Cornea

It is transparent, avascular ,watchglass structure forms 1/6thof outer fibrous coat of eye ball.

● Anterior surface is elliptical - 11.75mm horizontal & 11mm vertical

● Posterior surface is circular with diameter of 11.5mm.

● Thickness of cornea is 0.52mm in centre & 0.67mm in periphery.

● Radius of curvature: ant is 7.8mm & post. is 6.5mm.

● Refractive power of anterior surface is 48 D & posterior surface is -5D .net refractive

power is 43D● refractive index is 1.37

Histology

• Epithelium • Bowman’s membrane• Stroma• Descemet’s membrane• Endothelium

Epithelium : stratified squamous epithelium about 50-90μm thick and 5-6

layers of cells.it is replaced entirely in period of 6-8 days.

Basal layer :tall columnar polygonal shaped cells arranged in palisade like

manner on basement membrane.Wing cells: 2-3 layers of polyhedral shaped

cells.Flattened cells: constitute 2 most

superficial layers

Bowman’s membrane:Acellular mass of condensed

collagen fibrils 8-14 μm in thickness. Shows resistance to infection &

injury. Does not regenerate after destruction.

Stroma(substantia propria) : ● It consists of collagen fibrils and cells

embedded in hydrated matrix of proteoglycans.

●Lamellae are arranged in 200-250 layers. They are parallel to each other ,corneal

plane & continuous with scleral lamellae at limbus.

●cells among lamellae are keratocytes, wandering macrophages, histiocytes & few

lymphocytes.

Descemet’s membrane: It is strong homogenous layer which binds

stroma posteriorly. Made of collagen & glycoprotein thickness is 3μm at birth & 10-

12μm in young adults. Highly resistant to injury & has ability to regenerate. Posterior

surface at periphery show rounded wart like excrescences called hassell henle

bodies which ↑with age. Central excrescences are called guttatae in

fuch’s dystrophy .

Endothelium: It is a single layer of flat polygonal cells

form mosaic. Cell density is around 6000 cells /mm2 at birth cell count

falls by 26% in first year & further 26 % over next 11 years in young adults it is

2600-3000cells/mm2 .

Sl

slit lamp photo of normal cornea

Blood supply :Cornea is avascular tissue. Small loops in sub conjunctival tissue derived

from anterior ciliary vessels provide nourishment to peripheral 1mm.

Nerve supply :Rich nerve supply of sensory nerve endings derived from long ciliary nerves which are

branches of nasociliary nerve br. of ophthalmic division of V th nerve.

They run in suprachoroidal space, near limbus pierce sclera to leave eyeball divide

dichotomously & connect with each other & conjunctival nerves to form pericorneal

plexus of nerves.

About 60-80 myelinated trunks enter the cornea at various levels sclera , episclera and

conjunctiva. After having gone 1-2mm in stroma they lose myelin sheath branch

dichotomously & form stromal plexus. Some end in stroma ,most go anteriorly to form sub epithelial plexus. From here penetrate pores in bowman’s membrane, lose their schwann sheath form fibres & extend in all layers of epithelium to form intraepithelial plexus

ending as beaded filaments.

• Thus cornea has innervation density highest at centre ,gradually decreasing at periphery.No nerves are present in central posterior part, descemet’s membrane and endothelium.

Corneal physiologyPrimary physiologic functions:1)To act as powerful refracting lens of fixed focus2)To protect intraocular contents.

Processes concerned with functioning:1)Biochemical composition2)Metabolism3)Corneal transparency4)Drug permeability5)Cornel wound healing

Biochemical composition• Heterogenous in all layers• 80% water, 20% solids (19.8% organic matter,0.2%

inorganic salts)1)- Epithelium :it forms 10 % of total weight of cornea 70% water protein synthesis is 5 times hgher than stroma lipids (phospholipids & cholesterol) constitute

5.4% of dry weight enzymes for glycolysis,kreb’s cycle & Na⁺ K

activated ATPaseATP, glycogen, glutathione,acetylcholine,cholinesterases,

high K, low Na & Cl.

Stroma • collagen- Collagen fibrils embedded in matrix

of proteoglycans• Collagen forms 70 % of dry wet • Type 1 collagen is most prominent also collagen

types 5 ,6, 12 ,14 also present.• Diameter of collagen fibril 35 nm & spacing

between them is 55nm is constant• High content of glycin, proline, hydroxyproline in

collagen

• Soluble proteins – high levels of immounoglobulins IgG, IgA,IgM with albumin & glycoproteins.

