Corneal transparency

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.

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

5. 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).

6. 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.

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

8. 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.

9. 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

10. 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.

11. 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.

12. 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

13. 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 I n vivo- IP= IOP-SP i.e 17-60= -43mmHg SP has an interfibrillar tension causing the maintainence of normal arrangement in the cornea.

14. 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.

15. 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.

16. 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.

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

18. 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 .

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