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Eye and Ear Histology
The eye is a complex highly developed photosensitive organ responsible for light reception.
Each eye is located in a protective bony chamber in the skull, the orbit.
The eyeball is slightly flattened anteroposteriorly, with the anterior 1/6th of the sphere (the cornea) being
more convex.
Histologically, the eye consists of three concentric layers:
1. External (fibrous) layer: the tunica
fibrosa.
2. Middle (vascular) layer: the tunica
vascoulosa.
3. Internal (nervous) layer: the retina.
An eye has three chambers:
1. The anterior chamber lies between
the cornea and the iris.
2. The posterior chamber lies between
the iris and the lens.
3. The vitreous chamber lies between
the posterior surface of the lens and
the neural retina.
The Tunica Fibrosa
The external layer (tunica fibrosa) is subdivided into sclera and cornea, Cornea is a transparent layer
and covers the anterior one-sixth of the eye. Sclera covers the posterior five-sixths of the eye. The
junction between the cornea and the sclera is called limbus.
 The Sclera
It is the white part of the eye, an opaque layer of dense connective tissue that protects delicate
internal structures and gives the eye its shape. It is relatively avascular, consisting of: flattened
bundles of type I collagen fibers running in different directions parallel to the eyeball surface,
moderate amount of ground substance, & few fibroblasts.
The innermost layer of sclera (adjacent to the choroid) is less dense, with thinner collagen fibers,
more fibroblasts, melanocytes, & elastic fibers.
The sclera thickens posteriorly, reaching 1 mm at the optic nerve attachment, where it becomes
continuous with the epineurium of the nerve.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 The Cornea
The most anterior part of the eye, a highly transparent convex structure made of 5 layers
(anteroposteriorly): ABCDE
1. The Anterior Epithelium (Pavement Epithelium): stratified squamous epithelium (nonkeratinized) consisting of 5-6 cell layers. The basal layer regenerates other cells (corneal
epithelial turnover occurs each 7 days) & mitotic figures are seen especially at the perephery.
The surface cells have microvilli protruding into the tearfilm. The epithelium has a very rich
sensory nerve supply.
2. Anterior limiting (Bowmann's) membrane: the very thick (8-12 µm) basement membrane of
the epithelium, consisting of randomly runing collagen fibers, it is responsible for corneal
strength.
3. The Corneal stroma (Substantia Propria): about 60 layers of parallel highly organized
collagen bundles crossing at right angles to each other. The uniform orthogonal array of these
collagen fibrils contributes to the corneal transperacy. Fibroblast-like cells (keratocytes) have
flattened cytoplasmic extensions (like butterfly wings) between collagen fibrills, with
proteoglycan-rich extracellular substance. Lymphoid cells are seen in the stroma.
4. Posterior limiting (Descemet’s) membrane: thick homogenous layer composed of fine
interwoven collagen fibers organized in a 3D network.
5. The Posterior epithelum (Cornael Endothelium): a simple squamous epithelium, with the
cells showing the features of active transport & protein synthesis.
Note: The cornea is said to have three cellular layers (epithelial layers and stroma) and two
noncellular layers (Bowman membrane and Descemet membrane).
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 The corneoscleral junction (The Limbus)
It is the highly vascularized transitional zone between the cornea & sclera. In the stromal layer of the
limbus, there is the scleral venous sinus or Schlemmm's canal, an irregular endothelium-lined space
that communicate with the anterior chamber of the eye via many tiny openings at the iridocorneal
angle. Schlemm's canal is connected to the venous system & act to drain the aqueous humor to it.
The cornea is avascular, it is nourished by diffusion from:
1. Vessels in the limbus.
2. The aqueous humor.
LASEK
Laser refractive surgery reshapes the cornea in order to focus images more accurately onto the retina. The
most common procedures include laser in-situ keratomileusis (LASIK), and laser-assisted epithelial
keratomileusis (LASEK).
