About different factors affecting corneal transparency and its importance

1. CORNEAL
TRANSPARENCY
1
HIRA NATH DAHAL

2. Transparency is the quality or state of
transmitting light without appreciable
scattering so that bodies lying beyond are
entirely visible .
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3. The cornea is an exquisite layered composite
material with a structure ideally suited to its
function as the transparent window in the tough
outer tunic of the eye ball through which the
outside world is viewed.
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4. The Cornea is a transparent avascular tissue with
smooth outer convex surface and concave inner
surface which resembles a small watch glass.
Forms anterior one –sixth of the fibrous
coat of the eye.
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5. Corneal Dimension
Horizontal Vertical
Anterior surface-- 11.7mm 10.6mm
Posterior surface-11.7mm 11.7mm
Radius of curvature
Ant surface-7.8mm Post surface-6.5mm
Corneal thickness
Central-0.5-0.6mm Peripheral-1-1.2mm
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6. Functions of Cornea
Optical : It forms the principal refractive surface
accounting for some 70%(40-45 D) of total refractive
power(60D).
Barrier: Cornea provides a protective interference with
the outer environment and also contain intraocular
pressure.
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7. Layers of cornea
Epithelium.
Bowman layer.
Lamellar Stroma.
Descemet’s membrane.
Endothelium.
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8. 1. Epithelium (50-90 um)
3 types of cell :-
i) A single layer basal columnar cells stand as paliside like manner in
perfect alignment on the basement membrane.
Attached by hemidesmosomes to the epithelial basement membrane.
ii)2- 3 rows of wing cells.
iii) 2 layers of flattened surface cells
- microplicae and microvilli
- excellent ability to regenerate
- attached by desmosomes , and zonulae occludens
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9. 9

10. BIOCHEMICAL COMPOSITION OF
EPITHELIUM
Water (70% of wet weight) .
Protein synthesis is 5 times higher than stroma.
Lipid (Phospholipid and cholesterol).
Enzymes.
ATP,glycogen,glutathione,Ascorbic acid.
Acetylecholine, cholinesterase.
Electrolytes (K+,Na+,and Cl– ).
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11. 2. Bowman’s layer (8-14 um):
Acellular mass of condensed collagen fibril.
It is not true elastic membrane but simply continuation
of stroma.
Shows considerable resistance to infection and injury
Does not regenerate.
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12. 3.3. Lamellar Stroma (0.5mm):(0.5mm):
Consists collagen fibrils (lamellae) (200-250)and cells embedded in matrix
(proteoglycans).
Mean diameter of collagen fibril between
22.5&32nm,predominantly type I collagen.
 Fibril within the lamellae are parallel to each other & also parallel to corneal
plane
Fibril within adjacent lamellae make various angles with respect to one other.
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13.  Stromal cells are keratocytes, wandering
macrophages, histocytes etc.
 Keratocytes are fibroblast which produce ground
substance (keratin & chondrotin sulfate) and collagen
fibril.
The highly negatively charged keratin sulfate around
collagen fibril maintain spatial relationship between
them.
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14. BIOCHEMICAL COMPOSITION OF STROMA :
-Water(78%)
- Collagen (15%)
- other protein (5%)
- Glycosaminoglycan
# Keratan sulfate(0.7%)
# Chondroitin sulfate(0.3)
# Chondroitin
- Salts
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15. 4.4. Descemet’s membrane (3-40um):(3-40um):
Homogeneous layer.
Made up of collagen & glycoprotein.
Resistant to chemical agent, trauma infection and
pathological process .
Can regenerate.
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16. 5.5. Endothelium ::
-- Single layer polygonal cells
- Attached to Descemet’s membrane by
hemidesmosomes and laterally to each other
by tight junctional complex .
- barrier function is calcium dependent
-Contains active pump mechanism
-Involved in active secretion and protein
synthesis
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17. 17
FACTORS AFFECTING CORNEAL TRANSPERANCY
Anatomical factors:
* Uniform & regular arrangement of corneal
epithelium .
* Peculiar arrangement of corneal stromal lamellae.
* Corneal avascularity.
Physiological factors:
Relative state of corneal dehydration.

