The eye is a complex and sensor organ that is gathering
of visual information.
The eye is made up of
three main parts: eyeball (globe), orbit (eye socket), accessory (adnexal)
The eye’s accessory
structures contain the eyelids, conjunctiva, caruncle and lacrimal glands.
The orbit (eye socket) is made up of bone. It contains
the eyeball and the connective tissues that protect the eye. The muscles attach
to the eyeball make it easily move in different directions.
The eyeball (globe) is rich in blood
vessels. Human eyes are roughly spherical, ?lled with a transparent gel-like
substance called the vitreous humor. It
supports the internal structures and maintains the shape of the eye.
The eyeball consists of
three concentric layers (tunics), whose names re?ect their basic functions: a
?brous tunic, consisting of the sclera and the cornea; a vascular pigmented
tunic, comprising the choroid, ciliary body, and iris; and a nervous tunic, the
retina, see Figure(3).
ü The Fibrous Tunic
The ?brous tunic is the outermost layer
of eye tissue. It is a tough and inelastic corneoscleral envelope. The cornea
is dense fibrous connective tissue. It is transparent
to allow passing the light. The sclera is
tough and white connective tissue. Function: protection and support.
ü Vascular tunic
The vascular tunic is the middle layer, also known as
uvea. It consists of the choroid, the ciliary body and the iris, which is
perforated by the pupil. The iris is thin, colored part of the eye. It is
located in front of the eye between the lens and the cornea.
secreted by the ciliary body, ?ows into the posterior chamber, through the
pupil and out of globe through drainage apparatus at the angle of the anterior
chamber. The choroid contains blood vessels that supply oxygen and nutrient and
remove the waste products of the retinal cells. Function: nutritive.
ü The nervous tunic
tunic is the inner layer which includes the retina and lens. The retina
consists of receptors and neurons and concerned with the initial processing of visual information. Function:
The cornea and sclera together form the outer
fibrous tunic of the eye and withstand both internal and external forces to
maintain the shape of the eyeball. Although both of these structures consist
mainly of collagen fibrils, their optical properties are different.
The cornea must be transparent, refract light,
contain the IOP and provide a protective interface with the environment. Each
of these functions is provided by a highly specialized substructural
organization. The cornea does not contain blood vessels,
it receives nutrients from tears and the aqueous humor in the anterior chamber.
The cornea acts as the eye’s outermost lens. It
acts as a window that focuses and controls the amount of light to the eye.
When light strikes the cornea, it refracts the
incoming light onto the lens. The lens further refocuses that light onto the
retina. The retina starts the translation of light into vision.
The cornea also acts as a filter, screening out some
of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this
protection, the lens and the retina would be highly sensitive to injury from UV radiation.
The corneal tissue is arranged in ?ve layers. They
are: epithelium, Bowman’s membrane, the stroma, Descemet’s membrane and the
endothelium, see Figure (4).
The epithelium is the cornea’s outermost
region. It is ?lled with tiny nerve endings that make the cornea highly
sensitive to pain. The epithelium functions primarily to: (1) block the passage
of foreign material, such as water, dust, and bacteria, into other layers of
the cornea and the eye. (2) provide a smooth surface that absorbs oxygen and
cell nutrients from tears then distribute these nutrients to the rest of the
cornea’s layer. The basement membrane is the part of the epithelium where the
epithelial cells anchor and organize.
Bowman’s membrane lies directly below
the basement membrane of the epithelium. It is a transparent sheet of tissue,
is composed of strong layered of collagen ?brils. Bowman’s membrane protects
the cornea from injury. Once injured, it resiliently regenerates, leaving a
scar when the injury is deeper. If these scars are large and located in the
center, some vision loss can occur.
stroma lies under Bowman’s layer. It is the thickest layer about 90 % of the
cornea’s thickness. It consists primarily of water 78%, collagen 16%, and non-collagenous proteins 7%. It does not contain
any blood vessels. Collagen gives the cornea its elasticity, strength, and
form. The unique shape and arrangement of
collagen are essential in producing the cornea’s light-conducting transparency.
