Corneal anatomy and anatomy of limbus !

This is a presentation about anatomy of cornea

1. ANATOMY OF CORNEA Trishna Thapa Bachelor of Optometry

2. Contents Origin Introduction Embryology Dimensions Layers and their functions Nerve Supply Blood supply Nutrition Limbus

3. Origin ‘Kerato’ in Greek means horn or shield like Ancient Greek used to believe that cornea is derived from same material as thinly sliced horn of animal The word cornea comes from ‘kerato’

4. Introduction Anterior most structure of eyeball that covers iris ,pupil and anterior chamber Transparent , avascular ,watch glass like structure Forms anterior one sixth part of the outer fibrous coat of the eyeball

5. Embryology Of Cornea

6. Embryonic origin of cornea The formation of cornea is induced by the lens and optic cup . Corneal epithelium -Surface ectoderm Bowman’s membrane -Mesenchyme Stroma -Mesenchyme and Neural crest Descemet’s membrane-Synthesized by endothelium Endothelium -Neural crest

7. Anomalies of Cornea Microcornea : Horizontal diameter <9mm at 1 year of age Overgrowth of anterior tip of optic cup Megalocornea : >12mm horizontal diameter at 1 year of age Failure of optic cup to grow and of its anterior tips        to close

8. Dimensions Shape : Aspheric prolate shape - (Flatter in the periphery and steeper centrally ) Anterior Surface : elliptical Vertical diameter - 11.0 mm Horizontal diameter - 11.7 mm Posterior Surface :circular Average diameter -11.5 mm

9. Dimensions contd Thickness Central 0.52 mm/520 microns Peripheral 0.67mm/760 microns Radius of curvature - The anterior and posterior radii of curvature of the central part of cornea are 7.8 mm and 6.5 mm, respectively Surface Area -1.3 cm^2

10. Shape : Aspheric prolate (Flatter in the periphery and steeper centrally )

11. Refractive Power The ability to refract the light • Total refractive power of the eye: ~60 diopter -Anterior surface : +48 D -Posterior surface : -5 D -Net refractive power: +43 D (Central 5 mm area forms the powerful refractive surface) - Accounts for 70% of the total refractive power of the eye (+ 43D)

12. Layers Anterior Epithelium Bowman’s Layer Central Stroma Dua’s Layer Descemet’s membrane Endothelium

13. 1.Anterior epithelium Outer multilayered Stratified ,squamous and non keratinized Continuous with epithelium of bulbar conjunctiva at limbus 50 to 90 microns thick 5-7 layers of cells

14. Anterior corneal epithelial cells: a. Basal cells b. Wing cells c. Superficial Squamous cells

15. a. Basal cell layer Comprises tall columnar polygonal shaped cells Width of 12 micrometer Germinal layer of epithelium Basal cells firmly joined laterally to the other basal cells and anteriorly to wing cells

16. b. Wing cells Forms 2-3 layers of polyhedral shaped cells Nuclei flattened

17. c. Superficial cells Flattened cells contribute to two most superficial cell layers Has flattened nucleus The surface area of the outermost cells is increased by micro plicae and microvilli that facilitate the attachment of the tear film and mucin

18. XYZ hypothesis Proposed by Richard A Thoft and Judith Friend Based on experimental evidence that both limbal basal and corneal basal cells are the source for corneal epithelial cells X -Basal cell proliferation Y - Migration of cells from periphery to central cornea Z – Loss of cells from epithelial surface

19. Functions of epithelium Fights against infection and injury Receives oxygen and nutrients from tear film and distributed it to other corneal parts Major source to respond to wound healing Barrier to fluid loss and pathological entrance to other organisms

20. 2.Bowman’s membrane Also known as anterior limiting lamina 8-14 µm thickness Binds corneal stroma anteriorly with basement membrane of epithelium Modified region of anterior stroma Acellular homogenous zone Resistance to infection and injury But once destroyed, it does not regenerate

21. Function of bowman’s membrane Prevents from injury and swelling Once eroded, it does not regenerate and leaves superficial corneal scars

22. 3.Stroma 0.5 mm thickness Constitutes 90 % of corneal thickness Composed of collagen fibrils arranged in sets of lamella Cells embedded in ground substance

23. Components of stroma a. Corneal lamella b. Stromal cells c. Ground substance

24. a. Corneal lamellae Bundle of collagen fibrils Arranged in many layers(200 to 250 ) Becomes continuous with scleral lamellae at limbus

25. b. Stromal cells Keratocytes Wandering macrophages Histiocytes Few lymphocytes Keratocytes – 2-3 % of stromal volume Produce ground cells and collagen fibrils during embryogenesis

26. c. Ground substance Mainly consists of hydrated matrix of proteoglycans Keratan sulphate (max concentration in center) Chondroitin sulphate(periphery) Dermatan sulphate

