
EYES: THE PHOTORECEPTOR SYSTEM
The eye is a highly specialized organ of photoreception. It analyses the form, intensity, and
colour of light reflected from objects. The eyes are located in protective areas of the skull,
the orbits. The eye measures approximately 25 mm in diameter. It is suspended in the bony
orbital socket by six extrinsic muscles that control its movement. A thick adipose tissue
muscles are coordinated so that the eye moves symmetrically about their own central axis.
Each eye is composed of three concentric tunics or layers
A tough external layer consisting of Sclera
Cornea
A more vascular layer consisting of Choroid
Ciliary body
Iris
The inner sensory layer, the retina, consisting of Outer pigmented epithelium
Inner retina proper
The photosensitive inner layer of the retina communicates with the cerebrum through the
optic nerve on the posterior side, and the anterior edge is called the ora serrate. Within the
eyeball, the lens and the vitreous body are the transparent refractive elements in addition to
the cornea anteriorly. The cornea and lens concentrate and focus light on the retina,
photoreceptors in the retina detect the intensity and colour of the light.
Fibrous Layer
SCLERA
The sclera is the opaque white posterior five-sixth of the external layer .It is the fibrous
external layer of the eyeball protecting the delicate internal structures and providing sites
for muscle insertion. The sclera is relatively avascular with dense connective tissue
composed of collagen fibers, ground substance, and fibroblasts. The tendons of the
extraocular muscles that move the eyes are inserted into the anterior areas of the sclera.

CORNEA
The cornea forms the anterior one-sixth of the eyeball. It is colourless, transparent, and
completely avascular. In histological section, the cornea consists of five layers:
An
external stratified squamous epithelium
An
anterior limiting membrane
(
Bowman's membrane
of the basement
membrane of the stratified epithelium)
The
stroma
A
posterior limiting membrane
(
Descemet's membrane
, the basement
membrane of endothelium), and
An
inner simple squamous endothelium
The stratified surface epithelium is nonkeratinized, with five layers of cells comprising about
10% of the corneal thickness. The basal layer shows high mitotic activity and the flattened
surface cells are covered with a protective layer of tear film of lipid, glycoprotein and water.
The corneal epithelium has the richest nerve supply of any tissue.
The thick stroma, comprising about 90% of the corneal thickness is composed of collagen
bundles, arranged in uniform arrays contributing to the transparency of this tissue. Between
the collagen lamellae, flattened fibroblast-like cells and intercellular substance rich in
proteoglycans to maintain the organization and spacing of the collagen fibrils.
The posterior surface of the stroma bounded by a thick structure (Descemet's membrane),
composed of fine collagen fibers, upon which lies the corneal endothelium. The endothelial
cells are active in protein synthesis to maintain the basement membrane, and in pumping
sodium ions into the anterior chamber.
LIMBUS
The
limbus
, or
corneoscleral junction
, encircles the cornea, and is a transitional zone
where the transparent cornea joins the opaque sclera. At the corneoscleral junction,
Descemet's membrane and its simple endothelium are replaced with a system of irregular
endothelium-lined channels and called
trabecular meshwork
, which penetrate the stroma
and allow slow continuous drainage of aqueous humor from the anterior cavity. The fluid is
pumped from these channels into the adjacent larger space of the
scleral venous sinus
, or
canal of Schlemm
, from which it drains into the aqueous and episcleral veins of the sclera.
Vascular Layer
The more vascular middle layer or the
uvea
. It consists of three parts, from the posterior to
the anterior: the
choroid
, the
ciliary body
, and the
iris
.
CHOROID
The choroid is a highly vascular tunic in the posterior two-thirds of the eye. It is composed of
loose connective tissue, rich in collagen, blood vessels, elastic fibers, fibroblasts,

