The eye is composed of three concentric layers: 1. The outer fibrous layer consisting of the sclera and cornea. 2. The middle vascular layer including the choroid, ciliary body, and iris. 3. The inner sensory layer called the retina, which contains light-sensitive cells and communicates with the brain through the optic nerve. Within these layers are additional structures including the lens, aqueous and vitreous humors, lacrimal and meibomian glands, eyelids, and conjunctiva that help support vision and protect the eye.

1. E Y E histology

2. EYE LAYERS:
FIBROUS
Each eye is composed of three
concentric tunics or layers:
1.A tough external fibrous layer
consisting of the sclera and the
transparent cornea, which join
at the region called Limbus.
Note the outermost sclera, which covers the whole eyeball except
anterior part, iris (colorful part of the eye). This region is covered by
cornea.

3. EYE LAYERS:
VASCULAR
2.A middle vascular layer that includes the
choroid , ciliary body , and iris ;
On the image above find ciliary muscles and their continuation,
choroid, which is shown in reddish color, because of its high
vascularity.
Find iris. Note that those three together make middle layer of the
eye. Note that iris is covered by cornea, and the other parts are
covered by sclera on the outer surface.

4. EYE LAYERS:SENSORY
3.An inner sensory layer, the retina ,
which communicates with the cerebrum
through the posterior optic nerve;
Retina does not cover the anterior surface
of the eye. It contains light sensitive
receptors on its surface, which “translate”
light into electrical impulses to send it to
brain.
Note the optic nerve on the posterior
surface of the eye. That’s there the fibers
derived from retina collect to deliver
impulses to brain.

5. OTHER
PARTS OF
THE EYE
lens is a perfectly transparent biconvex structure held in
place by a circular system of zonular fibers that attach it to
the ciliary body and by close apposition to the posterior
vitreous body. iris surrounds a central opening, the pupil.
Find lens on the picture. Note that it’s held in its place by
zonular fibers, which attaches it to ciliary body.
Ciliary body consists of ciliary muscle and ciliary
processes covered by ciliary epithelium.
Accommodation reflex- in response to focus on
near objects, lens shape and pupil size change.
Those changes are mediated by CN III, which
innervates ciliary muscle.

6. CHAMBERS OF
THE EYE
❖ Anterior chamber is located between
cornea and iris;
❖ Posterior chamber is located between
iris and lens;
✔ both of them are filled with aqueous
humor.
❖ Note the posterior vitreous chamber,
surrounded by the retina. It lies behind
the lens and its zonular fibers and
contains a large gelatinous mass of
transparent connective tissue called the
vitreous body ;
❖ Vitreous body consists of transparent,
gel-like connective tissue that is 99%
water (vitreous humor), with collagen
fibrils and hyaluronate, synthesized by a
small amount of mesenchymal
hyalocytes. Vitrous body also contains a
few macrophages.

7. FIBROUS
LAYER: SCLERA
Limbus- the edge of the cornea,
where it transforms into sclera.
⮚Mainly consists of dense
connective tissue;
⮚Unlike cornea, sclera is
vascular;
⮚exposed, anterior portion of
the sclera is covered by
conjunctiva, thin mucus
layer;
⮚Tendons of the extraocular
muscles which move the eyes
insert into the anterior region
of the sclera;
⮚Posteriorly the sclera thickens
and joins with the epineurium
covering the optic nerve.
Note the insertion places of extraocular
muscles (innervated by CN III, IV, Vi) in
sclera.
Red arrow shows insertion point of sclera
with epineurium covering CN II.

8. FIBROUS LAYER: CORNEA
Cornea is transparent and completely
avascular.
Histologically, it consist of five layers:
1. An external nonkeratinized stratified
squamous epithelium; basal cells of this
layer have a high proliferative capacity
important for renewal and repair of the
corneal surface; They emerge from stem
cells in the limbus;
2. Bowman’s membrane, which is the
basement membrane of the external
stratified epithelium;
3. The thick stroma, consists of parallel
collagen fibers and scattered fibroblasts;
4. Descemet’s membrane - basement
membrane of the endothelium;
5. An inner simple squamous
endothelium.

9. LIMBUS
⮚ Limbus is a transitional area
between sclera and cornea;
⮚ surface epithelium becomes
more stratified transforming into
conjunctiva, covering the
anterior (visible) part of the
sclera;
⮚ Stroma becomes vascular and
less well-organized at the
limbus, as the collagen bundles
merge with those of the sclera.
❖ the limbus Descemet’s membrane and its simple endothelium are replaced
with a system of irregular endothelium- lined channels called the trabecular
meshwork; they open into scleral venous sinus, called canal of Schlemm.
Those structures makes a way for drainage of aqueous humor.
❖ Ciliary process epitheium secretes aqueous humor in posterior chamber. This
fluid reaches anterior chamber through pupil, moves from anterior chamber
through trabecular meshwork and then through Schlemm’s canal drains into
episcleral veins.

