CORNEAL TRANSPARENCY

Dr. CHRISTINA SAMUEL

PG- M.S OPHTHALMOLOGY

MMCH & RI

• 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.

• Therefore, any process which upsets the anatomy or

physiology of cornea will cause LOSS OF TRANSPARENCY

to some degree.

FACTORS AFFECTING CORNEAL TRANSPARENCY

• CORNEAL EPITHELIUM &TEAR FLIM

• ARRANGEMENT OF STROMAL LAMELLA

• CORNEAL VASCULARIZATION

• CORNEAL HYDRATION

• CELLULAR FACTORS AFFECTING TRANSPARENCY

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).

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.

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.

Maurice theory

• 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.

• 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

• 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.

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.

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

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.

• 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.

• 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.

• 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.

• Passive ion movement

K, Cl, HCO3 ions diffuse into the aqueous humor

Na, Cl, HCO3 ions diffuse from the aqueous humor to the

cornea

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.

THANK YOU