The cornea is the transparent front part of the eye that allows light to enter. It has 5 layers - epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. The epithelium and endothelium help regulate hydration to maintain transparency. The stroma contains collagen fibrils that provide strength and refractive power. The endothelium actively pumps fluid from the stroma into the aqueous humor. Together, the layers and pumping action keep the cornea clear for vision. Metabolism relies on diffusion of nutrients from tear film and aqueous humor due to lack of blood vessels. Nerves innervate the cornea for high sensitivity.

1. ANATOMY & PHYSIOLOGY OF
CORNEA
By
Lavanya Kalikivayi, M.Opt.,
Associate Professor
Ahalia School of Optometry & Research Centre
Palakkad

2. • CORNEA
Transparent avascular tissue with a convex
anterior surface & concave posterior surface.
Main function is OPTICAL
Accounts for 70% of the total refractive power
of the eye (+ 43D)
Other functions are: -STRUCTURAL
INTEGRITTY
-PROTECTION FOR THE EYE

4. DIMENSIONS
• Anterior Surface :
Vertical – 11.7 mm
Horizontal – 10.6 mm
• Posterior Surface :
Both 11.7 mm
• Thickness :
Central 0.52 mm
Peripheral 0.67 mm
• Surface Area:
1.3 cm2

5. • Radius of Curvature
Anterior – 7.8 mm Central 1/3
Posterior – 6.5 mm
- Peripheral cornea is more flattened
• Topography
Anterior curvature is spherical in 2-4 mm zone
decentered upwards & outwards relative to
visual axis but centered to the pupillary aperture(
lies 0.4 mm temporally) -- CORNEAL CAP or
APEX
• Curvature varies from apex to limbus , greater
flattening seen nasally & in upper part of cornea

6. STRUCTURES
5 LAYERS
A nterior Epithelium
B owman’s layer
C entral stroma
D escemets membrane
E ndothelium

7. EPITHELIUM
Stratified , Squamous & Non Keratinized
Continuous with conjunctiva , but no goblet
cells
50-90 u
5-6 layers
Posterior to anterior
1. BASAL CELLS
2. WING or UMBRELLA cells
3. SURFACE CELLS

8. • Desmosomes- lateral adhesion b/w cells, mainly at the
basal level.
• Surface cells contain MICROVILLI & MICROPLICAE–
Helps in stabilizing precorneal tearfilm
• BASAL LAMINA:
 2 LAYERS
• Superficial LAMINA LUCIDA
• Deep LAMINA DENSA
 Thicker peripherally
 Thickened in Diabetes , Corneal pathology, Old age
 Integrated with the underlying Bowmans layer through
ANCHORING FILAMENTS & ANCHORING PLAQUES
 Cohesion between Basal Lamina & Bowman’s
loosened by
• Lipid solvents
• Stromal edema
• Inflammation

9. Physiology of Epithelium
• Rich in glycogen , serves as energy store in
aerobic conditions
Glycogen levels
• Hypoxia
Corneal sensitivity

10. BOWMAN’S
 Anterior Limiting Lamina
8-14 u
Modified region of anterior stroma
Acellular homogenous zone
Normally attached to Basal Lamina
In pathological conditions
Corneal edema , Dystrophy
After death
-Epithelium readily seperates from this layer

11. STROMA
• 500u
• Regularly arranged lamellae of collagen
bundles
• Contains keratocytes between lamellae
• Keratocytes – production of COLLAGEN &
PROTEOGLYCANS during development

12. DESCEMET’S
• Posterior Limiting Lamina
• 2.2- 4.5 u
• It is Basal Lamina of Endothelium
• Appears at 2nd month of gestation
• Strong resistant sheet
• Sharply defined & the plane of seperation is
used in LAMELLAR KERATOPLASTY
• Thickens with age , endothelial degenerations
• Type 4 collagen

13. • Anterior 1/3 :
– Oldest
– Irregular banded pattern in cross section
– Banding develops at 5th month IUL
• POSTERIOR 2/3:
– Formed after birth
– Homogenous fibrillogranular material

14. ENDOTHELIUM
• Single layer of hexagonal / cuboidal cells
• Counts
– At birth : 6000/mm2
– 1 yr : falls by 26%
– 11yr : another 26%
• Gradual decrease in density & increase in
shape variation – POLYMEGATHISM

15. • The corneal endothelium governs fluid and
solute transport across the posterior
surface of the cornea and actively
maintains the cornea in the slightly
dehydrated state that is required for optical
transparency.
Hexagonal cells of corneal endothelium
visualized by specular microscopy

16. PHYSIOLOGY
1. NUTRITION :
– Glucose & amino acid
2. FLUID REGULATION:
– Maintains relative deturgescence by
1. Provides barrier to prevent ingress of salt &
metabolites into stroma
2. Decreases osmotic pressure of stroma by active
pumping out of bicarbonate.

