CORNEA
Meenank
Anatomy Dimensions Topography Histology Blood Supply Nerve Supply
Physiology Functions Transparency Hydration
CORNEA
CORNEA
The cornea is a transparent, avascular, watch-glass (outer - convex and inner – concave) smooth structure which forms the outer 1/6th of eye ball.Covers: iris, pupil and the anterior chamberGreek name : kerato
CO
RN
EA
L DIM
EN
SIO
N’S
Anterior surface – elliptical , 11.7mm / 10.6mm
Posterior surface – circular , 11.7mm
As V>H = astigmatism
Optical zone: center
› Ant. Radius – 7.8mm
› Post. Radius – 6.5mm
Thickness: center – 0.5 to 0.6mm
periphery – 0.6 to 0..8 mm
Refractive power : ant. Surface +48D,
post. Surface -5D = +43D
Refractive index: 1.37
Border: limbus
(A) ant. & post. Diameters The diff in v and H in ant is due to conj. And sclera
(B) thickness and the depth in relation to A.C and post. chamber
Topography
Corneal shape important for contact lens
fitting done by keratometry
Aveg. ant surface – 7.8mm to 8.4mm
post surface – 5.8mm
Flatter in males
Ant curvature – spherical, 2-4mm decentered up and
out towards visual axis but, correctly placed for
pupillary aperture = corneal cap/apex
Corneal curvature - limbus to apex is flattened
nasally and above
Corneal gutter – limbus - helps in CL fitting
Histology
Behind the pre-corneal tear film the cornea shows 5 tissue layer’s namely
EpitheliumBowmans layer (ant. Limiting lamina )Stroma (substantia propria)Decements layer (post. Limiting lamina )endothelium
Corneal Epithelium Stratified, squamous and non-keratanized nucleated cells of 5-6 layers
Basal cells: deepest, palisade on the basal lamina, germinative layer
Columnar with flat base, round head and oval nuclei oriented parallel to the long axis
Winged/ umbrella cell’s: Polyhedral cells
Convex ant. Cap
Converging base
Post. Process b/w the basal cell
Nuclei parallel to corneal surface
Next 2-3 layers are polyhedral cells whose base keeps inc. towards the surface
Surface cells – largest in area, non-keratanized and nucleated
Ultrastructure
Epithelial cell show cell organelles of actively metabolizing cell distributed in variable no. in different layersMitochondria: scares in basal but, abundant in middle and wingedtonofibrills : cells of electron dens cytoplasmic meshwork
Desmosomes :
adhesion
Abundant – basal
Scarce - wing and surface
Zonulae occludents +
desmosomes impermeable
to all
but,
semipermeable in bathing
pre-corneal tear film
Hemi-desmosomes – basal
cell to basal lamina
Microvilli:Superficial hexagonal cell folds Stabilizes tear film
Dendritic cell :Langerhans cellsID and representation for lymphocytesAbsent centrally
Repair : germinating layer Mitosis – inhibited by injury, adrg, anesth.associated with cAMP
Centripetal cell slide - actin fibrils rearranged– amoeboid manner – halt at inhibition – mitosis resume
Bowman’s layer Narrow, homogenous Modified zone of ant. stromaAnt- basement membranePost- stromaBoundary- junct. b/w cornea and limbus
Ultrastructure Collagen fibrils - strengthPost- more progressive and blend into stromaCannot regenerate – coarse scar.Non-myelinated nerves
Stroma
Regularly arranged Collagen bundle
lamellae
Central (200-300)
Peripheral (500)
Proteoglycan ground and keratocytes
Lamellae – parallel, limbus to limbus
Ant. ⅓ - oblique, runs into bowman's
Deep stromal – strap like
right angles, at periphery runs into
sclera and rectus muscle
Limbus – circular course
Ultrastructure
In each stromal lamellae collagen bundles run parallel Variation b/w the lamellar thicknessC. Fibrils causes – corneal transparency
Keratocytes : sys. and maintain stromal collagen + proteoglycanFound b/w not in lamellaeMaculae occludentas bindsNo ant. Post Nuclei – flat, long Cytoplasm – scares Cell organelle – complete but few
Descement’s MembraneBasal lamina of endothelium
Syn. All life, from 2nd gest.
Birth – 3-4μm
Childhood - 5μm
Adult – 10-12μm
Sharply defined strong resistant sheet
Thickens – age and degen contd.
Major protein – type IV collagen
Glycoproteins +proteoglycans = pink on
acid Schiff
Ultrastructure
Ant. 1/3rd - oldest – produced in fetal life
irregular bands, unlike type I collagen
Banding – 5th IUL
Post. 2/3rd – after birth
homogenous fibro-granular material
zone next to endo – new
Aging – long spacing collagen –
polymerization
Hassal-Henel Wart – focal over-production of
basal lamina like material – aging
fissured and cytoplasmic invagination on
endo faces
resembles descements wart/corneal
guttate(fusch dyst.)
Peripheral rim: landmark for corneal limbus viz
schwalbe’s line
Despite its non-elastic nature – rolls up to
stroma upon injury – resurfaces – endothelium
covers defect synth. Descement’s like basal
lamina
Endothelium
Single layer, cuboidal, hexagonal
Not vascular in origin like rest
Derived from neural crest
Young – mitosis
Birth – 6000 cells/mm²
Adult – fixed (500,000)
With age – polymerization + polymorphism
Injury – adjoining zone (area ↑*3, ht ↓)
Nuclei – flat, oval, central
Ultrastructure
Lateral border – convoluted-complex integration
Ant. (basal) – descement’s – HD
focal areas of inc. density - pinocytotic vesicles
Lateral memb. runs ant. and post.
