physiology of cornea
TRANSCRIPT
CORNEAL PHYSIOLOGY
-Riddhi Solanki
B.OptomAditya Jyot Institute Of
Optometry
Layers of cornea
Primarily concerned with:The sources of energy which fuel the cornea’s metabolic activity.Corneal transparency & its maintenance.
Corneal PermeabilityWater Endothelium> EpitheliumOxygen Derived from atmosphereCarbon dioxide Permeability is 7times more than oxygen
Corneal permeability & other
substancesSodium- endothelium > epithelium 100 timesGlucose & amino acids:metabolically activeAssociated molecules fluorescein
Low sodium permeabilityRelatively impermeable to water,lactic acid,amino acid, glucose,& large moleculesRelatively impermeable to associated and fat soluble entities
Role of cell junctions
Communication Electrical couplingBarrier to - electrolytes
- fluid - macromolecules
General classification of
junctionsOccluding or tightAdheringEach is further divided into shape & size of cell contact
- zonulae(belts) - fasciae (bands) - maculae(focal)
FibronectinCell surface glycoproteinInvolved in cell adhesion to surfacesReleased beneath generating epitheliumSynthesized by corneaFound in basal & apical surface of cultured endothelium cells.
Oxygen supply to cornea
It is the most important metaboliteDrives the supply from several surfaces
- atmosphere via tear film - capillaries of the limbal region - aqueous humor via corneal endothelium - capillaries of palpebral conjunctiva
Carbon dioxide efflux
Open eye from cornea & aqueous humor into tear film Closed eye
into the aqueous humor
Metabolism corneal energy by
carbohydrate metabolismGlucose enters cornea from aqueous humorEnergy:ATP
anaerobic:breakdown of glucose into lactic acid
aerobic: breakdown of glucose by TCA cycle into carbon dioxide & water
Sources of glucose to corneal epitheliumAqueous humor (90%)
Limbal blood vessels & tears(less than 10%)
Glucose consumption
30-90mg /hr40-60% of total consumption is by the epithelium
Glucose metabolic pathwayEMBEDEN-MAYERHAOFF
PATHWAY produces lactate (anaerobic)+2ATP
TRICARBOXYLIC ACID CYCLE aerobic
HEXOSE MONOPHOSPHATE SHUNT
aerobic produces NADPH,CO2,H2O& 6ATP
Lactic acidNot metabolized by corneaRemoved by diffusion into aqueous humorAccumulation results in epithelial & stromal oedemaHypoxia doubles lactic acid concentration resulting in an osmotic gradient
CORNEAL TRANSPARENCYFactors Affecting Corneal
Transparency:
Corneal Epithelium and Tear Film Arrangement of Stromal LamellaeCorneal VascularisationCorneal Hydration
Corneal transparency: stroma
Transmits 90% of incident lightPotentially a non-transparent layerFibrils: n=1.47Ground substance: n=1.37Regular fibril spacing :60 µm
Diffraction theory of Maurice
Depends on ordered arrangement of collagen fibrilsTransparency is maintained if the disruption is less than few wavelengthScattering effect increases as swelling increases(fibrils become larger optically)
Corneal swellingLactate & metabolite accumulation- osmotic gradient causes water imbibitionHydrophilicity of GAGs causes a natural water imbibitionSwelling sleep is due to : hypoxia (50%) lower water osmolarity increased temperature & humidity
It effects:Change in refractive index of intra & extracellular spacesSattler’s vielhaloes
Histochemical examination has shown presence of the endothelium of enzymes
needed for glycolysis & krebs cycle
The endothelial pump
Drug Permiability Across Cornea
Lipid And Water Solubility Of The DrugMolecular SizeWeight And Concentration Of The DrugsIonic Form Of The Drugs