• Proteoglycans – family of glycosylated proteins that contains one glycosaminoglycan chain covalently bonded to protein core.

• 3 fractions- keratan sulphate 50 %, chondroitin sulphate A ,chondroitin

• Responsible for stromal swelling pessure 60 mm Hg i.e. tendancy to imbibe water.

• Enzymes- Glycolytic and Krebs cycle enzymes are present.

• Matrix metalloproteinases- family of enzymes that breakdown of extracellular matrix. They helps to maintain normal corneal framework and crucial role in remodelling after injury.

• MMP-1 ( COLLAGENASE 1)- active against collagen types 1,2,3.

• MMP-2 ( GELATENASE A ) & MMP-9 ( GELATENASE B )- against types 4,5,6 & gelatins , fibronectins

• MMP-3 ( STROMELYCINS 1) – breaks down proteoglycans and fibronectin.

• Only MMP-2 is detected in normal cornea, others found after injury.

• MMPs 1,2,3- produced by stromal cells.• MMP 9- by corneal epithelium• Electrolytes – conc. Of Na is high that of K is

low as compared to epithelium.

Descemet’s membrane

• Made up of collagen and glycoproteins• Does not contain GAGs.

Endothelium

• Due to single layer delicate nature biochemical composition could not be studied yet withou inducing artifactual changes.

• However histiochemical examination shows presence of enzymes needed for glycolsis & kreb’s cycle

Metabolism of cornea

• Source of nutrients- 1. Oxygen- epithelium derives oxygen mainly

from atmosphere through the tear film ( active process) as well as through limbal capillaries.

• Endothelium derives its oxygen from aqueous humour.

• Mean total oxygen consumption of cornea is approx. 9.5 ml O2/ccm/hr

2. Glucose- respiratory quotient for cornea is 1• Aqueous humour is the source of glucose supply

to endohelium as well as stroma & epithelium.• Minimal requirement of epithelial glucose

consumption are 50-60 mg/ccm/hr.• In absence of exogenous supply of glucose

glycogen stored in epithelium is broken down to glucose.

• Amino acids- continous supply of amio acids is needed for protein synthesis for constant shedding and replacement of epithelial cells of cornea.

• Major source is again aqueous humour by passive diffusion.

• Metabolic pathways1. Glycolysis- enzymes of EMP & KREBS (TCA) are

pesent in all corneal cells. Maximum glucose consumption is by epithelium.

One mole of glucose gives 2 ATP by EM pathway i.e glycolysis 36 ATP by Krebs cycle2. HMP shunt- produces no ATP but NADPH utilised for

lipid synthesis by epithelium & ribose produced is utilised for DNA &RNA synthesis

Corneal transpeancy

Factors affecting corneal transperancy-• Corneal epithelium & tear film• Arrangement of stromal lamellae• Corneal vascularisation• Corneal hydration• Cellular factors

1)Corneal epithelium & tear film:normal epithelium is transparent due to

homogenicity of refractive index. The tight inter cellular junctions account for epithelium’s transparency & resistance to flow of water ,glucose & electrolytes i.e barrier function normal pre corneal

tear film also plays important role. Conditions associated with abnormality of both leads to loss of transparency .

2) Arrangement of stromal lamellae• Maurice theory: cornea is transparent because

uniform collagen fibrils are arranged in regular lattice so that scattered light is destroyed by mutual interference. Fibrils are seperated by less than wavelength of light 4000 to 7000 Ao . Stromal oedema or mechanical stress alter regular arrangement & cause loss of transparency

• Goldmann theory: stated cornea is transparent as fibrils are small compared to light & do not interfere transmission unless they are more than half wavelength of light 2000Ao. In oedematous corneas lakes-areas devoid of collagen of dimension greater than 2000Ao .

Both theories do not explain clouding of cornea with rise in IOP & its rapid clearing with reduction

of IOP

Corneal vascularisation• Cornea is avascular. But in some disease processes to bring

defence mechanisms in play against noxious agents vascularisation takes place. Thus hampering its functional properties esp. transparency.

• Chemical theory: presence of vasostimulatory factor( unknown..probably LMW amine- campbell & michaelson) or breakdown of pre existing vasoinhibitory factor(stromal GAG – meyer &chafre).