Post. (apical)
Apicolateral interface marginal fold
Tight junction’s – maculae adherentes and
maculae occlundentes
Desmosomes – rare
Post. Cell wall – microvilli
Cilia – rare, to A.C., more in periphery
Cell wall – pinocytotic vesicles on inner surface
Cell organelle:
Mitochondrion - around nucleus
(like RPE, and ellipsoid of R. photoreceptors)
RER, SER,
Golgi apparetus – peri-nuclear facing A.C.
Cytoplasum – condens, actin rich
Terminal web: close to post. Memb.
ass. With location of tight junction
Blood Supply
Cornea is avascular Ant. Ciliary – 1 mmSub-conjuctival
Nerve SupplyTrigeminal → ophthalmic
Descements and endothelium show no innervations
Functions of Cornea
Functions of cornea are :
1. Refraction of light
2. Transparency
3. Containing of intra-ocular pressure
4. Protection (corneal reflex)
The collagen fibrils matrix found in the stromal layer is
responsible for the containing IOP
Transparency
Transparency is due to
› Anatomical
› Avascularity
› Epithelial non-keratinization
› Stromal lamellar packing
› Non-myelinated nerves
› Pre-corneal tear film
› Physiological
› Corneal dehydration
› Uniform refractive index
water from endothelium maintains optical homogeneity
Maurice theory: Explained on the basis of stromal lattice arrangement of collagen fibrils Small diameter – regular spacing – light back scatter suppressed – destructive interface
Goldman theory: Fibril separation and a diameter ↓ ⅓ of the wave length of incident light – perfect transparency
Loss of transparency - Corneal scaring – new collagen – irregular interweaving
Stromal – corneal oedema - ↑ spaces – fluid lakes – stromal cloud → irregular surface viz irregular astigmatisum Epithelial oedema: ill fit CL/ IOP → seperation of basal cells by oedema → diffraction grating effect Imp. Symp in sub ac. Angle closure glaucoma
Main function→ optics Forms principle refracting surface ( 70% ) Factors such as -
Transparency Smooth anterior surface Uniform arrangement of epithelial cells Closely packed stromal lamellae of uniform size Avascularity
Help in maintaining a clear cornea Factors that effect cornel hydration viz transparency corneal epithelium corneal stroma corneal endothelium
Epithelium
5-7 layers, 5µm, 10% of corneaNon-keratanized sq. epithelium – regenerating Mech. Barrier – tight junct. ; electric resistance – impermeableTransparency – homogeneity Edema – surface irregular , Vn ↓ Sympt – glare, photophobia, halos due to scattered light min. in mesopic condt
Stroma
90% of cornea, uniformly arranged collagen fibrils Ground subs – glycosaminoglycans
keratan sulfatedermatan sulfatechondroitin
Stroma – water (70%), keratocytes(5%)Role – strength and shapeStroma+endo = preserve transparencyStromal oedema – epi/ endo malfunction A.P. spatial separation of ground subs corneal diameter doesn't swell
Endothelium
Monolayer, homogeneous, hexagonal cells 5μmMaintains transparency by endothelial barrier function
endothelial pump mechanismEpithelial barrier betterBarrier - cornea and aq. CompartmentPump mech – active Na-K-ATPase aq. Leak into stroma freed
Young – 3000-5000 cells/mm²→2/3 in adults ↓500 cells/mm² - corneal oedema
Corneal Hydration
Transparency depends on hydrationTo remain transparent – thin and dehydratedAq. medium – cornea swell – GAGDehydration –
stromal swelling pressure (SP)barrier function, epi and endo endothelial pumpevaporation from corneal surfaceintra-ocular pressure (IOP)
Stromal Swelling Pressure
Stroma – excised (78%) hydrated aq. Medium (98%) hydrated
Glycosaminoglycan's – major cause of hydration Keratan sulfate and chondroitin – electrostatic repulsion – swelling Collagen fibrils – cross-link– expand with repulsionSP (excised) – 50mmHg, GAG imbibition of fluid by neg. pressure – IPExcised – SP=IP ; normally IP↓ than SP due to IOPThus, IP= IOP – SP ( 17 – 50 = aveg. 30-40 )
GAG – resist’s flow across resistance ↓ if hydration↑ - oedema↑
no lateral flow except at limbus
Barrier Function
Epithelium and endothelium – semipermeable for flow of water and diffusion of electrolyte’s Epi. – 200 ↑ for electrolyte’s than endo. zonula occludes – intre-cellular spaces – sup. Epi cellsEndo – semipermiable – small ions + water from aq. – IOP
Pump Mechanism Endothelium – imp. Pump mech (active process)
Na/K-ATPase – qubain ATP inhibitor – block endo. Fluid transport – over hydration Bicarbonate – thgh neg electrical potential – thiocyanate Carbonic anhydrase – carbonic anhydrase inhibitors – stroma to aq.
Evaporation
Evaporation of water → con. And increase osmolarity Hypertonicity of tears draw the water from cornea Readily replaced by aqueous Aveg loss – 4%
Intra-ocular Pressure
Doesn't cause epi. Oedema, not associated with corneal
thickness
But, when IP is +ve i.e
IOP ↑ - SP = epithelial oedema
Eg: ↑IOP and SP normal = epi. Thickening – glaucoma
normal IP and ↓ SP = endo. Dystrophy.