melanocytes, macrophages, lymphocytes, mast cells and plasma cells. The abundant
melanocytes give the layer its characteristic black colour and block the light from entering
the eye. The inner region of the choroid (
choriocapillary lamina
) is rich in
microvasculature for the nutrition and maintenance of the retina. A thin amorphous hyaline
sheet,
Bruch's membrane
separates the choriocapillary layer from the retina.
CILIARY BODY
The ciliary body, an anterior expansion of the choroid at the level of the lens, is a thickened
ring of tissue inside the anterior portion of the sclera. The ciliary body has a stroma of loose
connective tissue, rich in microvasculature, elastic fibers, and melanocytes, and smooth
muscle. The
ciliary muscle is
inserted into the sclera, and when contracts, decreases the
diameter of the ciliary body, reducing the tension of the fibers (
the
zonule
) that connects
this body to the lens. This allows the lens to be more rounded and focus light from nearby
objects onto the retina. The ciliary muscle contraction is under parasympathetic stimulation
and it is important in visual accommodation.
The ciliary body, with its
ciliary processes
is covered by a double layer of low columnar
epithelium, the inner layer of which is rich in melanin and corresponds with the retinal
pigment epithelium. The surface layer of cells lacks melanin and is contiguous with the
sensory layer of the retina. The surface nonpimented cells have the characteristic features of
ion-transporting cells, they actively transport fluid from the vascular stroma into the
posterior chamber forming the
aqueous humor
. This fluid similar to the plasma, with very
low protein (less than 0.1%).The aqueous humor is secreted into the posterior chamber,
flows towards the lens, passing between it and the iris to reach the anterior chamber
through the pupil. The aqueous then flows into the angle formed by the cornea with the
basal part of the iris and penetrates the channels of the trabecular meshwork at the
corneoscleral junction (limbus), from which it is pumped into the scleral venous sinus.
The surface epithelial cells in the grooves between the ciliary processes secret elastin,
fibrillin, and proteoglycans, which assemble as thin fibers attached to the surface of the lens
capsule. The lens is anchored within the lumen of the ciliary body by the zonular fibers
which are called the
ciliary zonule
or the
suspensory ligament of the lens
IRIS
The
iris
is the most anterior extension of the uvea that partially covers the lens, leaving a
round opening in the center called the
pupil
. The anterior surface of the iris, exposed to the
anterior chamber, is not covered by epithelium, but consists of an irregular layer of
fibroblasts and melanocytes. The stroma consists of loose connective tissue with
microvasculature. The posterior surface of the iris is smooth, with a two-layered epithelium
continuous with that covering the ciliary body. The internal layer is composed of partially
pigmented myoepithelial cells. The external layer cells are filled with melanin granules, and
are in direct contact with the posterior chamber. The pigment epithelium of the iris prevents
the light from entering the eye except through the pupil.

The abundant melanocytes in the vascular layer of the eye act to keep stray light rays from
interfering with image formation. Melanocytes of the iris stroma also provide the colour of
the eyes. In individuals with very few light pigmented cells in the stroma, light with a blue
colour is reflected back from the black pigmented epithelium on the posterior iris surface. As
the number of melanocytes and amount of collagen increases in the stroma, the iris colour
changes through various shades of green, gray, and brown. Individuals with albinism have
almost no pigment and the pink colour of their irises is due to the reflection of incident light
from the blood vessels of the stroma.
Dilator pupillae muscle
is a radial extension of myoepithelial cells along the posterior
surface of the iris, while the
sphincter pupillae muscle
bundles are disposed in a circular
array near the pupillary margin. The dilator pupillae is innervated by the sympathetic
innervation, while the sphincter pupillae fibers have parasympathetic innervation.
Lens
The lens is a transparent biconvex structure immediately behind the iris to focus the light on
the retina. It is avascular and is highly elastic. The lens has three components:
LENS CAPSULE
: is thick and rich in proteoglycans and type IV collagen. The lens capsule
protects the underlying cells and for the attachment of zonular fibers.
LENS EPITHELIUM
: A single layer of cuboidal epithelium
LENS FIBERS
: are highly elongated, thin, flattened structures. The fibers are densely packed
together forming a transparent tissue for light refraction.
Vitreous Body
The vitreous body occupies the vitreous chamber behind the lens. It is composed of
transparent connective tissue containing 99% water. It is a gel-like connective tissue
contains typeIV collagen. The only cells in the vitreous body are few macrophages and
hyalocytes which synthesize the hyaluronate and collagen
Retina
The retina is the inner layer of the eye, externally it rests on the choroid, and internally it is
associated with the vitreous body, and it consists of two layers. The inner one, the neural
retina, contains the neurons and photoreceptors; the visual region of this layer extends
anteriorly as far as the ora serrata continues as a cuboidal epithelium lining the surface of
the ciliary body and posterior iris. The outer pigmented layer is an epithelium resting on
Bruch's membrane just inside the choroid. This pigmented, cuboidal epithelium also lines
the ciliary body and posterior iris. The pigmented epithelium consists of low columnar cells
with basal nuclei. The cells have well-developed junctional complexes, gap junctions and
numerous invaginations of the basal membrane with mitochondria. The apical ends of the

