anatomy and physiology of cornea

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  1. 1. Anatomy and Physiology of cornea Dr.Lhacha Wangdi 1st year Resident Department of Ophthalmology JDWNRH/KGUMSB
  2. 2. Outline Gross anatomy of cornea -Surface anatomy -clinical application Microanatomy of cornea -anatomy of ultra structure Physiology of corneal tissue
  3. 3. Introduction Word cornea originated from Latin word-Cornu (horn) Cornea tela(horny tissue) Transparent avascular tissue with a convex anterior surface & concave posterior surface. Main function is OPTICAL Other functions are: -STRUCTURAL INTEGRITTY -PROTECTION FOR THE EYE
  4. 4. Gross anatomy of cornea Covers the anterior scleral foramen Limbus is transition zone between cornea and sclera The cornea protrudes slightly beyond the scleral globe because of the different curvatures of the two structures Radius of curvature (cornea)6.7- 9.4mm whereas scleral-11.5mm Cornea appears elliptical in shape measuring 11-12mm horizontally and 10-11mm vertically Surface area: About 1.3 cm (one-sixth of the globe) 11-12mm 10-11mm limbus
  5. 5. Corneal thickness Posterior surface of cornea is curved more than anterior surface Central zone- 0.52mm Paracentral zone- 0.52- 0.57mm(outer diameter-7-8mm) Periphery zone- 0.63-0.67mm The thinnest zone is 1.50mm temporal to the geographic center about 0.505-0.51mm In endothelial/ epithelial cell dysfunction, corneal becomes unusually thick due to stromal edma Thick cornea will give false IOP reading
  6. 6. Surface anatomy- cornea Cornea is not a true sphere It has a central spherical (optical zone-4mm)with uniform flattening towards the periphery prolate shape Flattening is more extensive nasally and superiorly Surface cornea is divided into three zones- 1. Central zone of 1-2mm-spherical(red area) 2. Paracentral zone of 3-4mm around the central zone with progressive flattening 3. Peripheral zone which is flattened more. Clinical application- 1. Topographic information of cornea is important in contact lens fitting 2. Flattening of cornea at periphery helps to reduce the spherical aberration of optical system 3. Alteration of the uniform curvature of cornea will induce astimatism
  7. 7. Spherical aberration In total spherical object peripheral parallel rays of light refract more and focus in front of ideal image point. Effects- blurred vision Spherical aberration is minimized by aspheric (prolate) shapes of eye surfaces- due to peripheral flattening
  8. 8. Cornea as a optical system Main function of cornea- optical( refraction of light for clear vision) Contributed by its special characteristics; 1. Transparency 2.Avascularity 3. Controlled hydration 4. High refractive power The optical power of the cornea=42.0 D Is equal to 2/3 of the total optical power of the human eye (57 62 D) Refractive power of cornea=(refractive index of cornea-refractive index of air) (radius of curvature of cornea) Refractive index of cornea is more -Cornea- 1.33765 -Air- 1.000 Radius of curvature is inversely proportional to curvature: - Anterior surface about 7.8 mm -Post. Surface about 6.5 mm
  9. 9. Microanatomy of cornea Cornea has five defined layers 1. Epithelium and basal lamina- 5% of thickness 2. Bowmans layer 3. Stroma-90% of total thickness 4. Descemets Membrane 5. Endothelium single cellular layer (germinal layer)
  10. 10. Composition of cornea Water: 78 % Collagen: 15 % of which: Type-I : 50-55 % Type-III : 1 % Type-IV : 8-10 % Type-VI : 25-30 % Other protein: 5 % Ground substances -Keratan sulphate: 0.7 % -Condroitin/dermatan sulphate: 0.3 % Hyaluronic acid: + Salts: 1 %
  11. 11. Embryonic origin of cornea Clinical significance-cell line originating from surface ectoderm has regenerative capacity whereas those from neural crest has little regenerative capacity Disease affecting other organ such as in atopic dermatitis may cause keratitis due to similar embryonic origin Epithelium-Derived from surface ectoderm Bowmans layer- mesenchyme(neural crest cell) Endothelium- mesenchyme(neural crest cell) 1st wave Stroma - mesenchyme(neural crest cell) 2nd wave descemets synthesised by endothelium
  12. 12. Epithelium Derived from surface ectoderm Constitutes of 5-6 layers of cell accounting for about 5% of corneal thickness-(0.05mm/50um) Its has three cell layers 1. Apical cells- nonkeratinised Squamous epithelium 2. Wing cells- 2-3 layers of polygonal cell 3. Basal columnar cells (germinative layer) Nonepithelial cells- histocytes, macrophages, lymphocytes, antigen presenting langerhans cells are also present which becomes more numeruous during keratitis
  13. 13. Ultrastructure of epithelium Apical cells layers consists of 2-3 layers of flattened hexagonal cells Surface cells contain microvilli & microplicae coated with 300-nm thick glycocalyx/glycoprotein (buffy cell coat) The mucin layer of tear binds with glycocalyx and helps in uniform spreading of tear film,