Laser techniques have been continuously modified to reduce complications and enhance surgical
outcomes. Unlike with LASIK, the most recent LASEK surgery saves the epithelium by using an alcohol
solution to weaken epithelial cell adhesion so the epithelial layer can be lifted as a flap. After the
epithelial flap is moved out of the way, excimer laser energy is applied through the Bowman membrane
layer and into the upper stroma to reshape the cornea. The epithelial flap is then returned to its original
position. The advantages of LASEK are a reduction of postoperative discomfort, a decreased risk of
infection, and an increase in the overall thickness of the untouched area of the cornea.
The Tunica Vasculosa (Uveal Tract)
Tunica vasculosa consists of three parts: choroid, ciliary body and iris
 The Choroid
The choroid is a highly vascularized thin layer, with a loose C.T between its blood vessels. It lines
the posterior 3/4s of sclera (approximately to the level of ora serrata). The choroid is rich in C.T
cells, collagen & elastic fibers & melanocytes (that give the choroid its dark colour).
The outer layer of choroid beneath the sclera is the suprachoroidal lamina.
The inner layer (the choriocapillary lamina) is richer in small blood vessels and has a major role in
the nutrition of the retina, from which it is separated by the hyalin Bruch's membrane. This
membrane consists of three layers: elastic fibers network in the middle, & collagen layers on each
side. Bruch's membrane is covered externally by basement membrane of choriocapillary vessels, &
internally by basement membrane of the pigmented epithelium of retina.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 The Ciliary Body
This middle part of the uveal tract extends from ora serrata to the root of iris. It is a thick ring with a
triangular cross section, having one surface in contact with the sclera, one with the vitreous body &
the third irregular surface facing the posterior chamber of the eye.
Histologically, ciliary body consists of loose C.T rich in blood vessels, elastic fibers & melanocytes.
Apart from the surface adjacent to the sclera, the ciliary
body is covered by 2 layers of simple columnar
epithelium, both are derived from the retina. The first
(inner) layer (directly adjacent to ciliary stroma) consists
of melanin-rich (pigmented) cells & represents the
anterior continuation of the pigment epithelium of the
retina. The second layer covers the first & consists of
non-pigmented
cells
that
represent
the
anterior
continuation of the sensory layer of the retina.
The ciliary body is divided into:
1. Ciliary muscles: 2 bundles of smooth muscle fibers divided into thick inner circular & thin outer
longitudinal layers, they are important in visual accomodation.
2. Ciliary processes: about 75 ridge-like or finger-like projections from the ciliary body, each
consists of a loose connective tissue core rich in fenestrated capillaries & covered by 2 layers of
epithelium.
Ciliary processes serve two functions:
1) Give attachment to the fibers of suspensary ligament of the lens (that extend from the basement
membrane of the pigmented epithelium to the capsule of the lens).
2) Secrete the aqueous humor (by the non-pigmented epithelium) into the posterior chamber.
 Aqueous humor secretion and circulation:
Non-pigmented cells of the cilliary processes have tight junctions and
extensive basal & Na+/K+-ATPase in their lateral membrane, they
filterate blood in the ciliary process vessels secreting aqueous humor (1).
Aqueous humor is similar to plasma but with a minimal amount of
proteins. It is secreted into the posterior chamber & flows via the pupil
(2)
to the anterior chamber to supply the cornea by nutrients & then
enters the canal of Schlemm (through the trabecular meshwork in the
iridocorneal angle)(3), then drained to the veins in the limbus.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
Glaucoma
Glaucoma is a group of eye diseases that produce elevated intraocular pressure, usually because of
obstruction of the aqueous humor outflow.
Glaucoma results in damage to the optic nerve and is a major cause of blindness.
(1) Open-angle glaucoma is the most common type. Fragments from normal cell degeneration become
deposited within the trabecular meshwork and endothelial lining of the canal of Schlemm and reduce the
absorption of aqueous fluid.
(2) Acute angle-closure glaucoma is also common. Occlusion of the anterior chamber angle occurs when
the peripheral iris obstructs aqueous outflow.
