Anatomy of cornea
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ANATOMY OF CORNEA
Dr Nithin Keshav
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Introduction
CORNEA – Medieval Latin “ cornea tela “ HORNY WEB (latin ,cornu = horn)
Transparent avascular tissue with a convex anterior surface & concave posterior surface.
Main function is OPTICAL Accounts for 70% of the total refractive
power of the eye (+ 43D) Other functions are: -STRUCTURAL
INTEGRITTY -PROTECTION FOR THE
EYE
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DIMENSIONS
Anterior Surface : Vertical – 11.7 mm Horizontal – 10.6 mm Posterior Surface : Both 11.7 mm Thickness : Central 0.52 mm Peripheral 0.67 mm Surface Area: 1.3 cm2
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Radius of Curvature Anterior – 7.8 mm Central 1/3 Posterior – 6.5 mm - Peripheral cornea is more flattened
Topography Anterior curvature is spherical in 2-4 mm zone
decentered upwards & outwards relative to visual axis but centered to the pupillary aperture( lies 0.4 mm temporally) -- CORNEAL CAP or APEX
Curvature varies from apex to limbus , greater flattening seen nasally & in upper part of cornea
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STRUCTURES
5 LAYERS
A nterior EpitheliumB owman’s layerC entral stromaD escemets membraneE ndothelium
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EPITHELIUM
Stratified , Squamous & Non Keratinized Continuous with conjunctiva , but no
goblet cells 50-90 u 5-6 layers
Posterior to anterior
1. BASAL CELLS Arranged in pallisade manner Germinative layer Columnar with an oval nucleus
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2. WING or UMBRELLA cells Polyhedral Convex anteriorly
3. SURFACE CELLS 2-3 layers Polyhedral
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Ultrastructural features
Abundant mitochondria in wing & middle cell layers
High glycogen content (Wing & Superficial layers)
Tonofibrils ( Intermediate filaments)
Desmosomes- lateral adhesion b/w cells, mainly at the basal level.
Zona Occludens- Tight jn seen at surface cells in addition to desmosomes.
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Tight jn are impermeable to Na ions & confer semipermeable membrane properties to the epithelium
Surface cells contain MICROVILLI & MICROPLICAE– Helps in stabilizing precorneal tearfilm
Dendritic cells ( langerhans cells )- present in fetal epithelium but disappears in mature cornea.
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BASAL LAMINA
2 LAYERS Superficial LAMINA LUCIDA Deep LAMINA DENSA
Thicker peripherally Thickened in Diabetes , Corneal pathology, Old
age Integrated with the underlying Bowmans layer
through ANCHORING FILAMENTS & ANCHORING PLAQUES
Cohesion between Basal Lamina & Bowman’s loosened by Lipid solvents Stromal edema Inflammation
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Physiology of Epithelium
Rich in glycogen , serves as energy store in aerobic conditions
Glycogen levels Hypoxia Corneal sensitivity
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Turn Over
Limbal stem cells migrate towards centre
XYZ Hypothesis : Limbal & Corneal basal epithelial
cells are source for CORNEAL EPITHELIAL CELLS
TRANSIENT AMPLIFYING CELLS : Daughter cells of limbal stem cells
TRANSITIONAL CELLS: Basal cells lying between limbus & peripheral cornea commonly seen at Superior Cornea
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Markers
Epithelial cells – CK3
Cells of regenerative regions (limbal, transient amplifying cells & transitional cells)- CK19 , VIMENTIN
Hemidesmosome – a6b4 integrin
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Repair
Mitosis inhibited by
1. Injury
2. Adrenergic agents
3. Surface anesthetics
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Repair occurs by CENTRIPETAL SLIDE Rearrangement of Actin fibrils
Amoeboid migration
Halted by CONTACT INHIBITION
Anchor
MITOSIS resumes until epithelial thickness is
re-established
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TOTAL EPITHELIAL LOSS
Adjacent Conjunctival epithelium
resurfaces Cornea
Vascularised conjunctival type of epithelium containing GOBLET CELLS
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BOWMAN’S
Aka Anterior Limiting Lamina 8-14 u Modified region of anterior stroma Acellular homogenous zone Normally attached to Basal Lamina In pathological conditions
Corneal edema , Dystrophy After death
-Epithelium readily seperates from this layer
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Ultrastructural features
Fine collagen fibrils of uniform size in ground substance
Relatively resistant to trauma (mechanical & infective)
Convex ridges can be seen when relaxed – POLYGONAL / CHICKEN WIRE PATTERN Responsible for Anterior Corneal Mosaic
In Prolonged Hypotony & Atrophic Bulbi degenerative changes in the ridges contributes to Secondary Anterior Crocodile Shagreen
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STROMA
500u
Regularly arranged lamellae of collagen bundles
Contains keratocytes between lamellae
Keratocytes – production of COLLAGEN & PROTEOGLYCANS during development
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Stromal repair
Keratocyte Activation
Migration
Transformation into Fibroblasts
Requires presence of overlying epithelium
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DESCEMET’S Aka Posterior Limiting Lamina 2.2- 4.5 u It is Basal Lamina of Endothelium Appears at 2nd month of gestation Strong resistant sheet Sharply defined & the plane of seperation
is used in LAMELLAR KERATOPLASTY Thickens with age , endothelial
degenerations Type 4 collagen
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Anterior 1/3 : Oldest Irregular banded pattern in cross section Banding develops at 5th month IUL
POSTERIOR 2/3: Formed after birth Homogenous fibrillogranular material
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In Endothelial diseases where morphology & thickness of Descemets is altered , presence of normal anterior banded layer can be used to signify onset of disorder after birth.