18. Anatomical factors
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19. 1. Corneal epithelium
 Epithelial cells are closely packed.
 Uniformity & regularity in arrangement.
 Homogenicity in refractive index.
 Tight intercellular junction.
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20. 2. Tear film
 Keeps epithelial surface smooth.
 Provides high quality optical surface.
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21. 3.3. Arrangement of stromal lamellae:
Maurice Theory:
Collagen fibrils of uniform diameter (275-
350Å)are packed regularly and thus
creates a lattice pattern.
 The fibril axis are located at the position
of a perfect crystalline lattice.
 Interfibrillar spacing is about 50-60nm
and is smaller than wavelength of light
(400-700nm).
 Scattered light is destroyed by mutual or
destructive interference
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22. Interference:
Interference is the phenomenon by virtue of which
there is a modification in the distribution of energy due
to superimposition of two or more waves.
‘ Principle of superimposition’. This can be stated as: “
Whenever two or more sets of waves pass through and
cross one another in the same medium, they behave
independent of each other and net displacement of a
particle , at any instant , is equal to the algebraic sum of
the individual displacements due to all the waves.”
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23. Interference may be ‘constructive’ or ‘destructive’
In constructive interference the crest of one wave coincides with
the crest of another and the net amplitude is equal to the sum of
individual amplitudes.
In destructive interference the crest of one wave coincides with the
trough of another and the net amplitude is equal to the difference
between the individual amplitudes.
Scattered rays - destructive interference
Rays on the line of incident
light- Constructive interference
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Destructive interference

24. 24
LATTICE ARRANGEMENT
SWOLLEN CORNEA

25. Findings contradictory to Maurice theory
i) The cornea is not perfectly transparent, otherwise an
ophthalmologist could not view cornea in the slit lamp..
ii) Shark’s cornea with regions of disorganised fibres and
random distribution of interfibrillar distances is also quite
transparent.
.
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26. Hard core cylinder model of Twersky:
Twersky proposed a model in which the fibrils had a composite
structure consisting of an inner core, composed of collagen fibrils
and outer coating of a material that matches the refractive index
of the ground substance. The coating would increase correlations
but not affect the light scattering properties of a fibril.
The loss of transparency observed when the cornea swells would
be explained in the hard core model on the basis of increased
area available per fibril.
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27. Theory of Goldman & Benedek :
Fibrils are small in relationship to
wavelength of light and do not interfere
with light transmission unless they are larger
then one-half of a wavelength of light
(2000Å).
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28. 4. Endotheliumis transparent because it is
* Single layer.
* Homogeneous.
* Closely packed cells .
* It has deturgescence function.
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29. 5. Corneal Vascularisation:
Except for capillary palisade of limbus, normal cornea is
avascular.
Corneal avascularity factors are not known.
When vessels present are due to corneal pathology.
Vascularisation -Loss of transparency.
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30. Mechanical Theory : According to Cogan (1948)
 Blood vessel can not invade normal cornea.
 Loosening of compactness of corneal tissue
due to oedema is a must for neovsacularisation.
Interstitial keratitis where oedema always precedes
vascularisation.
Some vesostimulatory factor may be needed along with
corneal oedema for neovascularisation to occur.
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31. Chemical theory:
Role of vasoinhibitory factors(VIF)
Sulfate ester of hyaluronic acidSulfate ester of hyaluronic acid
Role of vasostimulatory factors (VSF)
Low mol wt. AminesLow mol wt. Amines
Corneal hypoxia→VSF stimulation→ Neovascularisation
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32. Physiological factors
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33. Corneal hydration :
Normal cornea maintains itself in a state of relative dehydration (80% water
content ) which is essential for corneal transparency.
It is kept constant by---
1) Factors which draw water in the cornea, like
-Stromal swelling pressure (SP)
-Intraocular pressure (IOP)
2 ) Factors which prevent flow of water in the cornea
- Mechanical barrier function of epithelium &
endothelium.
3) Factors which draw water out of cornea
-active pumping action of endothelium.
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34. FACTORS AFFECTING CORNEAL HYDRATION:
i) STROMAL SWELLING PRESSURE (SP, 60 mmHg):
Pressure exerted by glycosaminoglycans(GAGs) of
the corneal stroma which act like a sponge.
Electrostatic repulsion of the anionic charges on
the GAG molecule expands the tissue, sucking in the
fluid with equal but negative pressure called,
imbibition pressure (IP).
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35. In vitro, IP=SP
In vivo, IP is reduced by values
equivalent to IOP.
i.e. IP=IOP- SP
i.e. IP=17-60= - 43mmHg.
Negative imbibation pressure
draws out water from stroma.
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36. ii) Barrier function of epithelium and endothelium:
Epithelium & endothelium are semipermiable in nature.
Function as barriers to excessive flow of water and
diffusion of electrolytes into the stroma.
Epithelium offers most resistance to flow of water.
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37. iii) Hydration control by active pump mechanism:
a) Na+
/K+
ATPase pump system:
•Endothelium is more active than &epithelium,
•Pumps are located in basolateral membrane of endothelial cell.
•Stromal transparency develops 13-20 days after birth due to greatest increase in
pump sites/cell.
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38. •Enzyme “Na+
/K+
activated ATPase” mediate pump causes extrusion of the Na+
& water from the stroma and thus maintain corneal transparency.
•Corneal hydration depend on extent to which endothelial barrier and pump
function can be reestablished.
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39. b) Bicarbonate dependent ATPase present in endothelium are also reported to have
role in
fluid /ion balance in the cornea.
c) Carbonic enhydrase enzyme catalyzes the conversion of CO2and water
into HCO3
-
and H+
, thus provides important source for HCO3
-
for endothelial
pump.
d) Na+
/H+
pump has also been postulated.
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40. 40

