The document provides information on the physiology and biochemistry of the cornea. It discusses the cornea's structure, composition, function, metabolism, wound healing, and factors that affect transparency. The cornea's layers are composed primarily of water, collagen, and proteoglycans. It maintains transparency through its regular stromal spacing and metabolic pumps that regulate hydration. The cornea relies on limbal stem cells and tight epithelial/endothelial barriers to replenish and prevent edema.

1. PHYSIOLOGY OF
CORNEA
Amrit Acharya
1st Year
Resident, NEH
2016, Jan. 24

2. 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

3. 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.

4. Layers of cornea

6. 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%

7. 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 .

8. 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

9. 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

10. 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

11. Oxygen
 Epithelium: derives O2 from tear film and
limbal capillaries
 Oxygen 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

12. 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

13. = Anaerobic glycolysis
 Glucose
Lactic
acid 2 ATP

14. = 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

15. = Hexose monophosphate(HMP)
shunt.
NADPH produced is utilized in lipids
synthesis by cornea
Ribose produced is used to build
DNA,RNA & nucleic Acid

16. ….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

17. 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.

18.  Maurice theory
 Theory of goldman
Arrangement of stromal lamellae

19. 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.

20. 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.

21. Corneal transparency
Physiological factors :
-stromal swelling pressure
-metabolic pump
-barrier function
-evaporation from the corneal
surface
-intraocular pressure.

22. 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)

23. 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 .

24. ii. Barrier function of epithelium and endothelium
- semi permeable in nature.
- calcium dependent

25. 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.

26.  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.

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

28. 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

29. 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

30. Limbal epithelial stem cells
Progenitors 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

31. …...…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

32. Causes of
limbal stem
cells
deficiency
Congenital
-Aniridia
Aquired
-chemical burn
-thermal burn
-Multiple corneal
surface surgery
-prolonged contact
lens use

33. 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 .

34.  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.

35. 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)

36. Mitosis:
 Occurs only in basal cell layer
Daughter cells move upward from basal
layer, differenciating into wing cells and
finally into superficial cells

37. Shedding of superficial cells:
Corneal epithelium: Stratified Squamous
epithelium from which terminally
differentiated, superficial cells
continuously shed
Epithelium turn over completely every
week

38. 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

39. Stromal wound healing
Deposition 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

40. 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.

41. Corneal wound healing stages

42. Corneal injury

43. Corneal opacities
Nebular scar
Macular scar
Leucomatous

44. 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

45. 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.

46. 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

47. 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

48. 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

49. 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

50. 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

51.  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.

52. 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.

53.  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.

54. Drug deposits in cornea
 Vortex keratopathy : whorl like corneal epithelial
deposits.
 Antimalarials
 Amiodarone
 Chlorpromazine:yellowish brown deposits in
endothelium and deep stroma

55.  Argyrosis
 due to silver deposit
 greyish brown deposits in
descemets
membrane
 Chrysiasis:dust like
deposits in corneal stroma

56. Aging changes in cornea
 By 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.

57. 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