cornea physiology
TRANSCRIPT
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.
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
= 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(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 .
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.
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