EYES: THE PHOTORECEPTOR
Each eye is composed of three concentric tunics or layers:
A tough external fibrous layer consisting of the sclera
and the transparent cornea
■ A middle vascular layer that includes the choroid , ciliary
body, and iris; and
■ An inner sensory layer, the retina , which communicates
with the cerebrum through the posterior optic nerve.
Not part of these layers, the lens is a perfectly transparent biconvex structure held in place by a circular system of zonular
fibers that attach to the ciliary body and by close apposition
to the posterior vitreous body.
Partly covering the
anterior surface of the lens is an opaque pigmented extension of
the middle layer called the iris , which surrounds a central opening,
Located in the anterior portion of the eye, the iris and lens
are bathed in clear aqueous humor that fills both the anterior
chamber between the cornea and iris and the posterior chamber
between the iris and lens.
Aqueous humor flows through the pupil that connects these two chambers. The posterior vitreous chamber, surrounded by the retina, lies behind the lens and its zonular fibers and contains a large gelatinous mass of transparent connective tissue called the
In the 4-week embryo, epithelial optic vesicles bulge
bilaterally from the forebrain, then turn into the optic
stalks bearing optic cups.
Inductive interactions between the optic cups and the
overlying surface ectoderm cause the latter to invaginate
and eventually detach as the initially hollow lens vesicles.
The optic stalk develops as the optic nerve and in an
inferior groove called the choroid fissure encloses the
hyaloid vessels that supply blood for the developing lens
and optic cup.
In the ensuing weeks, head mesenchyme differentiates to
form most of the tissue in the eye’s two outer layers and the
vitreous. Ectoderm of the optic cup differentiates as the
retina and surface ectoderm creates the corneal epithelium.
When the lens is fully formed, the distal
hyaloid artery and vein disappear, leaving only the blood
supply to the retina.
Fibrous Layer: Has 2 parts
1. Sclera: Mainly dense irregular connective tissue with flat bundles of type I collagen parallel to to the organ surface.
Protects delicate internals, supports shape.
Extrinsic eye muscles attachment site.
Tendons of the extraocular muscles which move the eyes
insert into the anterior region of the sclera. Posteriorly the sclera
thickens to approximately 1 mm and joins with the epineurium
covering the optic nerve. Where it surrounds the choroid, the
sclera includes an inner suprachoroid lamina, with less collagen,
more fibroblasts, elastic fibers, and melanocytes.
Fibrous Layer: 2 parts
In contrast to the sclera, the anterior one-sixth of the eye—the
cornea—is transparent and completely avascular.
Protects anterior surface of the eye
refracts (bends) incoming light.
A section of the cornea shows five distinct layers:
■■ An external stratified squamous epithelium;
■■ An anterior limiting membrane (Bowman’s membrane),
which is the basement membrane of the external
■■ The thick stroma;
■■ A posterior limiting membrane (Descemet’s membrane),
which is the basement membrane of the endothelium;
■■ An inner simple squamous endothelium.
Vascular Tunic (Middle Layer) of eye.
Areolar connective tissue;
Supplies nourishment to retina
Pigment absorbs extraneous light
Vascular Tunic (Middle Layer) of eye.
Ciliary smooth muscle and ciliary processes;
covered with a secretory epithelium.
Epithelium secretes aqueous humor
Holds suspensory ligaments that
attach to the lens and change lens
shape for far and near vision.
Vascular Tunic (Middle Layer) of eye.
Two layers of smooth muscle (sphincter pupillae
and dilator pupillae) and connective tissue, with a
Controls pupil diameter and thus
the amount of light entering the
Retina (Internal Layer) of the eye.
Pigmented epithelial cells
Absorbs extraneous light.
Provides vitamin A for
Retina (Internal Layer) of the eye.
Photoreceptors, bipolar neurons, ganglion cells,
and supporting Müller cells.
Detects incoming light rays; light
rays are converted to nerve signals
and transmitted to the brain
The basal cells have a high proliferative
capacity important for renewal and repair of the corneal surface
and emerge from stem cells in the corneoscleral limbus that
encircles the cornea.
As another protective adaptation, the corneal epithelium
also has one of the richest sensory nerve supplies of any tissue.
has one of the richest sensory nerve supplies of any tissue.
Th e basement membrane of this epithelium, often called
contributes to the stability and strength of the cornea, helping to protect
against infection of the underlying stroma.
The stroma, or substantia propria, makes up 90% of
the cornea’s thickness and consists of approximately 60 layers
of parallel collagen bundles aligned at approximately right
angles to each other and extending almost the full diameter
of the cornea.
Between the collagen lamellae are cytoplasmic extensions
of flattened fibroblast-like cells called keratocytes.
The ground substance around these cells contains proteoglycans
such as lumican, with keratan sulfate and chondroitin
sulfate, which help maintain the precise organization and
spacing of the collagen fibrils.
The posterior surface of the stroma is bounded by another
thick basement membrane, called Descemet’s membrane,
which supports the internal simple squamous corneal endothelium.
This endothelium maintains Descemet’s membrane and
includes the most metabolically active cells of the cornea.
*responsible for regulating the proper hydration state
of the corneal stroma to provide maximal transparency and
optimal light refraction.
Encircling the cornea is the limbus, a transitional area
where the transparent cornea merges with the opaque sclera.
Here Bowman’s membrane ends
and the surface epithelium becomes more stratified as the conjunctiva that covers the anterior part of the sclera.
Also at the limbus Descemet’s membrane and its simple
endothelium change into a system of irregular endothelium-
lined channels called the trabecular meshwork.
they allow slow, continuous drainage of aqueous
humor from the anterior chamber. This fluid moves from
these channels into the adjacent larger space of the scleral
venous sinus, or canal of Schlemm.