Patients may present with severe headache and eye pain, malaise, nausea, and vomiting. Immediate
medical intervention is necessary to prevent vision loss.
 The Iris:
It is the anterior part of the uveal tract (the coloured part of the eye), a disc-like stracture attached to
the ciliary body peripherally and having a rounded appearture (the pupil) centrally. It has the
following layers:
1. Anterior Iridial border ( anterior surface of the iris):
this is not covered by epithelium, but formed by a
discontinuous layer of
fibroblasts & melanocytes, with
interdigitating processes giving an irregular, rough grooved
appearance.
2. The Iris Stroma (stroma iridis):: a loose C.T with an
anterior zone poorly vascularized & rich in fibroblasts &
melanocytes, & a highly vascularized posterior zone.
Around the pupil, the stroma contains circularly-arranged smooth muscle fibers innervated
parasympathetically, the sphincter pupillae muscle.
3. Posterior surface of the iris: a smooth surface covered by the same two epithelial layers
covering the ciliary body.
i.
The posterior layer (facing the posterior chamber) cells are heavily pigmented, preventing
light from entering the eye except via the pupil.
ii.
The anterior layer (adjacent to the stroma) consists of less pigmented myoepithelial cells that
havs radially arranged processes forming the dilator pupillae muscle (innervated
sympathetically).
The colour of the eye
Posterior pigmented epithelium and melanocytes in the stroma of the iris are responsible for the eye
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colour.
People with H.
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melanocytes
blue–eyes,
more melanocytes and collagen 2017
in the
Dr. Mohammed
MBChBhave
– MSc
DCHpeople
(UK)with
– MRCPCH
iris stroma have darker eyes. People with albinism lack pigment in their cells, they have pink eyes from
the visible blood vessels of the iris.
Eye and Ear Histology
The Referacive Media of the Eye
The eye has four refractile structures: Cornea, Aqueous humor, Lens, and Viterous body.
 The Lens
The lens is a biconvex transparent structure with great elasticity (that decreases with age). The lens
has three components:
1. Lens capsule: a thick homogenous
refractile external layer that represents the
basement
membrane
of
the
lens
epithelium.
2. Subcapsular epithelium: simple cuboidal
or columnar cells lining the anterior half
of lens capsule. Cells at the lens periphery
divide to give new lens fibers.
3. Lens fibers: extremely elongated highly
differentiated epithelial cells that fill the
lens. They originate from the subcapsular epithelium and loose their nuclei & organelles to
become very long, thin, flattened structures filled with proteins (crystallins).
 The suspensary ligament of the lens (Zonule): a group of radially oriented fibers extending
from the ciliary processes to the lens capsule. Zonular fibers are similar to the microfibrills of
elastic fibers.
Cataract
Cataract is a condition of opacity in the lens of the eye. Crystallin protein in the lens degenerates,
becoming insoluble and opaque. In cataractous lenses, lens fibers are edematous and sometimes
necrotic.These changes alter the normal continuity of the lens fibers. Cataract is usually an age-related
disorder that causes partial or total blindness if left untreated. The visual acuity decrease is directly
related to the density of the cataract. Medical intervention consists of removing the opacified lens from
the eye and implanting an artificial intraocular lens.
 The Vitreous Body
It is a transparent gelatinous medium filling the vitreous space between the lens & retina. It consists
of water (99%) with hyaluronate & small amounts of collagen. Vitreous body is surrounded by the
vitreous membrane, made by type IV collagen. The only cells in the vitreous body are few
macrophages & a small number of hyaluronate- producing cells (hyalocytes) near the membrane.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
The Retina (Nervous Layer)
The retina consists of two basic layers:
1) The neural retina or retina proper is the inner layer that contains the photoreceptor cells.
2) The retinal pigementary epithelium (RPE) is the outer layer that rests on and is firmly attached
through the Bruch’s membrane to the choriocapillary layer of the choroid.
