Cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye. Cornea forms the transparent and anterior 1/6th of the external fibrous coat of the globe of the eyeball. The cornea is the eye's most powerful structure for focusing light that provides approximately 65 to 75 percent of the focusing power of the eye. The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid.

1. Anatomy and physiology of
Cornea
By Getachew Kassahun
Gondar, Ethiopia
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2. CORNEA
 Cornea forms the transparent and anterior 1/6th of
the external fibrous coat of the globe of the
eyeball.
 It is just like glass cover of a watch which is set on
the sclera.
 It is oval from front and circular from behind.
 It is the main refracting medium of the eye.
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3. DIMENSIONS
Front/anterior
Back/posterior
Thickness
Horizontal diameter
Vertical diameter
Horizontal diameter
Vertical diameter
At the center
— 12 mm
— 11 mm
— 11.5 mm
— 11.5 mm
— 0.50 to0.58 mm
Radius of curvature
Refractive index
At the periphery
Anterior surface
Posterior surface
— 1.376
— 1 mm
— 7.8 mm
— 6.5 to 7 mm
Dioptric strength — +42.5 Diopter
At birth the size of the cornea is
80% of it’s adult size and it reaches
it’s adult size at 3 years of age.
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4. Cornea consists of six layers
• Epithelium
• Bowman's membrane
• Stroma
• Descemet's membrane
• Dua's layer
• Endothelium
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6. 1. Stratified Squamous Epithelium
It is composed of 5–6 layers of non keratinised mounted on a basement
membrane and continuous with the bulbar conjunctiva at the limbus.
i. Superficial cells (or Squamous cells): It is 2–3 cell layered and the
cells are polyhedral.
Outer surfaces of these cells have projections called microvilli and
microplicae.
These projections extend into the mucin layer of precorneal tearfilm and
help in retaining the tearfilm.
ii. Wing cells (or Umbrella cells): It consists of also 2–3 layered
polyhedral cells with their concave base fitting over the apex of the
basal cells. These cells send wings or process between the basal cells.
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7. iii. Basal cells: It is a single layer of columnar cells with
flat bases and rounded apices (or head).
These are germinal cells and show mitosis. New cells are
gradually pushed superficially.
iv. Basement membrane: It is formed by secretions from
the basal cells.
Despite lack of vascularity corneal epithelium is capable
of active regeneration after injury or abrasion.
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8. 2. Bowman’s Membrane
• It is the modified condensed anterior layers of the corneal
stroma,approximately 8 to 14 μm thick.
• It is acellular and composed of a dense, fibrous sheet of
interwoven collagen fibrils randomly arranged in a
mucoprotein ground substance.
• Bowman’s layer is produced prenatally by the epithelium
and is not believed to regenerate. Therefore, if injured, the
layer usually is replaced by epithelial cells or stromal scar
tissue. However, it exhibits strong resistance against
infection and injury.
• It is well-demarcated from the corneal epithelium.
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9. 3.Stroma (Substantia propria)
• It is the forward continuation of sclera.
• 500 μm thick, or about 90% of the total corneal thickness.
• It consists of approximately 200 layers of parallel collagen fibrils,
parallel to the surface of the cornea.
• Two types of cells are found in the stroma.
 Keratocytes (or Corneal Corpuscles): They secrete collagen and the
ground substance and lie within the collagen lamellae.
 Wandering leucocytes: They are derived from the limbal vessels.
• Cornea is the most sensitive structure in the body due to presence of
plenty non medullated nerve fibres in the stroma.
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10. 4.Descemet’s Membrane
• It is formed by secretion of corneal endothelium.
Hence, it is a modified basement membrane of the
endothelium.
• It is produced continually and therefore thickens throughout
life, such that it has doubled by age 40 years.
• It has wart-like elevations at the periphery termed as
Hassall-Henle bodies.
• It terminates peripherally at the Schwalbe’s line.
• It is strong and capable of regeneration after injury.
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11. 5.Dua’s layer
• Dua’s layer is well-defined, acellular and strong, consisting of
five to eight lamellae of type-1 collagen bundles total about six
to 15 microns thickness.
• The bundles are coarse and arranged in transverse, longitudinal
and oblique directions.
• Bundle spacing is similar to that in stromal tissue, but Dua’s
layer is entirely free of keratocytes in the zone that forms the
posterior wall of the bubble.
• These histological features also distinguish Dua’s layer from
the adjacent Descemet’s membrane, which consists of finer,
closer spaced, parallel collagen bundles in banded and non-
banded layers with endothelial cells.
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13. 6.Endothelium
• It is the deepest layer consisting of a mosaic of single
layer of hexagonal cells, bound together and continuous
with the endothelium of the anterior surface of the iris.
• Endothelial cells are responsible for maintaining relative
dehydration (deturgence) of corneal stroma and
transparency.
• Endothelial cells of the cornea can be seen by specular
reflection with the slit-lamp biomicroscope.
• Once damaged, the endothelial cells do not regenerate.
• At birth the endothelial cell count is 4500 cells/mm2. In
the first year of life 25% reduction in cell count occurs.
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14. COMPOSITION
• Water 75–80%
• Electrolytes—Electrolyte level varies in different layers
• Collagen—It is destroyed by the enzyme collagenase
• Soluble protein—Albumin glycoprotein
• Immunoglobulins—IgG, IgA and IgM
• Glycosaminoglycans (GAG)—It is highly concentrated in the
stroma than in the epithelium and endothelium.
• Glycoprotein.
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15. CORNEAL INNERVATION
• The cornea is densely innervated with sensory fibers.
