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

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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.