17. STRUCTURAL PROTEINS OF CORNEA
COLLAGEN
Basal lamina - type 4
Bowman’s - 5
Stroma - 1 (90%)
Descemets - 4

18. CORNEAL TRANSPARENCY
• MAURICE THEORY:
LATTICE ARRANGEMENT of collagen fibres
is responsible for transparency.
– Due to small diameter & regular seperation of collagen,
back scattered light would be suppressed by
DESTRUCTIVE INTERFERENCE
• GOLDMAN THEORY :
If fibril seperation & diameter is less than 1/3 of
wavelength of incident light –TRANSPARENCY
ensues

19. Other factors
• Absence of blood vessels & pigments
• Absence of myelinated nerve fibres
• Uniform refractive index of all layers &
uniform spacing of collagen fibrils

20. The factors that affect the corneal
transparency are:
1-Corneal an avascular.
2-Unmyelination of corneal nerve fibers.
3-Degeneration of epithelium.
(integrity of this layer and all layers of
cornea)
4-Higher difference between refractive index
of cornea and air

21. CORNEAL METABOLISM
• It is the series of chemical reaction that
plays in living tissue.
• The constant metabolic activity in cornea
is necessary to maintain on transparency,
temperature and hydration of cornea (78%)
• The metabolism occur in epithelium and
endothelium.
• The main substance for this metabolism is
glucose, oxygen, amino acid and vitamin.

22. • Due to avascularity of cornea, it
promotes to alternative routes of
metabolic supply.
• There are three possibilities one from
aqueous second from atmosphere via
tear film third from perilimbal blood
vessels.
• The oxygen is mainly derived from
atmosphere via tear film, assuming tears
are saturated with oxygen and therefore
the tension of oxygen crosspending to
atmosphere is 155 mm hg at sea level in
the open eye and when eye closed the
tension is about 55 mm hg in this state
the oxygen supply from perilimbal blood
vessels.

23. • The consumption rates of oxygen for layers
of the cornea are not equal.
• It is as follows: 40, 39, 21 epithelium, stroma,
endothelium respectively.
• The glucose is derived from aqueous; the
cornea derives the energy from the oxidative
breakdown of carbohydrates and the glucose
is primary substrate for generation of
adenosine triphosphate is catabolized by two
metabolic pathways as follows:
• Glycilytic ( Embden Meyerhof pathway)
• Hexose monophosphate (pentose
phosphate)

24. The effect of contact lenses on corneal metabolism
• Contact lens presents barrier between the cornea
and atmosphere therefore Contact lens deprivation
of oxygen to enter into cornea and consequently
reduction in aerobic glycolysis.
• Normally this situation is avoided with hard corneal
lenses because they move and produced tears
circulation this permitting some degree of oxygen
and carbon dioxide exchange between the cornea
and atmosphere.
• Soft contact lenses also move on the eye but the
circulation of the tear under soft contact lens is less
although they have the advantage of transmission o
oxygen in amount varying with the nature and
thickness of materials but with sclera lenses tear
exchange is less. The cornea can tolerated levels as
low as 11-19 mm hg.

25. • In ill fitting contact lenses & IOP, basal cells
which are regularly arranged are seperated by
edema fluid of differing refractive index to cells
DIFFRACTION GRATING EFFECT
HALOS AROUND LIGHT

26. Physiology of corneal endothelium
• The principal physiological function of the
corneal endothelium is to allow leakage of
solutes and nutrients from the aqueous humor to
the more superficial layers of the cornea while at
the same time actively pumping water in the
opposite direction, from the stroma to the
aqueous.
• This dual function of the corneal endothelium is
described by the "pump-leak hypothesis."
• Since the cornea is avascular, which renders it
optimally transparent, the nutrition of the corneal
epithelium, stromal keratocytes, and corneal
endothelium must occur via diffusion of glucose
and other solutes from the aqueous humor,
across the corneal endothelium.

27. • The corneal endothelium then actively
transports water from the stromal-facing
surface to the aqueous-facing surface by
an interrelated series of active and passive
ion exchangers.
• Critical to this energy-driven process is
the role of Na+/K+ATPase and carbonic
anhydrase.
• Bicarbonate ions formed by the action of
carbonic anhydrase are translocated
across the cell membrane, allowing water
to passively follow.

28. NERVE SUPPLY
• Ophthalmic division of Trigeminal via Anterior
Ciliary Nerve
• Supply also from Cervical Sympathetic
• Anterior ciliary nerve enters sclera from
perichoroidal space just behind the limbus &
joins with the conjunctival nerve to form
PERICORNEAL PLEXUS

29. • Divides into 2 branches Anterior & Posterior
• Anterior passes subjacent to the BOWMANS
forming SUBEPITHELIAL PLEXUS
• Posterior innervates posterior stroma , does not
involve Descemets .

30. Corneal sensitivity
• The sensitivity of the corneal is probably
unsurpassed by that of any part of the
body.
• Its varies from a maximum apically to a
minimum at the periphery with
considerable drop in sensitivity at limbus.
• Sensitivity of the cornea is reduced with
age.
• The peak sensitivity is found in young
patient and this is 3 times that found in
older patients.

31. • Sensitivity also varies with iris color, in this
the blue-eye color have a greater sensitivity
than those with dark –brown color.
• Sensitivity is the same in both eye and sexes
in the normal circumstance.
• Cornea display a diurnal variation in
sensitivity with about greater sensitivity as
the day progresses from morning to evening.
• Diabetic, Albinism, and all disease affecting
the cone of the corneal causes reduction in
sensitivity

32. The corneal temperature
• The central has a temperature of 34°C,
which appears to increase towards the
periphery and is found to be nearly 0.50°C
warmer at the limbus.
• The cooling of the cornea following a blink
seems to be slower in those who exhibit a
lower blink rate than normal.
• The normal corneal temperature may alter
during contact lens wear.
• Corneal temperature can be measured by
a wide-field, colour-coded infra-red imaging
device, and a thermography-visual-system.