cells extend processes and projections that surround the tips of the photoreceptors. Melanin
granules are numerous in the extensions and apical cytoplasm. These cells contain
phagocytic vacuoles, secondary lysosomes, smooth ER and peroxisomes, with specialized
regions for isomerization of all-trans-retinal (derived from vitamin A) and its transport to
the photoreceptor. The function of the cells in the pigmented epithelium includes the
following:
An important part in blood-retina barrier
Absorb light passing through the retina to prevent its reflections
Phagocytose shed components from the adjacent rod and cones,
Remove free radicals
Isomerize and regenerate the retinoids used as chromophores by the rods and cones
The posterior, photosensitive part of the retina is a complex structure containing more than
30 subtypes of neurons interconnected via synapses. The neurons and supporting groups are
divided into four groups
Photoreceptors, the rods and cones
Conducting neurons, bipolar and ganglion cells
Association and other neurons-horizontal, centrifugal, and amacrine
supporting cells- Muller and neuroglial cells
The arrangement and association of the nuclei and processes of these cells result in the
retina being organized in ten layers seen in the light microscope. These ten layers from
outside inward are:
1.
Pigment epithelium- the outer layer of the retina, it is not part of the neural retina,
but intimately related to it
2.
Layer of rods and cones- contains the inner and outer segments of photoreceptor
cells
3.
Outer limiting membrane- the apical boundary of Muller cells
4.
Outer nuclear layer- contains the cell bodies (nuclei) of retinal rods and cones
5.
Outer plexiform layer- contains the processes of the rods and cones and processes of
the horizontal, amacrine, and bipolar cells that connect to them
6.
Inner nuclear layer- contain the cell bodies (nuclei) of horizontal, amacrine, bipolar,
and ganglion cells that connect to each other
7.
Inner plexiform layer
-
contains the processes of horizontal, amacrine, bipolar,
and ganglion cells that connect to each other
8.
Ganglion cell layer- contain the cell bodies (nuclei) of ganglion cells
9.
Layer of optic nerve fibers- contains processes of ganglion cells that lead from the
retina to the brain
10.
Inner limiting membrane- composed of the basal lamina of Muller's cell

ROD CELLS
The human retina contains approximately 120million
rod cells
. They are extremely sensitive
to light, they allow vision at dusk or nighttime. Rod cells are thin, elongated cells
(50microns+3microns), composed of two distinct segments. The outer segment is
photosensitive; the inner segment is for the metabolic and energy production.
When examined with electron microscope, the outer rod-shaped segment consists mainly of
600-1000 flattened membranous discs, surrounded by plasma membrane. The connecting
stalk is a constriction between the inner and outer segment. It is a modified cilium with a
basal body. The inner segment is rich in glycogen, mitochondria, and polyribosomes for the
synthesis of
rhodopsin (visual purple)
CONE CELLS
The human retina has 6 or 7 million cone cells, which are less sensitive to low light than the
rod cells and are specialized for colour vision in bright light. The cone cells contain the visual
pigment
iodopsin
with sensitivity to red, blue, or green regions of the visible spectrum.
Cone cells are similar to rod ones, but the outer segment of the cones is shorter and more
conical.
SPECILIZED AREAS OF THE RETINA
The posterior area of the retina where the optic nerve leaves the retina is devoid of
photoreceptors and is known as the
blind spot
of the retina, or the
optic disc
.
On the temporal side of the optic disc, lies a specialized area of the retina called
fovea
centralis
. The fovea is a shallow depression having only cone cells at its center, with the
bipolar and ganglion cells located only at the periphery. Blood vessels do not cross over this
area and the light falls directly on the cones in the central part of the fovea, which helps
account for the precise visual acuity of this region.
Macula lutea
or
macula
is 5.5mm in diameter. Here all layers of the retina are present and
it contains carotenoids, which gives this area its yellowish colour
CONJUNCTIVA, EYELIDS, and LACRIMAL GLANDS: Read them in your textbook