In the neural retina, two regions or portions that differ in function are recognized:
1) The nonphotosensitive region (nonvisual part), located anterior to the ora serrata, lines the inner
aspect of the ciliary body and the posterior surface of the iris.
2) The photosensitive region (optic part) lines the inner surface of the eye posterior to the ora
serrata except where it is pierced by the optic nerve.
Retenal Detackment
A potential space exists in the retina between the retinal pigment epithelium (RPE) and the neural retina
( because embryologically these two structures arries from different different layers of the optic vesicle).
If this space expands, the neural retina separates from the RPE, which remains attached to the choroid
layer. As a result of retinal detachment, the photoreceptor cells are no longer supplied by nutrients from
the underlying vessels in the choriocapillary plexus of the choroid.
Clinical symptoms of retinal detachment include visual sensations commonly described as a “shower of
pepper” or floaters. A detached retina can be observed and diagnosed during ophthalmoscopic eye
examination. If not repositioned quickly, the detached area of the retina will undergo necrosis, resulting
in blindness. An argon laser is often used to repair retinal detachment by photocoagulating the edges of
the detachment and producing scar tissue. This method prevents the retina from further detachment and
facilitates the repositioning of photoreceptor cells.
 Layers of the Retina
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
Before discussing the ten layers of the retina, it is important to identify the types of cells found there.
For convenience, neurons and supporting cells can be classified into four groups of cells
1) Photoreceptor cells—the retinal rods and cones
2) Conducting neurons—bipolar neurons and ganglion cells
3) Association neurons and others—horizontal and amacrine neurons
4) Supporting (neuroglial) cells—Müller’s cells
The specific arrangement and associations of the nuclei and processes of these cells result in the
retina being organized in ten layers that are seen with the light microscope. The ten layers of the
retina, from outside inward, are:
1. Retinal pigment epithelium (RPE).
2. Photoreceptor layer: contains the outer and inner segments of photoreceptor cells.
3. Outer limiting membrane: the apical boundary of Müller’s cells.
4. Outer nuclear layer: contains the cell bodies (nuclei) of retinal rods and cones.
5. Outer plexiform layer: contains the processes of retinal rods and cones and processes of the
horizontal, amacrine, and bipolar cells that connect to them.
6. Inner nuclear layer: contains the nuclei of horizontal, amacrine, bipolar, and Müller’s cells.
7. Inner plexiform layer: contains the processes of horizontal, amacrine, bipolar, and ganglion
cells that connect to each other.
8. Ganglion cell layer: contains the cell bodies (nuclei) of ganglion cells
9. Optic nerve fibers layer: contains ganglion cells processes that lead from the retina to the brain.
10. Inner limiting membrane: composed of the basal lamina of Müller’s cells
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 The Retinal Pigment Epithelium (RPE)
The RPE is a single layer of cuboidal cells rest on Bruch’s membrane of the choroid layer. The
pigment cells are tallest in the fovea and adjacent regions, which accounts for the darker color of
this region. Adjacent RPE cells are connected by a junctional complex consisting of gap
junctions and elaborate zonulae occludentes and adherentes. This junctional complex is the site
of the blood–retina barrier.
The pigment cells have cylindrical sheaths on their apical surface that are associated with, but do
not directly contact, the tip of the photoreceptor processes of the adjacent rod and cone cells.
Complex cytoplasmic processes project for a short distance between the photoreceptor cells of
the rods and cones. Numerous elongated melanin granules, unlike those found elsewhere in the
eye, are present in many of these processes.
The RPE serves several important functions, including the following:
1. It absorbs light passing through the neural retina to prevent reflection and resultant glare.
2. It isolates the retinal cells from blood-borne substances. It serves as a major component of
the blood–retina barrier via tight junctions between RPE cells.
3. It participates in restoring photosensitivity to visual pigments that were dissociated in
response to light. The metabolic apparatus for visual pigment resynthesis is present in the
RPE cells.