• Mechanical theory: Cogan states vessels cannot invade normal cornea due to its compactness. Loosening due to oedema is mandatory for neovascularization

• Maurice et al gives combined theory that vasostimulatory factor & loosening of compact cornea are necessary for neovascularisation

Corneal hydration• Cornea maintains relative state of

dehydration. Water content is 80%.1) Stromal swelling pressure: swelling pressure

(SP) exerted by GAGs of stroma (60 mm of Hg) acting like sponge. Anionic charges on GAG molecule repel each other sucking in fluid with equal but negative pressure imbibition pressure (IP). IP=IOP - SP

IP=17-60= - 43mmof Hg

2) Barrier function of epithelium & endothelium:Epithelium is twice resistant to water flow than

endothelium. Epithelium is semipermeable to small solutes

NaCl& urea. While in endothelium these solutes diffuse across

layer & water is extracted osmotically. Barrier function of endothelium is calcium

dependent. Corneal transparency & thickness is affected

more when endothelium is damaged as compared to epithelium.

3) Hydration control by active pump mechanisms

• Endothelium has several enzyme pump mechanisms controlling fluid transport.

• Na⁺ K ⁺ATPase pump mediates active extrusion of Na⁺ . Ouabain acts as its inhibitor blocking endothelial fluid transport resulting in overhydration

• Bicarbonate dependent ATPase: depletion of bicarbonate induces swelling

• Carbonic anhydrase: It decreases fluid flow from stroma to aqueous humor

• Na⁺ H⁺ pump

4) Evaporation of water from corneal surface: Evaporation of precorneal tear film

leads to its hypertonicity drawing water from cornea. However fluid lost is replaced from

aqueous so less role in dehydration5) Intraocular pressure: when IOP exceeds

SP oedema occurs as seen when IOP>50 mm of Hg. When swelling pressure is low edema occurs even with normal IOP e.g endothelial

dystrophy

Cellular factors affecting transparency

• Corneal fibroblasts (keratocytes) as they are source of stromal collagens & proteoglycans

Drug permeability across cornea

• Lipid & water solubility of drug • Molecular size , weight & concentration of

drug • Ionic form of drugs• pH of solution• Tonicity of solution• Surface active agents • Pro drug form

Effect of contact lens wear on corneal physiology

• Hard lenses made up of PMMA- restrict oxygen availability, deplete oxygen stores.

• Hard lens induced inhibition of aerobic enzymes such as hexokinase reduces direct glucose utilization by cornea.

• Soft contact lenses made up of HEMA, Poly HEMA, silicon permeable to oxygen and CO2.

• Oxygen flux = DK/L*DP• D-Diffusion coeff. K- Solubility, L- Thickness of lens

material, DP- Change in partial pressure of oxygen across the material.

• HEMA, PMMA have low oxygen flux while hydrogels and silicon have high flux.

• Contact lenses cause epithelial thinning, reduction in hemidesmosomes density and no. of anchoring fibrils and reduced adhesion of epithelium to basement membrane. Severe cases excessive use of contact lens produces epithelial oedema and keratopathy in form of punctate epithelial erosions.

• Rigid contact lenses also produce tear film instability by damging mucin layer.

Cell turnover and wound healing

1. Epithelium- epithelium is constantly regenerated by mitotic activity in basal cells.

• After epithelial debridement hemidesmosomes & other junctions reform and gradually single layer is restored to its 6 layered architecture by mitosis.

• Migration of epithelial cells is achieved by marked cytoskeletal and cell shape changes involving actin- myosin fibrils. Migration of cells is also dependant on intracellular signalling via fibrinectin / fibrin, laminin, collagen peptides through cell surface integrins.

• Most of the mitotic activity in epithelium takes place at limbus where stem cells undergo rounds of cell division to repopulate entire corneal surface

• The stroma- incisional wounds involving stroma: immediate effect is wound gaping & imbibition of water from tears by GAGs f/b

deposition of fibrin, rapid epithelization , activation of keratocyte to divide & synthesize collagen.

• During early phase of corneal wound healing keratocytes revert to fibroblast and lay down collagen and GAGs e.g. hyaluronic acid, collagen types I & III , matrix glycoproteins.

• Endothelium- cannot undergo mitosis. Response to direct injury is to undergo cell sliding as occurs in epithelium in early stages of migration.

• Vascularisation- occurs when vessels from conjunctiva or deep scleral plexus invade the periphery of cornea during healing.

• MMPs and plasminogen activators ( uPA, tPA) are released by incoming leucocytes and resident epithelial and stromal cells. Cytokines such as IL1 IL6, TNFa , TGFb , GM-CSF liberated from inflammatory cells stimulate further ingress of inflammatory cells and initiate vascularisation.

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