41. Corneal thickness is increased and transparency is
decreased when there is endothelial damage and to a
lesser extend when epithelium is damaged.
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42. iv) Passive ion movement, like
- K+
, Cl-
and HCO3
-
ions diffuse into
aqueous humor.
- Na+
, Cl-
and HCO3
-
diffusion in
contra lateral direction.
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43. v) Hypoxia, pH and changes in the temperature
can alter the metabolic activity of the cornea and
thereby may cause alteration in corneal thickness
and loss of transparency.
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44. vi) Intraocular pressure (IOP) :
As we know ,
I P = IOP - SP, i.e. 17- 60= - 43 mmHg,
i.e. I P is a negative pressure.
When IOP exceeds SP, i.eWhen IOP exceeds SP, i.e. when I Pwhen I P becomes positive , corneal oedemabecomes positive , corneal oedema resul
It can occur when there is
- high IOP and normal SP,as in acute glaucoma,
- Normal IOP and low SP, as in endothelial dystrophy.
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45. 45

46. Corneal Swelling:
Electron micrograph of a swollen cornea shows fibril
distribution with region completely devoid of fibrils.
These voids are called lakes, which have larger dimensions &
spoils the interference that is critical to transparency.
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47. Photorefractive keratectomy using argon fluoride excimer
laser:
. Despite the excimer laser’s ability to ablate corneal tissue
with good precision and with little collateral damage, the
treatments commonly result in the development of increased
subepithelial light scattering that gives the cornea a hazy
appearance in the treated area.
increased numbers of activated keratocytes and vacuoles within
and around keratocytes adjacent to the treated area. Activated
keratocytes may have different effective refractive indices than
normal keratocytes, thus increasing their contribution to
scattering. Vacuoles within and around keratocytes may act like
the lakes seen in swollen cornea.
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48. vii) Evaporation of water from corneal surface:
Evaporation of water from the pre-corneal tear film fluid →
increase in osmolarity relative to cornea→hyper tonicity of
tear film → flow of water from the cornea.
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49. Corneal transparency results from-----
Interference among the residual waves scattered
by different fibrils.
The inefficiency of the fibrils as scatters.
Avascularity of the cornea.
The thinness of cornea which is maintained by a
complex series of metabolically dependent reaction
in the corneal endothelium and epithelium.
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