10. VASCULAR LAYER
(UVEA)
▪ Uvea, the middle, vascular layer consists
of three parts, from posterior to anterior:
⮚the choroid,
⮚the ciliary body,
⮚the iris.
Choroid is a highly vascularized loose connective
tissue. High amount of melanocytes form a
characteristic black appearance, which prevents
light from entering the eye except through the pupil.
Choroid is made of two layers:
✔ Choido-capillary lamina: highly vascularized
tissue;
✔ Brush’s membrane: a thin extracellular sheet,
is composed of collagen and elastic fibers (see
the arrow).

11. Ciliary body represents the anterior expansion of the uvea
that encircles the lens and lies posterior to the limbus.
Ciliary body consists of:
❖ Ciliary muscle-Contraction of these muscles affects
the shape of the lens and is important in visual
accommodation (innervated by CN III);
❖ Ciliary processes- a radially arranged series of
ridges extending from the inner highly vascular region
of the ciliary body, its surface epithelium is specialized
for aqueous humor secretion;
❖ ciliary zonule is a system of many radially oriented
fibers, which held lens in its place.
Epithelium of ciliary processes.
Zonular fibers

12. IRIS
⮚ Iris is the most anterior part of the uvea which forms a central hole, pupil;
⮚ The anterior surface of the iris, exposed to aqueous humor in the anterior chamber
(AC), has no epithelium and consists only of a matted layer of interdigitating
fibroblasts and melanocytes.
⮚ Cells of the external pigmented epithelium (PE) are very rich in melanin granules to
protect the eye’s interior from an excess of light.
⮚ Cells of the other layer are myoepithelial, less heavily pigmented, and comprise the
dilator pupillae muscle (DPM).
⮚ Near the pupil, fascicles of smooth muscle make up the sphincter pupillae muscle
(SPM).
❖ The arrows show blood vessels. Iris is highly vascularized.

13. L
E
N
S
▪ The lens is a transparent biconvex avascular, highly
elastic structure which focuses light on the retina;
it has three principal components:
❖lens capsule- composed of proteoglycans and type
IV collagen surrounds the lens and provides the
place of attachment for the fibers of the ciliary zonule;
❖ subcapsular epithelium- consists of a single layer
of cuboidal cells present only on the anterior surface
of the lens; those epithelial cells divide to provide new
cells that differentiate as lens fibers.
❖Lens fibers are highly elongated, terminally
differentiated cells that appear as thin, flattened
structuresare packed tightly together and form a
perfectly transparent tissue highly specialized for light
refraction.
LC =lens capsule; LE = lens epithelium;
At the equator of the lens, near the ciliary
zonulae, the epithelial cells proliferate and give
rise to cells that align parallel to the epithelium
and become the lens fibers. During
differentiation process they lose their
organelles and nuclei and become filled with
groups of proteins called crystalline. DLF
represents differentiating fibers. The core fibers
✔ Incoming light rays are
refracted onto the retina by
transparent parts of the eye
which includes cornea,
lens, aqueous and
vitreous fluid.

14. ACCOMMODA
TION OF THE
EYE
⮚ The ciliary muscle, innervated by CN III is
responsible for changing the distention rate of the
zonules;
❑ When the eye is at rest or gazing at distant objects,
ciliary muscles relax and the resulting shape of the
ciliary body puts tension on the zonule fibers, which
pulls the lens into a flatter shape. (see a)
❑ To focus on a close object the ciliary muscles
contract, causing forward displacement of the ciliary
body, which relieves some of the tension on the
zonule and allows the lens to return to a more
rounded shape and keep the object in focus. (see b)
⮚ this process which permits focusing on near and far
objects by changing the curvature of the lens is
called visual accommodation.

15. RETINA
the innermost tunic of the eye, retina, consists
of:
⮚ The outer pigmented layer- a simple
cuboidal epithelium
⮚ attached to Bruch’s membrane;
The inner neural layer - thick and stratified
with various neurons and photoreceptors.
Note that the
neural structure
and visual
function of retina
extend
anterior only as
far as the ora
serrata, which
represents the
junction between
retina and ciliary
body.

16. PIGMENTED
EPITHELIUM OF
RETINA
Functions of pigmented epithelium include:
✔absorbs scattered light;
✔cells of the pigmented epithelium have many tight
junctions and form an important part of the protective
blood-retina barrier
✔plays key roles in the visual cycle of retinal
regeneration, isomerize all-trans-retinal into 11-cis-
retinal;
✔Phagocytosis shed components;
✔removes free radicals by various protective
antioxidant activities and support the neural retina by
secretion of ATP, various polypeptide growth factors,
and immunomodulatory factors.

