cornea physiology

PHYSIOLOGY OF CORNEA

Amrit Acharya1st Year Resident, NEH2016, Jan. 24

History

The word cornea has come from kerato.

Kerato in greek mean horn or shield like

Ancient greek used to believe that cornea is derived from thinly sliced horn of animal

Functions of cornea

It is the most important refractive media in the eye .

The transparency of the cornea results from the uniform spacing of the collagen fibrils in the substantia propria.

Any increase in tissue fluid between the fibrils cause cloudiness of the cornea.

The endothelium and epithelium play a major role in limiting fluid uptake by the corneal stroma.

Layers of cornea

Composition of cornea

Water 78% Collagen 15% Type I 50-55% type III <1% Type IV 8-10% TypeVI 25-30% other protein 5% Keratan sulphate 0.7% Chondroitin/dermatan sulphate 0.3% Hyaluronic acid and salts 1%

Biochemical composition of epithelium

10% of the total weight of cornea.

Water - 70%of wet weight.

protein synthesis in epithelium is highest.

Lipids:phospholipid and cholesterol

Contain enzymes necessary for krebs cycle, glycolysis

Acetylcholine and ATP, glycogen ,glutathione

Electrolyte:K, Na ,Cl .

Biochemical composition of stroma Contains 80% water and 20 % solids

Collagen(type I,V,XII,XIV)

Soluble proteins- albumin,immunoglobulins and glycoprotein.

Proteoglycans(GAG fractions- keratan sulfate 50%,chondroitin sulfate 25% and chondroitin 25%)

Glycolytic and Krebs cycle enzymes

Matrix metalloproteinases :MMP-1(collagenase-I),MMP-2(Gelatinase A,MMP-3(Stromelysin I)

Electrolytes and salts

Biochemical composition of descemet´s membrane

Composed of collagen(73%)and glycoproteins

Collagen of descemet’s membrane is insoluble and extremely resistant to chemical and enzymatic action.

Descemet’s membrane doesn’t contain GAG

Single cell layered structure

Contains enzymes for glycolysis and Krebs cycle.

Biochemical composition of endothelium

Metabolism in cornea

The most active layers for metabolism are epithelium and endothelium.Energy in the form of ATP is generated by breakdown of glucose.

Sources of nutrients for the corneal metabolism and metabolic pathway are –

= Oxygen = Glucose = Amino acids

OxygenEpithelium: derives O2 from tear film and

limbal capillariesOxygen required by epithelium is 1/10th of that

available from atmosphere when the eyes are open and about ¼ of that available from palprebral conjunctiva when eyes are closed

Endothelium: derives from aqueous humour

Total corneal oxygen consumption:9.5ml 02 /cm2 /hr

Glucose

• Primary metabolic substrate for epithelial cells,stromal keratocytes and endothelium

In absence of an exogenous supply of glucose ,glycogen stored in corneal epithelium is broken down

Rate of glucose consumption of cornea: 100mg/cm2/hr 90% being consumed by epithelium

Glucose is metabolized in the cornea by 3 metabolic pathway

= Anaerobic glycolysis

Glucose Lactic acid 2 ATP

= Tricarboxylic acid( Krebs's cycle)

36 ATP produced from a molecule of glucose

Only 12% of glucose metabolise through TCA cycle

Accumulation of lactate even in aerobic condition

Lacate is eliminated from cornea by diffusion through

epithelium

Glucose 36 ATP CO2 & H2O

= Hexose monophosphate(HMP) shunt.

NADPH produced is utilized in lipids synthesis by cornea

Ribose produced is used to build DNA,RNA & nucleic Acid

….continued

In epithelium and endothelium, the HMP pathway breaks down 35%-65% of the glucose but stroma metabolize very little via this pathway

Pyruvic acid is end product of glucose

Amino acids

Supplied from aqueous humour by passive diffusion

• Requirement: For synthesis 10 mg/hr of protein , for constant shedding and replacement of epithelial cells of cornea.

Maurice theory

Theory of goldman

Arrangement of stromal lamellae

Maurice theory(1957)The stromal collagen fibrills are of equal diameter (275-350 Å) and are equally distant from each other, arranged as a lattice with the inner fibrillar spacing less than a wave length of light (4000-7000 Å)

He explains , because of their small diameter and regularity

of separation ,back scattered light would be almost completely supressed by destructive interference.

Goldman and Benedek’s theory (1967) :

He suggest that , a perfect crystalline lattice periodicity

Is not always necessary for the sufficient destructive interference .

He explains, If fibril separation and diameter is less

than a third of the wavelength of the incident light, almost perfect transparency will issue.

The stromal fibrils are small in relation to the light

and do not interfere with the light transmission , unless they are larger than one half a wave length of light.

Corneal transparencyPhysiological factors :

-stromal swelling pressure

-metabolic pump

-barrier function

-evaporation from the corneal surface

-intraocular pressure.

