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The Cornea
The cornea is the eye's outermost
layer. It is the clear, dome-shaped
surface that covers the front of the
eye.
Structure of the Cornea
Although the cornea is clear and
seems to lack substance, it is
actually a highly organized group of
cells and proteins. Unlike most
tissues in the body, the cornea
contains no blood vessels to nourish
or protect it against infection.
Instead, the cornea receives its
nourishment from the tears and
aqueous humor that fills the chamber
behind it. The cornea must remain
transparent to refract light
properly, and the presence of even
the tiniest blood vessels can
interfere with this process. To see
well, all layers of the cornea must
be free of any cloudy or opaque
areas.
The corneal tissue is arranged in
five basic layers, each having an
important function. These five
layers are:
Epithelium
The epithelium is the cornea's
outermost region, comprising about
10 percent of the tissue's
thickness. The epithelium functions
primarily to: (1) Block the passage
of foreign material, such as dust,
water, and bacteria, into the eye
and other layers of the cornea; and
(2) Provide a smooth surface that
absorbs oxygen and cell nutrients
from tears, then distributes these
nutrients to the rest of the cornea.
The epithelium is filled with
thousands of tiny nerve endings that
make the cornea extremely sensitive
to pain when rubbed or scratched.
The part of the epithelium that
serves as the foundation on which
the epithelial cells anchor and
organize themselves is called the
basement membrane.
Bowman's Layer
Lying directly below the basement
membrane of the epithelium is a
transparent sheet of tissue known as
Bowman's layer. It is composed of
strong layered protein fibers called
collagen. Once injured, Bowman's
layer can form a scar as it heals.
If these scars are large and
centrally located, some vision loss
can occur.
Stroma
Beneath Bowman's layer is the stroma,
which comprises about 90 percent of
the cornea's thickness. It consists
primarily of water (78 percent) and
collagen (16 percent), and does not
contain any blood vessels. Collagen
gives the cornea its strength,
elasticity, and form. The collagen's
unique shape, arrangement, and
spacing are essential in producing
the cornea's light-conducting
transparency.
Descemet's Membrane
Under the stroma is Descemet's
membrane, a thin but strong sheet of
tissue that serves as a protective
barrier against infection and
injuries. Descemet's membrane 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 is regenerated readily
after injury.
Endothelium
The endothelium is the extremely
thin, innermost layer of the cornea.
Endothelial cells are essential in
keeping the cornea clear. Normally,
fluid leaks slowly from inside the
eye into the middle corneal layer (stroma).
The endothelium's primary task is to
pump this excess fluid out of the
stroma. Without this pumping action,
the stroma would swell with water,
become hazy, and ultimately opaque.
In a healthy eye, a perfect balance
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 are lost
forever. If too many endothelial
cells are destroyed, corneal edema
and blindness ensue, with corneal
transplantation the only available
therapy.
Content Courtesy : www.nei.nih.gov
Note : Information herein is
provided for informational purposes
only and is not a substitute for
professional medical advice. You
should not use this information for
diagnosing or treating a medical or
health condition. If you have or
suspect you have a medical problem,
promptly contact your professional
healthcare provider. Please consult
your healthcare provider before
beginning any course of
supplementation or treatment.
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