• Seventy to 80 large nerves, branches of the long and
short ciliary nerves, enter the peripheral stroma.
• Approximately 2 to 3 mm after they pass into the cornea,
the nerves lose their myelin sheath, but the covering
from the Schwann cell remains.
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16. Cont…
• The density of sensory nerve endings in the epithelium is
approximately 400 times that of the epidermis of the skin, with
approximately 7000 nociceptors per square millimeter in the
cornea.
• Stimulation of the cornea, even just touch, is recognized as pain
because of the density of nociceptors.
• The cornea also recognizes changes in temperature.
• Contact lens wear over time and aging cause a decrease in
sensitivity
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17. Assessing Corneal Sensitivity
• can be “measured” clinically by touching the cornea
gently with a wisp of cotton from a swab and initiating
a blink response.
• It can be measured quantitatively by using a measuring
device, an esthesiometer.
• A small, fine filament is introduced from the side to
just touch the cornea.
• Because rigidity depends on the length of the filament,
the longer the filament (the more flexible the filament.)
that initiates a blink, the more sensitive the cornea.
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18. NUTRITION SOURCE
• Oxygen from the air via diffusion across tear film
• Glucose and amino acids from
– Aqueous humour
– Perilimbal capillaries
• Tearfilm.
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19. METABOLISM
• Cornea requires energy for
renewal of tissues and
maintenance of transparency.
• Energy is derived in the form of ATP
1. Anaerobic glycolysis: This is essentially an anaerobic pathway
and accounts for majority of corneal metabolism.
2. Tricarboxylic acid (or TCA/Kreb’s cycle)—This is essentially
an aerobic pathway and results in a net yield of 36 molecules of
ATP (greater energy yield).
3. Hexose monophosphate shunt (or Pentose phosphate shunt)
plays a significant role in both the epithelium and the
endothelium.
It gives a net yield of 6 molecules of ATP
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20. CORNEAL TRANSPARENCY
• Cornea maintains high level of transparency to transmit more
than90% of incident light.
• Several factors contribute to the corneal transparency.
• Epithelial
– Uniform regular arrangement of epithelium
– Homogenous refractive index throughout the epithelium
– Presence of precorneal tearfilm
– Tight junctions between superficial epithelial cells.
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21. Stromal
– Regular crystalline lattice arrangement of corneal
collagen fibrils in a mucopolysaccharide ground
substance, separated by less than the wavelength of
light (Maurice Theory)
– Absence of blood vessels
– The diameter of the corneal collagen fibrils are
smaller than the wavelength of light and therefore, do
not interfere with the light transmission
– Demyelination of corneal nerves.
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22. Deturgence (Relative Dehydration)
 Relative dehydration of stroma which is responsible for
maintenance of normal corneal thickness is maintained by
following structures:
 Epithelium offers high resistance to flow of ions and water
 Endothelium
• Water imbibes into corneal stroma, but it is driven out by
metabolically driven pump, i.e. Na–K ATPase pump of the
endothelium.
• Zonulae occludentes, i.e. focal tight junctions of the adjacent
endothelial cells maintain barrier to intercellular fluid traffic.
 Stromal deturgence
• Water is driven out of the stroma by negative imbibition pressure
of the stroma
• Glycosaminoglycans (GAG) present in the corneal stroma act as a
sponge to suck in water. However, in a normally dehydrated
cornea they offer high resistance to inflow of water.
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23. CORNEAL WOUND HEALING
• Corneal wound healing is very essential to maintain
transparency of cornea for clear vision.
• Corneal healing process is quite different from other
tissues due to avascularity.
• It also varies according to site, suture material, nature of
injury and topical ophthalmic preparations.
• Epithelial level—Corneal epithelial repair is a complex
interaction between the cells and the extracellular matrix.
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24. • Bowman’s membrane level—Bowman’s membrane doesn’t
regenerate.
• Stromal level—Central avascular stromal wound takes longer time to
heal than the peripheral wound
• Descemet’s membrane level—Descemet’s membrane easily
regenerate from endothelial cells.
It is essentially the basement membrane of the endothelial cells.
• Endothelial level—Proliferation of endothelial cells by mitosis is
restricted only to younger group of peoples.
In adults the endothelial cells simply undergo enlargement and sliding
to cover the defects.
CORNEAL WOUND HEALING
Cont…
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25. CORNEAL BLOOD SUPPLY
• The cornea is avascular and obtains its nourishment by
diffusion from the aqueous humor and from the
conjunctival and episcleral capillary networks located in
the limbus.
• Absence of blood vessels is an important factor in
corneal transparency and although it is surrounded by
conjunctival capillary loops a balance between
angiogenic and antiangiogenic factors maintains its
avascular state.
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26. CORNEAL FUNCTION
• The cornea has two primary functions:
to refract light and
to transmit light.
The cornea transmits light with wavelengths between 310 and
2500 nm.
Wave lengths below 300 nm are absorbed by the epithelium
and Bowman’s layer and do not penetrate deeper; those
between 300 to 320 nm are absorbed by corneal stroma
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27. Refraction of light
Factors that affect the amount of corneal refraction
include (1) the curvature of the anterior corneal
surface,
(2) the change in refractive index from air to
cornea (actually the tear film),
(3) corneal thickness,
(4) the curvature of the posterior corneal surface,
and
(5) the change in refractive index from cornea to
aqueous humor.
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28. • The total refractive power of the eye focused at
infinity is between 60D, with 43 D attributable
to the cornea.
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29. References
• Clinical Anatomy and Physiology of the visual system
3rd edition
• Textbook of Visual Science and Clinical Optometry
by Bikas bhttacharyya
• Adler’s physiology of the eye
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