  14. 14. Ctn Epithelial cells are adhered together by tight junctions 1. Tight junctions & desmosomes surface cells 2. Desmosomes wings & superficial cells 3. Desmosomes & Hemidesmosomes in basal cells 4. Cells are anchored to deeper tissue by anchoring proteins Functions- 1. Maintains corneal homeostasis(impermeable to Na ions & confer semipermeable membrane properties to the epithelium) 2. Mechanical barrier protective function against infection/toxins 3. Tight junction ensure corneal transparency 4. Anchor epithelial cells to basal lamina and bowmans layer Anchoring protein desmosome hemidesmosomes
  15. 15. Basal lamina Fibrous layer consisting of 1V collagen and glycoprotein. Secreted by the basal cells 0.5 - 1 m wide Ultra structurally it is distinguished in to two parts 1. Lamina lucida (superficial)- electron lucen zone 2. Lamina densa (deep electron dense zone) 3. Anchored to bowmans layer with numerous anchoring fillaments Lipid solvent, stromal oedema and inflamation may loosened the cohesion between Bowmans zone,lamina and epithelial cells- eg mucus filaments due to epithelial instability With old age, in diabetes and in some corneal disorders it becomes thickened and multilamellar
  16. 16. Epithelial regeneration The epithelium is constantly in a state of turn-over with exfoliating apical cells being replaced by underlying wing cells-weekly Basals cells are only epithelial cells capable of mitosis During normal apical cell exfoliation basal cells proliferates and replace lost cells in 7-14 days Loss of basal cells and defective regeneration will lead to corneal scar formation Apical cells loss
  17. 17. Wound healing During epithelial defect either due to infection/trauma/infla mation there are extended proliferation and differentiation of basal cells If Bomans layer is intact the epithelium is regenerated in 7- 14 days
  18. 18. Cellular events Repair of corneal epithelial injury like abration/infection follows a distinctive sequence of events- 1. Cells at wound edge retract, thicken and lose attachment, produce various growth factors (egTGF-Bs) 2. Basal cells travel in an amoeboid movement to cover the defect 3. Migration process is halted by contact inhibition 4. They then anchor by secreting basal lamena 5. Mitosis resumes to re-establish epithelial thickness 6. Surface tight junctions re- establised 7. Adhesion with Bowmans layer within 7 days (if basal lamina intact) Toxin, trauma, infection, inflammation Spreading and dedifferentiation Cell migration Cell proliferation Regeneration redifferentiation
  19. 19. Germinative cells It is now recognized that the germinative region lies at the limbus Limbal stem cell migrate centrally to replace corneal epithelial basal cells The stem cells migrate at a very slower rate (123 m/week) to the center of the cornea which may be as long as a year Loss of limbal stem cells will result in corneal scar Limbal stem cells
  20. 20. The XYZ hypothesis Richard A. Thoft & Judith Friend(1983) proposed on the basis of experimental evidence that both limbal basal and corneal basal cells are the source for corneal epithelial cells. The corneal epithelium is maintained by a balance among- (Z)Sloughing of cells from the corneal surface is = (X)cell division in the basal layer + (Y) Migration of basal cells originating from the limbal stem cells
  21. 21. Ctn.. In normal healthy cornea there is a constant balance between; (cell turn over=regeneration) Regeneration= balanced basal cell proliferation + migration+maturation+secretion of basement membrane+regeneration. Abnormality of epithelial Cell turn over and regeneration causes epithelial opacity and haziness. Example, Corneal Epithelial Basement membrane Dystrophy(EBMD) aslo called map dot finger print/cogan microcystic dystrophy chateractised by; -dots/epithelail microcysts (due to abnormal epithelium) -fingerprint/geographic map lines (due to thicken basement membrane)
  22. 22. Bowmans layer Modified region of anterior stroma 8 14 m thick Acellular homogeneous zone It is perforated by many nerve axons which courses through toward the epithelium Ant. surface is smooth & parallel with corneal surface Posteriorly it becomes blended & interweaved with fibrils of ant. stroma Functions- 1. Anchoring site for epithelial cells to ensure its stability 2. Tough acellular layer provide mechanical supports 3. Prevents stromal keratocytes from exposure to epithelial growth factors- prevents keratocytes metaplasia to fibroblast and scar formation
  23. 23. Ultrastructural features Ultrastructurally it is a meshwork of fine collagen fibrils of uniform size in a ground substance (glycoprotein &proteoglecan) Compact arrangement of collagen types I, III, V, and VI it has great strength and relatively resistant to trauma both mechanical and infective It is acellular and lacks fibroblast