Corneal Hydration State of relative dehydration that is necessary

for corneal transparency. Normal water content of cornea

( 80%) is kept constant by balance of factors that draw water in cornea(swelling pressure and IOP), factor which prevent flow of water in cornea(epithelial barrier) and that draw water out of cornea(Endothelial pump)

Factors affecting corneal hydration

i. Stromal swelling pressure (SP)

- 50mmHG exerted by GAGs and collagen of corneal stroma.

- Imbibition pressure (IP) is equal to SP in vitro but not in vivo .

ii. Barrier function of epithelium and endothelium

- semi permeable in nature. - calcium dependent

iii. Active pump mechanisms-

Mechanism by which endothelium Removes fluid from stroma referred as endothelial pump.

a) Active process :

Na-K-ATPase pump - Essential component of endothelial pump function. - Integral membrane protein located in the basolateral aspect of endothelium – its action is vital in the maintenance of corneal hydration.

-Central to all the system as maintains the sodium gradient required for the Na -H exchange thereby promoting bicarbonate production.

Bicarbonate dependant ATPase: - Is essential for the maintenance of the corneal thickness -The bicarbonate transported by the endothelium is generated

intracellularly via the action of carbonic anhydrase.

- CO2 diffuses into the cell from the extra cellular space combines with the water in presence of the carbonic anhydrase . The carbonic anhydrase dissociates into H + and bicarbonate ions.

Carbonic anhydrase enzyme Inhibitation of this enzyme decreases flow of fluid from stroma to aqueous humour Na-H pump.

b) Passive process : K+, Cl-,Hco3- ions diffuse into the aqueous Na+,Cl-,Hco3- diffuse from aqueous into cornea

iv. Evaporation of water from corneal surface

Increase in concentration of precorneal fluid(osmolarity)

Water from cornea is drawn into tear film

Relative state of corneal dehydration

v. Intra ocular pressure IOP > Swelling pressure = corneal oedema occur- The relation of swelling pressure of stroma to IOP is- IP(imbibition pressure of corneal stroma) =IOP- SP- As stromal pressure decreases precipitiously by increase in corneal

thickness, mild corneal edema combined with increase in IOP leads to high imbibitions pressure and subsequent microbullous formation and epithelial edema.

Cellular factor affecting transparency - keratocytes maintain transparency by producing collagens and proteoglycans - keratocytes contain enzymes involved in assembly of stromal matrix

Specific enzyme defects are associated with corneal opacification eg- mucopolysaccharidoses

Limbal epithelial stem cellsProgenitors to replenish

themselves and form cells of other type.

Present in limbal palisade of vogt.

Finger like projection protects stem cells from shearing forces.

Only 5%-15% of the limbal cells are stem cells

…...…stem cells

Highly vascularized and potential source of nutrient and growth factors for stem cells.

Are slow cycling cells and retain DNA labels for longer time.But in events of injury they become highly proliferative.

Prevents corneal neovascu-larization and conjunctival ingrowth

Causes of limbal stem cells deficiency

Congenital-Aniridia

Aquired-chemical burn-thermal burn-Multiple corneal surface surgery-prolonged contact lens use

Corneal Epithelium Functions:

To form a Barrier between the environment and the stroma of the cornea.

Barrier formed as the cells move superficially to the surface of the cornea ,differentiating until the superficial cells form two layers of the flattened cells encircled by the tight junction, which serves as a high resistance, semi permeable membrane.

Barrier functions to prevent the movement of the fluid from tears to the stroma .

- To form a smooth refractive surface on the cornea

- Protects the cornea and the intraocular structures from infection by pathogens

smoothes the surface of the cornea, increases its ability to become wet by aqueous, thereby forms smooth optical surface required for clear vision.

Maintenance of the Epithelium:

Maintained by a balance among: (X,Y,Z Hypothesis)

Process of cell migration: . originate from stem cells in limbal

epithelium

. Migration of new basal cells into cornea from limbus.

.Migrate centripetally at about 120 μm/wk Prolifera

-tion of basal cells (x)

Centripetal

migration of cells

(Y)

Epithelial loss from corneal surface(Z)

Mitosis:

Occurs only in basal cell layerDaughter cells move upward from

basal layer, differenciating into wing cells and finally into superficial cells

Shedding of superficial cells:

Corneal epithelium: Stratified Squamous epithelium from which terminally differentiated, superficial cells continuously shed

Epithelium turn over completely every week

Epithelial wound repair

Mitosis resume and epithelial thickness is re-established

Re-establishment of adhesion between basal epithelial cells and bowman´s layer

Cells separate from basal lamina and travel in amoeboid manner unless halted by contact

inhibition

Injury inhibits mitosis of epithelial cells

Centripetal migration of marginal cells by rearangement of actin fibrils in filopodial

extension of cells

Stromal wound healingDeposition of fibrin within the stromal

wound

Rapid epithelization of wound

Activation of keratocytes to divide and synthesize collagen and GAGs

Initial lay out of irregular fibroblast

Production of normal corneal matrix to restore clarity in small wound

Stromal wound healing continued…….