4. It phagocytoses and disposes of membranous discs from the rods and cones of the retinal
photoreceptor cells.
 The Photoreceptors
The rods and cones are the outer segments of photoreceptor cells whose nuclei form the outer
nuclear layer of the retina. The retina contains approximately 120 million rods and 7 million
cones.
Functionally, the rods are more sensitive to light and are the receptors used during periods of low
light intensity (e.g., at dusk or at night). The rod pigments have a maximum absorption at 496
nm of visual spectrum, and the image provided is one composed of gray tones (a “black and
white picture”). In contrast, the cones contains a different visual pigment molecule that is
activated by the absorption of light at the blue (420 nm), green (531 nm), and red (588 nm)
ranges in the color spectrum. Cones provide a visual image composed of color by mixing the
appropriate proportion of red, green, and blue light.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
Each rod and cone photoreceptor consists of three parts:
1. The outer segment of the photoreceptor is
roughly cylindrical or conical (hence, the
descriptive name rod or cone). This portion
of the photoreceptor is intimately related to
microvilli projecting from the adjacent
pigment epithelial cells.
2. The connecting stalk contains a cilium
composed of nine peripheral microtubule
doublets extending from a basal body. The
connecting stalk appears as the constricted
region of the cell that joins the inner to the
outer segment.
3. The inner segment is divided into an outer
ellipsoid and an inner myoid portion. This
segment contains a typical complement of
organelles associated with a cell that
actively synthesize proteins.
The outer segment is the site of photosensitivity, and the inner segment contains the metabolic
machinery that supports the activity of the photoreceptor cells.
With the TEM, 600 to 1,000 regularly spaced horizontal discs are seen in the outer segment.
These discs contain the visual pigments. Rod cells contain the visual pigment rhodopsin (also
called visual purpule); cone cells contain the visual pigment iodopsin.
Both rhodopsin and iodopsin contain a membrane-bound subunit called an opsin and a second
component called a chromophore. The opsin of rods is scotopsin; the opsins of cones are
photopsins. The chromophore of rods is a vitamin A–derived carotenoid called retinal. Thus, an
adequate intake of vitamin A is essential for normal vision. Prolonged dietary deficiency of
vitamin A leads to the inability to see in dim light (night blindness)
 Müller’s cells
Müller’s cells form the scaffolding for the entire retina.Their processes invest the other cells of
the retina so completely that they fill most of the extracellular space. The basal and apical ends
of Müller’s cells form the inner and outer limiting membranes, respectively. Microvilli extendin
from their apical border lie between the photoreceptor cells of the rods and cones.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 Bipolar cells
Bipolar cells and their processes extend to both the inner and outer plexiform layer. In the
peripheral regions of the retina, the axons of bipolar cells pass to the inner plexiform layer where
they synapse with several ganglion cells. Through these connections, the bipolar cells establish
communication with multiple cells in each layer except in the fovea, where they may synapse
only with a single ganglion cell to provide greater visual acuity in this region.
 Horizontal cells
Horizontal cells and their processes extend to the outer plexiform layer where they intermingle
with processes of bipolar cells. The cells have synaptic connections with rods, cones, and bipolar
cells. This electrical coupling of cells is thought to affect the functional threshold between rods
and cones and bipolar cells.
 Amacrine cells
Amacrine cells processes pass inward, contributing to a complex interconnection of cells. Their
processes branch extensively to provide sites of synaptic connections with axonal endings of
bipolar cells and dendrites of ganglion cells. Besides bipolar and ganglion cells, the amacrin cells
synapse in the inner plexiform layer with interplexiform and other amacrine cells.
 Specialized Areas of the Retina
The visual part of the retina is behind the ora serrata. At the optic disc (the exit of the optic nerve,
slightly medial to the posterior pole of the eyeball),
there are no photoreceptors, hence, no vision occurs
here & it is called the blind spot of the retina.
