Corti: the inner
hair cells (IHCs) and the outer
hair cells (OHCs).
Each hair cell has a number of
small hair-like projections called
stereocilia (cilia) extending from
the top of the cell.
The group of stereocilia at the
top of a hair cell is called a
stereocilia bundle. The
stereocilia bundle of each hair
cell is organized in several rows
forming either a ““W”” or ““V””
pattern for OHCs and shallow
““U”” pattern for IHCs
Stereocilia in each row have
graduated heights (like stair
steps) and their tips are
connected together by thin
fibers called tip links.
Each type of hair cell in the ear
is connected to the nervous
system by both afferent
(ascending) and efferent
(descending) nerve endings.
Outer hair cell
Inner hair cell
Farther from modiolus
No. of rows
Shape of hair cells
no. of rows of cilia
6-7 per cell
2-4 rows per cell
Steriocilia arrangement W or v shape
Shallow U shape
Length of steriocilia
Long & thin
Short & fat
Outer hair cells
Inner hair cells
Modulate function of
inner hair cells.
The groups of IHCs and OHCs
are separated by two rods
(pillars) of Corti, which
structurally support the organ
The rods are attached at their
tips and more widely
separated at the base, forming
a triangular shape called the
tunnel of Corti.
The tunnel is filled with the
cortilymph fluid that has similar
properties to the perilymph
fluid found in the bony
The tops of the hair cells and
supporting cells of the organ of
Corti are tightly connected
together at their tips to form a
continuous layer called the
The reticular lamina isolates all
of the organ of Corti from the
endolymph of the scala media
except for stereocilia which
project through the reticular
lamina into the endolymph.
The OHCs are held in position by
the outer rod of Corti on one side
and by Deiters cells on the other
Each Deiters cell holds an OHC
at the bottom and through long
projections called phalangeal
processes from above.
The middle part of an OHC is not
firmly supported and is
surrounded by a perilymph-filled
space called the space of Nuel
Next to the Deiters cells, towards
the outer end of the cochlea,
there are several groups of
supporting cells, called Hensen
cells, Claudius cells, outer spiral
sulcus cells, and Boettcher cells.
Lateral to these support cells is
the Stria vascularis, a highly
vascular organ attached to the
outer surface of the scala media.
Stria vascularis recycles potassium
and produces endolymph for the
scala media, thus maintaining the
endocochlear potential of the
stria vascularis is composed of three cell
1.Marginal cells, which line the endolymphatic
canal and have an essential role in ion
2. Intermediate cells, which are rich in the
3. Basal cells.
The vestibular nerve supplies the utricle, the saccule, and the ampullæ of
the semicircular ducts.
On the trunk of the nerve, within the internal acoustic meatus, is a
ganglion, the vestibular ganglion (ganglion of Scarpa); the fibers of the
nerve arise from the cells of this ganglion.
On the distal side of the ganglion the nerve splits into a superior, an
inferior, and a posterior branch.
The filaments of the superior branchmacula of the utricle and in the
ampullae of the superior and lateral semicircular ducts
inferior branchmacula of the saccule.
The posterior branch runs through the foramen singulare at the posteroinferior part of the bottom of the meatus and divides into filaments for the
supply of the ampulla of the posterior semicircular duct.