Keratocyte at injured site undergo apoptosis peaking 4 hrs. after initial insult. Modulate wound healing by activating adjacent keratocytes

In 1-2 wks remodeling starts. Which sometime continue over prolong period and eventually restore corneal transparency.

Corneal wound healing stages

Corneal injury

Corneal opacities

Nebular scar

Macular scar

Leucomatous

Endothelial injury and repair

When endothelial cells are lost defect is covered by spreading of cells from adjacent areas of wound

If single cell lost the cells surrounding the defect spread to fill in the area left by missing cell

• If large defect-cell migration toward the center of wound followed by remodeling into hexagonal shape.

• The pump of endothelium are reestablished when monolayer is restored allowing cornea to return to normal thickness

Corneal Vascularization-

Chemical theory - Destruction of vasoinhibitory factor(VIF) - Release of vasostimulatory factor(VSF) Mechanical theory:( Cogan) blood vessels cannot invade

stroma due to its compactness Combined theory:( Maurice et al) role of both VIF &

compactness of the cornea. Role of leucocytes – stimulate corneal vascular growth.

Pathogenesis of corneal vascularization

New capillaries arise from the perilimbal capillaries and parent venules by focal degradation of the venular basement membrane.

Migration of endothelial cells toward angiogenic stimulus

Endothelial cells elongate and form solid sprout which later develop lumen

The outer surface is lined by pericytes and blood flow begin

Clinical correlation:• Cornea is immunologically privileged for

keratoplasty due to avascularity, absence of lymphatics and few antigen presenting cells.

• Degree and depth of corneal vascularization are prognostic in keratoplasty.

Deep vascularization of more than 2 quadrants is considered as high risk of graft rejection following keratoplasty

Types of corneal vascularisation

Superficial

Deep

Superficial

Deep

Present below epithelial layer

Lie in corneal stroma

Can be traced with conjunctival vessels

Continuity can´t be traced beyond limbus

Pattern- commonly arborising

Terminal loops , brush, umbel

Causes- Trachoma , Phlyctenular kerato -conjunctivitis, superficial corneal ulcer

Interestial keratitis , disciform keratitis, chemical burn, deep corneal ulcer

Corneal pharmacology The volume of normal adult tear film 7-9μL and the maximum amount cul de sac can maintain : 25 to 30μL

Volume of a drop is approx. 40 μL .

Most of the medication is immediately lost to eye lashes and runs out of the eye and the remainder is diluted by tear film to approx. 25%.

Both volume and concentration of drop are reduced.

Ointment is retained in cul de sac, gradually melts, releasing the drug into the tears, purpose is to increase the time the drug is present in tears

Corneal pharmacology….

After topical administration most of the drug enters the stroma, aqueous by corneal penetration

Drug penetrating to conjunctiva is carried away by blood vessels.

The corneal epithelium provides an initial barrier due to the tight junction

Epithelium composed of lipids so non polar substance penetrate readily

Stroma contain mainly H2O so polar group pass more readily

Drugs must pass through both barriers, those soluble in both lipids and water exhibit best penetration.

Some preservatives present in drug e.g. benzalkonium chloride impair the integrity of epithelial barrier and increase penetration.

Drug permeability across cornea

Depends on:

Solubility- epithelium and endothelium easily penetrated by lipid soluble substances. Stroma is hydrophilic so allow water soluble substance. Hence to go through drug should be amphipathic.

Ioniation- Drug must have the capacity to exist both in ionized and non-ionized form for better penetration,non ionised drugs can penetrate through epithelium and ionized through stroma.

pH: affect on electric charges and stability of solution. Solution outside the range of 4-10 increases the permeability.

Molecular size: not important for lipid soluble substance but for water soluble size should be less than 4 A.

Molecular weight: less than 500 can pass through

Wetting agents: They increases permeability by reducing surface tension.

Drug deposits in cornea

Vortex keratopathy : whorl like corneal epithelial deposits.

AntimalarialsAmiodarone

Chlorpromazine:yellowish brown deposits in endothelium and deep stroma

Argyrosis due to silver deposit greyish brown deposits

in descemets membrane

Chrysiasis:dust like deposits in corneal stroma

Aging changes in corneaBy advancing age cornea becomes less translucent & dust like

opacities due to condensation in stroma.

Bowman's & Descemet’s membranes also increase in thickness .

Arcus senilis is present due to infiltration of extra cellular lipid and in almost every person over 60yrs.

The small protrusion at the periphery of Descemet’s membrane occur and known as Hassal Henle bodies and do not interfere with vision.

References

Adler’s Physiology of eye. 7th ed.

Internet resources

Fundamentals and External disease & cornea- American academy of Ophthalmology. 2011-2012

A.K.Khurana Anatomy and Physiology of Eye

Clinical anatomy of eye Richard S. snell, Michael A.Lemp 2nd edition

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