Physiology of corneal transparency

1. CORNEAL TRANSPARENCY
Dr. CHRISTINA SAMUEL
PG- M.S OPHTHALMOLOGY
MMCH & RI

2. • Main physiologic function of cornea is to act as a major
refracting medium, so that a clear retinal image is formed.
• Normal corneal transparency is result of
• 1.anatomical factor
such as uniform and regular arrangement of corneal
epithelium, peculiar arrangement of corneal lamella and corneal
vascularity
2.Physiological factor
[ie] relative state of corneal dehydration.

3. • Therefore, any process which upsets the anatomy or
physiology of cornea will cause LOSS OF TRANSPARENCY
to some degree.

8. FACTORS AFFECTING CORNEAL TRANSPARENCY
• CORNEAL EPITHELIUM &TEAR FLIM
• ARRANGEMENT OF STROMAL LAMELLA
• CORNEAL VASCULARIZATION
• CORNEAL HYDRATION
• CELLULAR FACTORS AFFECTING TRANSPARENCY

9. CORNEAL EPITHELIUM
• Normal epithelium is transparent due to homogenicity of its
refractive index.
• Basal cells are firmly joined laterally to other basal cells and
anteriorly to the wing cells by desmosomes and macule
occludentes.
• Thse tight intercellular junction accounts for corneal
transparency.
• As well as it resistance to flow of water, electrolytes and
glucose(BARRIER FUNCTION).

11. TEAR FLIM
• PRECORNEAL TEAR FLIM plays important role in
maintaining the transparency.
• Therefore ,condition associated with tear flim
abnormalities and corneal epithelial abnormality may
result in loss of corneal transparency.

12. ARRANGEMENT OF STROMAL
LAMELLAE
• Two theories have been put forward to explain the role of
peculiar arrangement of stromal lamella in corneal
transparency.
1. Maurice theory.
2. Theory of Goldman et al.

14. • Corneal transparency is because of the uniform collagen
fibrils which are arranged in a regular lattice so that
scattered light is destroyed by the mutual interference.
• He stated that as long as the fibrils are regularly arranged
in a lattice, seperated by less than a wavelength of light
(4000-7000A) the cornea will remain transparent.
• Loss of transparency will result if this regular
arrangement is altered by stromal oedema or mechanical
stress.
Maurice theory

15. • Goldman et al in 1968 after applying diffraction theory
to the problem concluded that the lattice arrangement is
not a necessary condition for stromal transparency.
• Rather they postulated that the cornea is transparent
because the fibrils are small in relationship to the light
and do not interfere with light transmission unless they
are larger than one half a wavelength of light (2000 A).
• ‘Lakes’ – areas devoid of collagen, were found in non
transparent human corneas(>2000A)
Theory of goldman et al

16. • However, theory of maurice as well as that of Goldman et al
fail to explain the occurrence of rapid clouding of cornea
associated with acute raise of intraocular pressure and rapid
clearing of the cornea with reduction of intraocular pressure.

17. CORNEAL VASCULARIZATION
• Cornea is avascular except for small loops which invade
the periphery for about 1 mm.
• It facilitates nutrition, transport of systemic antibiotic
and drugs.
• Progressive vascularization, however is a harmful process
as it interfere with functional properties of cornea,
especially its transparency.

18. Chemical theory-
VIF- stromal GAG( sulfate ester of hyaluronic acid)
VSF- any corneal injury- release of VSF- stroma-limbus-
NV(hypoxia)
Mechanical theory-
Loosening of tissues by corneal edema-NV
Combined theory-
VSF + edema
Role of leucocytes-
Leucocytes- inflammation response-NV

19. CORNEAL HYDRATION
• 1) Stromal swelling Pressure
The pressure of GAG in the corneal stroma is 60mmHg, it acts like
a sponge. (SP)
These have an anionic effect on the tissue and therefore sucking the
fluid with equal negative pressure= Imbibition Pressure (IP)
In vitro- IP=SP
In vivo- IP= IOP-SP i.e 17-60= -43mmHg
SP has an interfibrillar tension causing the maintainence of normal
arrangement in the cornea.

20. • 2) Barrier function of epithelium& endothelium
The epithelium and endothelium acts like a semipermeable
membrane
Small solutes –urea and sod.chloride
The epithelium offers twice the resistance to flow of water.
In endothelium these solutes diffuse across the layer while
water is extracted osmotically.
Barrier function of endothelium is Calcium dependant.
Any damage to endothelium or epithelium causes decrease in
corneal transparency and increase in corneal thickness.

21. • 3) IOP
IOP > SP= corneal edema
4) Evaporation of water from corneal surface
The Evaporation of water from precorneal tear film would
concentrate this fluid and increase its osmolarity relative to the
cornea. The hypertonicity of the tear film would draw water
from the cornea. This loss of fluid is however replaced by the
aqueous.

23. • 5)Active pump mechanisms
<A> Na/K ATPase pump system: More active in the endothelium
than in the epithelium. There is active extrusion of Na from the tissue.
Oubain, ATPase inhibitor when applied to the eye blocks endothelial
fluid transport resulting in corneal overhydration.
<B> HCO3 dependent ATPase: This enzyme is present in the
mitochondria. Depletion of HCO3 from incubation/perfusion medium
induces swelling.
<C> Carbonic anhydrase enzyme: Present on the endothelium.
Regulates fluid transport. CA inhibitors decrease the flow of fluid from
stroma to aqueous. Produces HCO3 and H ions in the tissues.
<D> Na/H pump: present in lateral plasma membrane surface.

24. • Passive ion movement
K, Cl, HCO3 ions diffuse into the aqueous humor
Na, Cl, HCO3 ions diffuse from the aqueous humor to the
cornea

27. CELLULAR FACTORS
Keratocytes (corneal fibroblasts)- source of collagen and
proteoglycans. Enzymes required for the matrix assembly are
encoded in the genes of keratocytes for post transitional effect.
Any defect in the gene causes corneal opacification-mucopolysaccharidoses.

28. THANK YOU