Anatomy of cornea

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Anatomy of cornea

  1. 1. ANATOMY OF CORNEA Dr Nithin Keshav
  2. 2. Introduction CORNEA – Medieval Latin “ co rne a te la “ 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
  3. 3. 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
  4. 4.  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 CAPorAPEX  Curvature varies from apex to limbus , greater
  5. 5. STRUCTURES 5 LAYERS A nterior Epithelium Bowman’s layer C entral stroma Descemets membrane E ndothelium
  6. 6. 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
  7. 7. 2. WING or UMBRELLA cells  Polyhedral  Convex anteriorly 3. SURFACE CELLS  2-3 layers  Polyhedral
  8. 8. 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
  9. 9.  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.
  10. 10. 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 
  11. 11. Physiology of Epithelium  Rich in glycogen , serves as energy store in aerobic conditions Glycogen levels  Hypoxia Corneal sensitivity
  12. 12. Turn Over  Limbal stem cells migrate towards centre  XYZ Hypothesis : Lim bal& Co rne albasale pithe lialce lls are so urce fo r CO RNEAL 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
  13. 13. Markers  Epithelial cells – CK3  Cells of regenerative regions (limbal, transient amplifying cells & transitional cells)- CK19 , VIMENTIN  Hemidesmosome – a6b4 integrin
  14. 14. Repair  Mitosis inhibited by 1. Injury 2. Adrenergic agents 3. Surface anesthetics
  15. 15.  Repair occurs by CENTRIPETAL SLIDE Rearrangement of Actin fibrils Amoeboid migration Halted by CONTACT INHIBITION Anchor MITOSIS resumes until epithelial thickness is re-established
  16. 16. TOTAL EPITHELIAL LOSS Adjacent Conjunctival epithelium resurfaces Cornea Vascularised conjunctival type of epithelium containing GOBLET CELLS
  17. 17. 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
  18. 18. 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 Pro lo ng e d Hypo to ny & Atro phic Bulbi degenerative changes in the ridges contributes to Secondary Anterior Crocodile Shagreen
  19. 19. STROMA  500u  Regularly arranged lamellae of collagen bundles  Contains keratocytes between lamellae  Keratocytes – production of COLLAGEN & PROTEOGLYCANS during development
  20. 20. Stromal repair Keratocyte Activation Migration Transformation into Fibroblasts  Requires presence of overlying epithelium
  21. 21. 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
  22. 22.  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
  23. 23.  In Endothelial diseases where morphology & thickness of Descemets is altered , presence of no rm alante rio r bande d laye r can be used to signify o nse t o f diso rde r afte r birth.  In AGEING CORNEA:  Bands of long spacing collagen found  Focal overproduction of basal lamina like material produces peripheralexceresences HASSAL HENLE WARTS
  24. 24.  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
  25. 25.  Hypertrophied in congenital anomalies – POSTERIOREMBRYOTOXON  On stripping Descemet’s it ROLLS INTO STROMA  Lens capsule curls outwards  On injury endothelial cells resurfaces & deposits Basal Lamina identical to Descemets
  26. 26. 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
  27. 27. 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
  28. 28. PHYSIOLOGY 1. NUTRITION :  Glucose & aa 2. FLUID REGULATION:  Maintains relative deturgescence by 1. Provides barrier to prevent ingress of salt & metabolites into stroma 2. Decreases osmotic pressure of stroma by active pumping out of bicarbonate.
  29. 29. 3. INJURY & REPAIR:  Physical & chemical (ouabain)  SLIDING PHENOMENA
  30. 30. STRUCTURAL PROTEINS OF CORNEA COLLAGEN Basal lamina - type 4 Bowman’s - 5 Stroma - 1 (90%) Descemets - 4
  31. 31. PROTEOGLYCANS Keratan sulphate – 50% Chondoritin sulphate Chondroitin  Peripheral cornea Dermatan sulphate & Keratan sulphate
  32. 32. Stromal edema  Altered biosynthesis of ground substance  Dermatan sulphate present centrally Scarring  Keratan sulphate & Heparan sulphate and Hyaluronate
  33. 33. 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
  34. 34. Other factors  Absence of blood vessels & pigments  Absence of myelinated nerve fibres  Uniform refractive index of all layers & uniform spacing of collagen fibrils
  35. 35.  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
  36. 36. 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
  37. 37.  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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