At the posterior pole of the optical axis, there is a
shallow depression called the "fovea centralis"
where the retina becomes very thin because bipolar
& ganglion cells accumulate at the periphery of the
fovea. Its center consists only of cone cells with
long thin highly packed cones & no rods, in
addition, blood vessels do not cross over the photoreceptor cells here. These factors make the fovea
the region of the maximal visual acuity & colour perception.
Around the fovea centralis is the macula lutea (5.5 mm diam.), here all the layers of the retina are
present, & the two plexiform layers are rich in carotenoids (which give a yellowish colour) that
protect the fovea from dangerous short wavelength light by their antioxidant properties. When we
move towards the perephery of the retina, the number of rods increases & cones decrease.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 The optic nerve
The optic nerve is a nerve fiber trunk formed by the
convergence of retinal ganglion cell axons at the posterior
pole of the eye. From there, they leave the globe on their
way to the brain. Each optic nerve contains about one
million myelinated axons and even more neuroglial cells.
The surface of the optic nerve is covered by pia
mater,which is continuous with that on the surface of the
brain. The optic disk (optic nerve papilla) is the small
circular site in the retina where the retinal nerve fiber layer (nonmyelinated nerve fibers) continues
into the optic nerve. The nonmyelinated nerve fibers begin to acquire myelin at the level of the
lamina cribrosa (the thin dotted line in the figure) a perforated, sievelike region of the sclera through
which optic nerve fibers and blood vessels pass. The myelinated segments of ganglion cell axons,
therefore, form the optic nerve. The neuroglial cells include oligodendrocytes, which produce myelin
for axons in the CNS, and astrocytes, which perform several nutritive and supportive functions.
Accessory Structures of the Eye
 The Conjunctiva
The conjunctiva is a thin, transparent mucous membrane that extends from the corneoscleral limbus
located on the peripheral margin of the cornea across the sclera (bulbar conjunctiva) and covers the
internal surface of the eyelids (palpebral conjunctiva). It consists of a stratified columnar epithelium
containing numerous goblet cells and rests on a lamina propria composed of loose connective tissue.
The goblet cell secretion is a component of the tears that bathe the eye.
The upper and lower limits of the conjunctiva are the superior and inferior fornices.
 The Eyelids
Are mobile folds of tissue that protect the eye. Each eyelid consists of the following layers:
1. Skin: thin, elastic skin with eyelashes at its free margin.
2. Loose C.T containing two muscles: orbicularis oculi and levator palpebri superioris.
3. Tarsal plate: tough plate of dense connective tissue that contains the meibomian glands.
4. Conjunctiva.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
The eyelid contain four types of glands:
1. Meibomian glands: long sebaceous
glands in the tarsal plate. They don't
communicate with the hair follicles of
eyelashes and produce an oily layer on
the surface of the tearfilm preventing its
rapid evaporation.
2. Glands
of
Zeis:
smaller
modified
sebaceous glands connected to the hair
follicles.
3. Glands of Moll: spiral sweat glands that
open in the hair follicles of the eyelashes.
4. Acessory lacrimal glands of Krause
and of Wolfring
 The Lacrimal Apparatus
o Lacrimal gland: It is the tear secreting gland,
located in the antero-supero-lateral part of the orbit.
It consists of several lobes with several ducts that
open in the superior conjunctival fornix. The
lacrimal gland is tubuloalveolar gland composed of
serous columnar cells rich in secretory granules.
The
secretory
portion
is
surrounded
by
myoepithelial cells. Tear film moisturizes the front
of the eye and is drained by:
o Lacrimal canaliculi, that begin at two lacrimal
puncta (tiny openings at the medial end of the free margin of each eyelid), run medially about
8mm then unite forming one canaliculus (all lined with stratified squamous epithelium) before
opening into:
o Lacrimal sac, which passes tears to the nasal cavity via the:
o Nasolacrimal duct.
