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Anatomy of cornea

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    Anatomy of cornea Anatomy of cornea Presentation Transcript

    • ANATOMY OF CORNEA Dr Nithin Keshav
    • 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
    •  
    • 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
      • 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
    • STRUCTURES
      • 5 LAYERS
      • A nterior Epithelium
      • B owman’s layer
      • C entral stroma
      • D escemets membrane
      • E ndothelium
    •  
    • EPITHELIUM
      • Stratified , Squamous & Non Keratinized
      • Continuous with conjunctiva , but no goblet cells
      • 50-90 u
      • 5-6 layers
        • Posterior to anterior
      • BASAL CELLS
        • Arranged in pallisade manner
        • Germinative layer
        • Columnar with an oval nucleus
      • 2. WING or UMBRELLA cells
        • Polyhedral
        • Convex anteriorly
      • 3. SURFACE CELLS
        • 2-3 layers
        • Polyhedral
    • 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.
      • 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.
    • 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
    • Physiology of Epithelium
      • Rich in glycogen , serves as energy store in aerobic conditions
      • Glycogen levels
      • Hypoxia
      • Corneal sensitivity
    • 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
    • Markers
      • Epithelial cells – CK3
      • Cells of regenerative regions (limbal, transient amplifying cells & transitional cells)- CK19 , VIMENTIN
      • Hemidesmosome – a6b4 integrin
    • Repair
      • Mitosis inhibited by
        • Injury
        • Adrenergic agents
        • Surface anesthetics
      • Repair occurs by CENTRIPETAL SLIDE
      • Rearrangement of Actin fibrils
      • Amoeboid migration
      • Halted by CONTACT INHIBITION
      • Anchor
      • MITOSIS resumes until epithelial thickness is
      • re-established
      • TOTAL EPITHELIAL LOSS
      • Adjacent Conjunctival epithelium resurfaces Cornea
      • Vascularised conjunctival type of epithelium containing GOBLET CELLS
    • 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
    • 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
    • STROMA
      • 500u
      • Regularly arranged lamellae of collagen bundles
      • Contains keratocytes between lamellae
      • Keratocytes – production of COLLAGEN & PROTEOGLYCANS during development
    • Stromal repair
      • Keratocyte Activation
      • Migration
      • Transformation into Fibroblasts
      • Requires presence of overlying epithelium
    • DESCEMET’S
      • Aka Posterior Limiting Lamina
      • 2.2- 4.5 u
      • It is Basal Lamina of Endothelium
      • Appears at 2 nd 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
      • Anterior 1/3 :
        • Oldest
        • Irregular banded pattern in cross section
        • Banding develops at 5 th month IUL
      • POSTERIOR 2/3:
        • Formed after birth
        • Homogenous fibrillogranular material
      • 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
    •  
      • 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
      • 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
    •  
    • 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
    • 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
    •  
    • PHYSIOLOGY
      • NUTRITION :
        • Glucose & aa
      • FLUID REGULATION:
        • Maintains relative deturgescence by
        • Provides barrier to prevent ingress of salt & metabolites into stroma
        • Decreases osmotic pressure of stroma by active pumping out of bicarbonate.
      • INJURY & REPAIR:
        • Physical & chemical (ouabain)
        • SLIDING PHENOMENA
    • STRUCTURAL PROTEINS OF CORNEA
      • COLLAGEN
      • Basal lamina - type 4
      • Bowman’s - 5
      • Stroma - 1 (90%)
      • Descemets - 4
      • PROTEOGLYCANS
      • Keratan sulphate – 50%
      • Chondoritin sulphate
      • Chondroitin
      • Peripheral cornea Dermatan sulphate & Keratan sulphate
      • Stromal edema
      • Altered biosynthesis of ground substance
      • Dermatan sulphate present centrally
      • Scarring
      • Keratan sulphate & Heparan sulphate and Hyaluronate
    • 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
    • Other factors
      • Absence of blood vessels & pigments
      • Absence of myelinated nerve fibres
      • Uniform refractive index of all layers & uniform spacing of collagen fibrils
      • 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
    • 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
      • Divides into 2 branches Anterior & Posterior
      • Anterior passes subjacent to the BOWMANS forming SUBEPITHELIAL PLEXUS
      • Posterior innervates posterior stroma , does not involve Descemets .
    •