The Microscopic Organization of Collagen
The corneal stroma depends mostly on the
degree of its collagen ?brils (spatial order) which are narrow in diameter and
closely packed in a regular array. Di?erent types of the collagen present in
the human cornea (I, III, VI, XII). By scanning electron microscopy, it seems that the average diameter of collagen ?brils is
highly uniform (about 31nm), remaining constant across the cornea before rising
at the limbus. There is a significant increase in
spacing from the central cornea (about 57nm) to the peripheral cornea (about
62nm), followed larger increase at the limbus
itself. The proteoglycan matrix (also known as ground substance) is a gel-like
substance, consisting mainly of water 14.
The collagen in stroma also plays an
important role on the macroscopic level, where it confers shape and strength.
The stromal ?brils are organized into three hundred to ?ve hundred ?at bundles,
or lamellae, which run uninterrupted from limbus to limbus like thin belts up
to 0.2mm broad and about 1 ? 2cm thick. Fibrils within a given lamella run
approximately parallel but tend to make
large angles with those in adjacent lamellae, Figure (5).
Descemet’s membrane is located
beneath the stroma, it is a thin but strong sheet of tissue. It is composed of collagen fibers (different
from those of the stroma) and is made by the endothelial cells that lie below
it. Descemet’s membrane plays an important role in
corneal hydration and in the maintenance of the endothelium after wounding and
surgery, regenerating readily after injury. Considering its thickness (? 10µm) and unique composition, it maybe speculated whether
Descemet’s layer has a specialized function,
besides the function as a basement membrane that could be in mechanical
support, ?ltration or liquid barrier 15.
endothelium is the thin and the innermost layer of the cornea. Endothelial
cells are essential in keeping the cornea clear. Normally, fluid leaks slowly
from inside the eye into the stroma. The excess fluid pumps out of the stroma
by the endothelium. Without this pumping,
the stroma would swell with water. A perfect balance in a healthy eye is
maintained between the fluid moving into the cornea and fluid being pumped out
of the cornea. Once endothelium cells are destroyed by disease or trauma, they
do not recover. Too much damage to endothelial cells can lead to corneal edema
(swelling caused by excess ?uid) and blindness, with corneal transplantation
the only available therapy.
The limbus is the area
that the cornea meets the sclera and conjunctiva. Functions
of limbus are nourishing
the peripheral cornea, providing an outflow
for the aqueous humor, assisting in corneal epithelial regeneration.
In anatomically it is di?cult to
determine exactly where the cornea ends and the sclera begins. Transitional
zone is approximately 1.5?2mm. By using synchrotron
X-ray di?raction confirmed the presence of circumferential
annulus of collagen ?brils located in the limbus of the human eye 16. Since
the preferred orientation of the collagen ?brils is circular at the limbus, it
is the weakest region in the corneoscleral wrapper by IOP pressure. From a
consideration of the mechanics of the system,
it seems probable that the purpose of this annulus is to help maintain the
correct curvature of the cornea; studies
on bovine tissue, proposed microstructural models of possible integration
arrangements between the central cornea and the limbus 17 support this
The sclera gives the eye most of its white color. It is
relatively avascular and consists almost entirely of collagen. The sclera
protects the intraocular contents from injury and the function of the collagen
in the sclera is obviously structural. The strength and resilience of the
sclera are imparted by the close interlacing of the collagen fibers which
account for 80 % of the dry weight. Its mechanical
strength serves to contain the IOP and at the same time prevents deformations
of the globe by resisting the stresses induced by contractions of the
extraocular muscles. The thickness of sclera is not uniform, being thinner in females
than in males. There is also increase in scleral thickness, together with
opacity in relation to age 18.
The sclera is pierced by the optic nerve, forming a thin netlike lamina.
This structure provides support and anchorage for the optic nerve ?bres passing
through it and also reinforces the globe
at its weakest point. The bands of collagen bundles of the sclera are mostly
parallel to the surface, but they cross each other in all directions and may
divide and reunite, see Figure (6). Within each bundle,
the collagen ?brils are parallel and show wide variation in diameter and
spacing (ranging from 25–230 nm), which is distinctly di?erent from that of the
cornea and account for the opacity of the sclera 19.