Chalazion is a chronic eyelid inflammatory lesion that results from the obstruction of the ducts of either
the Zeis or meibomian glands,or both. Trapped sebaceous secretions leak into the surrounding tissue and
cause a granulomatous inflammation.This is frequently associated with blepharitis and occasionally
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becomes
secondarily infected.
warm – compresses
until
Dr. Mohammed
H. Assi Treatment
MBChB –options
MSc –include
DCH (UK)
MRCPCH applied to the outer lid
2017
acute symptoms disappear, topical antibiotics, and surgical incision if the lesion is large and disturbs vision.
Eye and Ear Histology
The ear is a complex structure that serves
two important sensory functions, hearing
(through the auditory system) and balanc
(through the vestibular system). Sensory
receptor
organs
that
serve
the
two
functions are supplied by two distinct
branches of the vetibulocochlear nerve
(cranial nerve VIII); the acoustic and the
vestibular branches.
The ear can be divided into three general
regions, the external ear, middle ear, and
inner ear
The External Ear
The external ear consists of:
1. The Auricle: an irregular plate of elastic cartillage covered by tightly adherent skin.
2. The External Auditory Meatus: a flattened canal extending from the auricle to the tympanic
membrane. It is lined with hairy thin skin, beneath which sebaceous glands and ceruminous
glands (modified sweat glands secreting cerumin or ear wax) are found. The wall of meatus is
formed by elastic cartillage (outer1/3rd) and temporal bone (inner 2/3rd). The external side of
tympanic membrane is covered by thin epidermis resting on a dense connective tissue layer.
The Middle Ear
This is a slit-like irregular cavity extending from the tympanic membrane to the internal ear. It
communicates anteriorly with the nasopharynx via the auditory tube & posteriorly with the mastoid
air cells. The middle ear is lined by simple squamous epithelium (that becomes cuboidal on the
tympanic membrane) resting on a thin lamina propria adherent to the periosteum. In the bony medial
wall of middle ear, there are two membrane-closed openings: oval and round windows. The
tympanic membrane is connected to the oval window by three small auditory ossicles: the malleus,
incus and stapes. These bones are covered by simple squamous cells and articulated by synovial
joints. Two muscles are connected to the ossicles to regulate sound conduction.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
The Internal Ear
The internal ear situated inside the petrous part of temporal bone, the internal ear consists of two
labyrinths (bony and memberanous).
 Bony labyrinth
The bony labrinth is a series of spaces within the
bone: the irregular central cavity is the vestibule,
from its posterior side, three semicircular canals
arise (each one is perpendicular to the other two).
From the anterolateral side of the vestibule, a spiral
canal arise (the cochlea), making 2.75 turns around
a bony core (the modiolus). The modiolus has a
thin spiral ridge (the osseous spiral lamina)
projecting into the cochlea along its length. Deep to
the attachment of this lamina, the modiolus contains the spiral ganglion (the sensory ganglion of the
cochlear part of the cranial nerve VIII) from which nerve fibers aggregate at the base of modiolus
forming the cochlear nerve, which enters the cranial cavity (with the vestibular nerve) through the
internal auditory meatus.
The bony labyrinth contains the membranous labyrinth, with the space between them filled with a
fluid called the "perilymph" that communicates with the CSF via the perilymphatic duct.
 Membranous labyrinth
Membernaous
labrinth
is
a
series
of
interconnected membranous cavities lined with
simple squamous epithelium and filled with fluid
(the
endolymph).
Those
cavities
roughly
resemble the bony labyrinth containing them.
They include the utricle and saccule (in the
vestibule), the utricle is connected to three
semicircular ducts (in the semicircular canals)
and the saccule is connected to the cochlear
duct (in the cochlea). From the junction of the utricle and saccule, the slender endolymphatic duct
arise and run inside the bone to end as the dilated endolymphatic sac beneath the dura mater.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 Saccule and Utricle
These are epithelium-lined connective tissue sheaths
that contain regions of spacialized epithelium called
the "maculae" (the macula of saccule lie in its floor
and that of utricle in its lateral wall).
