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Anatomy of cornea

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  • 1. ANATOMY OF CORNEA Dr Nithin Keshav
  • 2. 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
  • 3.  
  • 4. 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
  • 5.
    • 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
  • 6. STRUCTURES
    • 5 LAYERS
    • A nterior Epithelium
    • B owman’s layer
    • C entral stroma
    • D escemets membrane
    • E ndothelium
  • 7.  
  • 8. 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
  • 9.
    • 2. WING or UMBRELLA cells
      • Polyhedral
      • Convex anteriorly
    • 3. SURFACE CELLS
      • 2-3 layers
      • Polyhedral
  • 10. 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.
  • 11.
    • 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.
  • 12. 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
  • 13. Physiology of Epithelium
    • Rich in glycogen , serves as energy store in aerobic conditions
    • Glycogen levels
    • Hypoxia
    • Corneal sensitivity
  • 14. 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
  • 15. Markers
    • Epithelial cells – CK3
    • Cells of regenerative regions (limbal, transient amplifying cells & transitional cells)- CK19 , VIMENTIN
    • Hemidesmosome – a6b4 integrin
  • 16. Repair
    • Mitosis inhibited by
      • Injury
      • Adrenergic agents
      • Surface anesthetics
  • 17.
    • Repair occurs by CENTRIPETAL SLIDE
    • Rearrangement of Actin fibrils
    • Amoeboid migration
    • Halted by CONTACT INHIBITION
    • Anchor
    • MITOSIS resumes until epithelial thickness is
    • re-established
  • 18.
    • TOTAL EPITHELIAL LOSS
    • Adjacent Conjunctival epithelium resurfaces Cornea
    • Vascularised conjunctival type of epithelium containing GOBLET CELLS
  • 19. 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
  • 20. 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
  • 21. STROMA
    • 500u
    • Regularly arranged lamellae of collagen bundles
    • Contains keratocytes between lamellae
    • Keratocytes – production of COLLAGEN & PROTEOGLYCANS during development
  • 22. Stromal repair
    • Keratocyte Activation
    • Migration
    • Transformation into Fibroblasts
    • Requires presence of overlying epithelium
  • 23. 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
  • 24.
    • 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
  • 25.
    • 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
  • 26.  
  • 27.
    • 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
  • 28.
    • 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
  • 29.  
  • 30. 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
  • 31. 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
  • 32.  
  • 33. 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.
  • 34.
    • INJURY & REPAIR:
      • Physical & chemical (ouabain)
      • SLIDING PHENOMENA
  • 35. STRUCTURAL PROTEINS OF CORNEA
    • COLLAGEN
    • Basal lamina - type 4
    • Bowman’s - 5
    • Stroma - 1 (90%)
    • Descemets - 4
  • 36.
    • PROTEOGLYCANS
    • Keratan sulphate – 50%
    • Chondoritin sulphate
    • Chondroitin
    • Peripheral cornea Dermatan sulphate & Keratan sulphate
  • 37.
    • Stromal edema
    • Altered biosynthesis of ground substance
    • Dermatan sulphate present centrally
    • Scarring
    • Keratan sulphate & Heparan sulphate and Hyaluronate
  • 38. 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
  • 39. Other factors
    • Absence of blood vessels & pigments
    • Absence of myelinated nerve fibres
    • Uniform refractive index of all layers & uniform spacing of collagen fibrils
  • 40.
    • 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
  • 41. 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
  • 42.
    • Divides into 2 branches Anterior & Posterior
    • Anterior passes subjacent to the BOWMANS forming SUBEPITHELIAL PLEXUS
    • Posterior innervates posterior stroma , does not involve Descemets .
  • 43.