17. NEURAL
RETINA
▪ The neural retina functions as an outpost of the CNS
with glia and several interconnected neuronal subtypes
in well-organized strata, which contains nine distinct
layers;
▪ From those nine, three layers are major, containing the
nuclei of the interconnected neurons.
1. outer nuclear layer (ONL) located under pigmented retina, containing cell
bodies of photoreceptors (the rod and cone cells);
2. inner nuclear layer (INL) contains the nuclei of various neurons, notably the
bipolar cells, amacrine cells, and horizontal cells, all of which make specific
connections with other neurons and integrate signals from rods and cones over a
wide area of the retina;
3. ganglionic layer (GL) has ganglion neurons with much longer axons. These
axons make up the nerve fiber layer (NFL) and converge to form the optic nerve
which leaves the eye and passes to the brain.

18. between those three layers containing
cell bodies, their axons and dendrites
make two other layers:
1. The outer plexiform layer includes
axons of the outer nuclear layer
(rods and cones) and dendrites of
association neurons in the inner
nuclear layer.
2. The inner plexiform layer consists
of axons and dendrites connecting
neurons of the inner nuclear layer
with the ganglionic layer cells.
All neurons of the retina are supported physically
by glial cells called Müller cells (find them on the
schematic picture).
Müller cells organize two boundaries:
1. The outer limiting layer junctions formed
between the photoreceptors and Müller cell
processes;
2. The inner limiting layer consists of terminal
expansions of Müller cell processes that
cover the collagenous membrane of the
vitreous body.

19. Two remaining layers include:
⮚ The nerve fiber layer,
containing the ganglionic cell
axons that converge at the optic
disc and form the optic nerve (N8
on the picture);
⮚ The rod and cone layer, which
contains the outer segments of
these cells where the
photoreceptors are located. This
region is in tight connection with
pigmented epithelium(N2 on the
picture)
Look at the schematic picture on the left.
Find all nine layers of nervous retina,
plus pigmented epithelium ;
Differentiate three cell-body containing
layers and their fibers.
Find Muller cells and note their
connections to retinal neurons.

20. RODS AND
CONES
RODS
❖ rod cells are extremely sensitive to light;
❖ The rod-shaped outer segment represents
modified primary cilium and consists mainly of
flattened membranous discs;
❖ Proteins on the cytoplasmic surface of each
disc include abundant rhodopsin (or visual
purple) which is bleached by light and initiates
the visual stimulus.
CONES
⮚ cone cells in the human retina produce
color vision in adequately bright light;
⮚ There are three morphologically similar
classes of cones, each containing one type
of the visual pigment iodopsin (or
photopsins).
⮚ Each of the three iodopsins has maximal
sensitivity to light of a different wavelength,
in the red, blue, or green regions of the
visible spectrum, respectively.

21. OPTIC
DISC
▪ Optic disc, lacks photoreceptors and all
conducting neurons.
▪ It occurs where axons of ganglionic
cells converge to produce the optic
nerve which leaves the retina.
▪ The central artery and vein of the
retina enter at the optic disc.
⮚ “blind spot” (optic disc)
contains only fibers, none
of the other layer of retina
is present there. That’s the
place where optic nerve
(CN II) is formed.

22. FOVEA
CENTRALIS;
MACULA LUTEA
✔ Fovea centralis is the area of the retina near optic disc,
where visual acuity or sharpness is maximal; It is a
shallow depression with only cone cells at its center,
ganglion cells and other conducting neurons are located
only at its periphery.
✔ Blood vessels do not cross the fovea ;
✔ Surrounding the fovea centralis is the macula lutea.
✔ Within the GL of the entire retina a subset of ganglion
cells serve as nonvisual photoreceptors. These neurons
contain protein melanopsin and serve to detect
changes in light quantity and quality during each 24-
hour dawn/dusk cycle. Signals from these cells pass to
the pineal gland, where they help establish the body’s
physiologic circadian rhythms.
refracted light falls
directly on the cones of fovea
centralis.

23. CONJUNCTIVA
; EYELIDS
✔ Conjunctiva is a thin, transparent mucosa that
covers the exposed, anterior portion of the sclera
and continues as the lining on the inner surface
of the eyelids.
✔ Goblet cells scattered within stratified columnar
epithelium of conjunctiva produce mucus, which
is added to the tear film that coats this epithelium
and the cornea.
⮚ Eyelid is a pliable structures containing skin, muscle,
and conjunctiva that protect the eyes;
⮚ On the distal edge of it large follicles with eyelashes
are present;
⮚ Beneath the skin are fascicles of muscles that fold the
eyelids;
⮚ adjucent to the conjunctiva is a dense fibroelastic plate
called the tarsus that supports the other tissues;
⮚ The tarsus surrounds a series of glands called
Meibomian glands, which produce oil, that forms a
surface layer on the tear film, reduces its rate of
evaporation, and helps lubricate the ocular surface.

24. LACRIMAL
GLANDS
▪ The lacrimal glands produce fluid continuously for the tear film
that moistens and lubricates the cornea and conjunctiva and
supplies O2 to the corneal epithelial cells;
▪ Tear fluid also contains various metabolites, electrolytes, and
proteins of innate immunity such as lysozyme;
▪ The main lacrimal glands are located in the upper temporal
portion of the orbit;

25. THANKS FOR ATTENTION ☺