The macula consists of high epithelial cells covered
by a thick gelatinous glycoprotein layer, at the top
of it are many otoliths (calcium salts crystals).
The epithelial cells include:
1. Receptor cells: long spindle-shaped cells having sensory nerve endings at their base. The apical
surface of each cell contain a row of steriocilia (highly specialized microvilli) and a single
cilium. Receptor cells are either type I (with a large single nerve ending surrounding the cell
base) or type II (with many nerve endings).
2. Supportive cells: columnar cells with short microvilli. They may have a role in the secretion of
the gelatinous layer.
 Semicircular Ducts
Each one of these three ducts has a dilated
ampulla at one end, that contain the receptor area
(the crista ampullaris). Crista ampullaris has the
same structure of the macula, but the gelatinous
layer is conical in shape (called the cupula) and
has no otoliths. The cupula is so high that it
touches the opposite wall of the ampulla.
Note: Afferent nerve fibers from the maculae and cristae ampullares converge forming the vestibular
part of cranial nerve VIII.
o Endolymphatic duct: Extending from the vestibule to the subdural space, this duct is lined with
simple squamous epithelium that gradually changes into simple columnar near the
endolymphatic sac.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 Cochlear duct
This blindly ended tube is the part concerned
with sound reception. The duct extends from the
base of bony cochlea to the tip, hanging
between the spiral lamina and the peripheral
wall of the cochlea. Thus, it divides the cochlea
into three compartments: scala vestibuli (above
it), scala tympani (below it) and scala media
which is the cochlear duct itself. Scala vestibuli
is connected to the oval window, and scala
tympani
to
the
round
window,
they
communicate at the tip of cochlea via the
helicotrema.
The upper wall of cochlear duct is the thin vestibular membrane (a double layer of simple
squamous epithelium). The lateral wall is thickened as the stria vascularis, while the lower wall is
the thick basilar membrane extending from the osseous spiral lamina to the spiral ligament,
carrying the specialized hearing area: the organ of Corti.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017
Eye and Ear Histology
 Organ of Corti
A complex structure resting on the basilar membrane. It consists of two masses: the spiral limbus (on
the osseous spiral lamina) and the sensory epithelium laterally, separated by the internal spiral
tunnel. The sensory epithelium includes:
1. Hair cells: these are the receptor cells connected at their base to nerve terminals of the sensory
neurons in the spiral ganglion, and have apical stereocilia. Hair calls lie in two groups:
a- Outer hair cells: 3-5 rows of high cells with a W-shape arrangement of stereocilia. The tips of
the tallest stereocilia are embeded in the tectorial membrane (a gelatinous glycoprotein structure
secreted by -and attached to-the cells of the spiral limbus).
b- Inner hair cells: a single row of shorter sensory cells with free stereocilia arranged as a line.
These cells are separated from the outer cells by a triangular space (the inner tunnel) and have a
greater afferent innervation than them.
2. Supporting cells: simple columnar cells that support hair cells. The pillar cells are supportive cells
that limit the inner tunnel.
Sensorineural Hearing Loss
Extended exposure to loud sounds can impair hearing. Outer hair cells are more subject to damage than inner
hair cells.
Durations of loud noise as short as a few minutes can produce detectable damage to stereocilia; longer
duration exposure causes death of hair cells. Damaged hair cells are not replaced in mammels.
Hair cells are normally lost with advancing age (presbyacusis). Hair-cell damage can also be produced by
prolonged exposure to high dosages of aminoglycoside antibiotics (e.g., streptomycin, neomycin), some
diuretics (e.g., furosemide), and some chemotherapy agents.
When only outer hair cells are damaged, there is an overall loss of sensitivity and a profound loss of
frequency discrimination (e.g,.ability to understand speech).
A loss of both inner and outer hair cells leads to complete deafness, which cannot be ameliorated by hearing
aids.
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Dr. Mohammed H. Assi MBChB – MSc – DCH (UK) – MRCPCH
2017