In AGEING CORNEA: Bands of long spacing collagen found Focal overproduction of basal lamina like
material produces peripheral exceresences
HASSAL HENLE WARTS
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Physiological
Resemble Descemet’s warts of central cornea – CORNEA GUTTATA in Fuch’s
Peripheral rim of Descemets forms internal landmark of corneal limbus & marks anterior limit of angle – SCHWALBE’S LINE
Prominent in 15-20% of individuals
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Hypertrophied in congenital anomalies –POSTERIOR EMBRYOTOXON
On stripping Descemet’s it ROLLS INTO STROMA
Lens capsule curls outwards
On injury endothelial cells resurfaces & deposits Basal Lamina identical to Descemets
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ENDOTHELIUM
Single layer of hexagonal / cuboidal cells
Counts At birth : 6000/mm2 1 yr : falls by 26% 11yr : another 26%
Gradual decrease in density & increase in shape variation – POLYMEGATHISM
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Ultrastructural features
Lateral borders convoluted forming marked interdigitation
Cell junction Ant 2/3 : Maculae adherentes Post 1/3 : maculae occludentes
Posterior surface shows Microvilli - Absorptive surface area
Abundant mitochondria Condensation of cytoplasm rich in actin
lies close to posterior membrane – TERMINAL WEB
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PHYSIOLOGY
1. NUTRITION : Glucose & aa
2. FLUID REGULATION: Maintains relative deturgescence by 1. Provides barrier to prevent ingress of salt
& metabolites into stroma2. Decreases osmotic pressure of stroma by
active pumping out of bicarbonate.
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3. INJURY & REPAIR: Physical & chemical (ouabain)
SLIDING PHENOMENA
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STRUCTURAL PROTEINS OF CORNEA
COLLAGEN
Basal lamina - type 4Bowman’s - 5Stroma - 1 (90%)Descemets - 4
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PROTEOGLYCANS Keratan sulphate – 50%Chondoritin sulphate Chondroitin
Peripheral cornea Dermatan sulphate & Keratan sulphate
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Stromal edema
Altered biosynthesis of ground substance Dermatan sulphate present centrally
Scarring
Keratan sulphate & Heparan sulphate and Hyaluronate
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CORNEAL TRANSPARENCY
MAURICE THEORY: LATTICE ARRANGEMENT of collagen
fibres is responsible for transparency.
Due to small diameter & regular seperation of collagen, back scattered light would be suppressed by DESTRUCTIVE INTERFERENCE
GOLDMAN THEORY : If fibril seperation & diameter is less than 1/3
of wavelength of incident light –TRANSPARENCY ensues
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Other factors
Absence of blood vessels & pigments
Absence of myelinated nerve fibres
Uniform refractive index of all layers & uniform spacing of collagen fibrils
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In ill fitting contact lenses & IOP, basal cells which are regularly arranged are seperated by edema fluid of differing refractive index to cells
DIFFRACTION GRATING EFFECT
HALOS AROUND LIGHT
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NERVE SUPPLY
Ophthalmic division of Trigeminal via Anterior Ciliary Nerve
Supply also from Cervical Sympathetic
Anterior ciliary nerve enters sclera from perichoroidal space just behind the limbus & joins with the conjunctival nerve to form PERICORNEAL PLEXUS
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Divides into 2 branches Anterior & Posterior
Anterior passes subjacent to the BOWMANS forming SUBEPITHELIAL PLEXUS
Posterior innervates posterior stroma , does not involve Descemets .
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