The eye’s more vascular middle layer, known as the uvea, consists
of three parts:
from posterior to anterior:
and the iris
Located in the posterior two-thirds of the eye, the choroid consists
of loose, well-vascularized connective tissue and contains
These form a characteristic
black layer in the choroid and prevent light from entering the
eye except through the pupil. Two layers make up the choroid
The inner choroido-capillary lamina has a rich microvasculature
important for nutrition of the outer retinal layers.
■■ Bruch’s membrane, a thin extracellular sheet, is composed
of collagen and elastic fibers surrounding the
adjacent microvasculature and basal lamina of the retina’s
The ciliary body, the anterior expansion of the uvea that encircles
the lens, lies posterior to the limbus.
Like the choroid, most of the ciliary body rests on the sclera.
Important structures associated with the ciliary body include
Ciliary muscle makes up most of the ciliary body’s
stroma and consists of three groups of smooth muscle
fibers. Contraction of these muscles affects the shape of
the lens and is important in visual accommodation.
Ciliary processes: Cells of this dual epithelium have extensive basolateral folds with Na+/K+-ATPase activity and are
specialized for secretion of aqueous humor.
The ciliary zonule: is a system of many radially oriented
fibers composed largely of fibrillin-1 and 2 produced by
the nonpigmented epithelial cells on the ciliary processes. The fibers extend from grooves between the ciliary processesand attach to the surface of the lens,
holding that structure in place.
The iris is the most anterior extension of the middle uveal layer
which covers part of the lens, leaving a round central pupil.
The posterior surface of the iris has a two-layered epithelium
continuous with that covering the ciliary processes, but
very heavily filled with melanin. The highly pigmented posterior
epithelium of the iris blocks all light from entering the
eye except that passing through the pupil.
Myoepithelial cells form a partially pigmented epithelial layer and extend contractile processes radially as the very thin dilator pupillae muscle.
Smooth muscle fibers form a circular bundle
near the pupil as the sphincter pupillae muscle.
The dilator and sphincter muscles of the iris have sympathetic and parasympathetic
innervation, respectively, for enlarging and constricting the pupil.
Melanocytes of the iris stroma provide the color of one’s
The lens is a transparent biconvex structure suspended immediately
behind the iris, which focuses light on the retina.
the lens is a unique avascular
tissue and is highly elastic, a property that normally decreases
with age. The lens has three principal components:
A thick (10-20 μm), homogeneous 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 lens epithelium consists of a single layer
of cuboidal cells present only on the anterior surface
of the lens. Area here allows for growth of the
lens and continues at a slow, decreasing rate near the
equator of the lens throughout adult life.
Lens fibers are highly elongated, terminally differentiated
cells that appear as thin, flattened structures. Its cytoplasm
becomes filled with a group of proteins called crystallins,
and the organelles and nuclei undergo autophagy. Lens
fibers are packed tightly together and form a perfectly
transparent tissue highly specialized for light refraction.
The lens is held in place by fibers of the ciliary zonule,
which extend from the lens capsule to the ciliary body. Together with the ciliary muscles....
this structure allows the process of visual accommodation, which permits focusing on near and far objects by changing the curvature of the lens.
In the fourth decade of life presbyopia
normally causes the lenses to lose elasticity and their ability to undergo
The vitreous body occupies the large vitreous chamber behind
the lens. It is 99% water gel like.
T cells in the vitreous body are a small mesenchymal population near the membrane
called hyalocytes, which synthesize the hyaluronate and collagen,
and a few macrophages.
The retina, the innermost tunic of the eye, develops with
two fundamental sublayers from the inner and outer layers of
embryonic optic cup.
1. The outer pigmented layer is a simple cuboidal epithelium
attached to Bruch’s membrane and the choroidocapillary
lamina of the choroid.
2. The inner retinal region, the neural layer, is thick and
stratified with various neurons and photoreceptors.
Retina Pigmented Epithelium:
This cellular region also contains numerous
phagocytic vacuoles and secondary lysosomes, peroxisomes,
and abundant smooth ER (SER) specialized for retinal
(vitamin A) isomerization.
Some functions of the Retina Pigmented Epithlium are:
1. The pigmented layer absorbs scattered light that passes
through the neural layer, supplementing the choroid in
2. With many tight junctions, cells of the pigmented epithelium
form an important part of the protective blood-retina
barrier isolating retina photoreceptors from the highly
vascular choroid and regulating ion transport between
More Retina Pigmented Epithelium Functions:
3. After Phagocytosis of coponents,
The cells play key roles in the visual cycle of retinal
all-trans-retinal released from photoreceptors and produce
11-cis-retinal that is then transferred back to the photoreceptors.
5. Cells of pigmented epithelium remove free radicals by
various protective antioxidant activities and support the
neural retina by secretion of ATP, various polypeptide
growth factors, and immunomodulatory factors.
Neural retina functions as
an outpost of the CNS with glia and several interconnected
neuronal subtypes in well-organized strata. Nine distinct layers comprise the neural retina.
1. Near the pigmented epithelium, the outer nuclear layer
(ONL) contains cell bodies of photoreceptors (the rod and
2. The 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. Near the vitreous, the ganglionic layer (GL) has neurons
(ganglion cells) 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.
Neural Retina other layers
4. The outer plexiform layer (OPL) includes axons of the
photoreceptors and dendrites of association neurons in the
5. The inner plexiform layer (IPL) consists of axons and
dendrites connecting neurons of the INL with the ganglion
The rod and cone cells, named for the shape of their outer
segments, are polarized neurons with their photosensitive portions
aligned in the retina’s rod and cone layer (RCL) and their
axons in the IPL.
All neurons of the retina are supported physically by glial
cells called Müller cells.