27. Functions of stroma Provides transparency to cornea Gives cornea its strength and elasticity Lined by thinly distributed keratocytes which contribute to repair and maintain cornea

28. 4. Dua’s layer Discovered in 2013 by Dr. Harminder Dua 15 micro meter thick Acellular ,very strong and impervious to air Does not extend in periphery

29. 5. Descemet's membrane Thickness varies with age with 3 micrometer at birth,10-12 micrometer in young adults Thin , strong sheet of tissue Very resistant to chemical agents , trauma , infection and pathological process When destroyed , can be regenerated Thickens with age Type IV collagen Normally , it remains in a state of tension and when torn , it curls on inward on itself

30. Functions of Descemet’s membrane Serves as a protective barrier against infections and injuries Provides rigidity and stiffness to cornea Regenerated readily after injury

31. 6.Endothelium Single layer of flat polygonal cells Cell density : At birth : 6000/mm2 1 yr. : falls by 26% 11yr : another 26% Decreases with the advancing age Gradual decrease in density and increase in shape variation - POLYMEGATHISM

32. Functions of corneal endothelium : Pumps excess fluid out of stroma Keeps cornea clear /transparent

33. Overall functions of Cornea Most important refractive surface of eye Protection of intraocular contents Absorption of topically applied drugs Wound healing after anterior segment injury or trauma Corneal Reflex

34. Structural proteins of cornea : Collagen Basal lamina – type IV Bowman’s layer –type V Stroma –type I Descemet – type IV

35. Proteoglycans Keratan sulphate Chondroitin Chondroitin sulphate Peripheral cornea –Dermatan sulphate and keratan sulphate

36. Nerve Supply Trigeminal nerve Ophthalmic division Nasociliary nerve Long ciliary nerves Enter eyeball around optic nerve

37. Run forward in the suprachoroidal space A short distance from limbus, pierce the sclera to leave the eyeball Divide dichotomously and connect with each other and conjunctival nerves Pericorneal plexus : 60-80 trunks(myelinated) Enters the cornea

38. After going 1-2 mm in stroma, cornea nerves loose their myelin sheaths, branch dichotomously and form 3 nerve networks Stromal plexus (in mid stroma ) Subepithelial plexus (located between bowman’s layer and anterior stroma ) - lose their Schwann’s sheath Supplies all over epithelial cells

39. Blood supply of Cornea Avascular structure The anterior ciliary artery, which is derived from the ophthalmic artery, forms a vascular arcade in the limbal region

40. Nutrition of Cornea Perilimbal blood Aqueous humor—It supplies glucose and other nutrients by process of simple diffusion or active transport. Oxygen from atmospheric air is derived directly through the tear film.

41. Associated Conditions Peter’s anomaly Sclerocornea

42. Keratoconus

43. Fungal keratitis

44. Bullous keratopathy

45. Fuch’s corneal dystrophy

46. Limbus Grayish ring at the corneal periphery Transition zone between cornea and sclera Approximately 1.2 mm wide Elliptical in shape as its long axis is directed horizontally

47. Limbal stem cells Present in the basal epithelium of palisades of vogt Palisades of vogt are the radial infoldings at limbus

48. Histological Limbus Identifiable junction between sclera and cornea seen on histological cross section

49. At Limbus … Corneal epithelium becomes bulbar conjunctival epithelium Bowman’s membrane becomes lamina propria of conjunctiva Corneal stroma becomes sclera Descemet’s membrane becomes Schwalbe’s line Endothelium lines trabecular meshwork

50. Anatomical limbus Circumcorneal transitional zone of the conjunctivocorneal and corneoscleral junction Conjunctivocorneal junction : bulbar conjunctiva is multilayered and continuous with the corneal epithelium Corneoscleral junction : transparent corneal lamellae is continuous with the opaque, circular and oblique scleral lamellae

51. Anatomical Limbus Anterior anatomical limbus : Formed by joining end point of Bowman’s membrane to end point of Descemet’s membrane Posterior anatomical limbal border : Extends from scleral spur and up to the surface of globe perpendicularly Anatomical limbus is the area between anterior and posterior limbal border

52. Surgical Limbus 1. Anterior limbal border: overlies the termination of Bowman’s membrane Under microscope , represents the area where fine conjunctival blood vessels terminate 2. Mid limbal line: overlies the termination of Descemet’s membrane Border between blue and grey zone 3. Posterior limbal border: overlies the scleral spur

53. Zones of limbus Blue limbal zone Bluish translucent zone posterior to anterior limbal border Extent of blue zone varies in different quadrants -1mm in superior quadrant , 0.8 mm in inferior quadrant , 0.4 mm in nasal and temporal quadrant (Due to insertion of conjunctiva into cornea being different in different quadrants )

54. White limbus -1 mm whitish area between b/w mid limbal and posterior limbal border -Overlies trabecular meshwork

55. Limbal vasculature Derived from anterior ciliary arteries Supplies: Peripheral cornea Conjunctiva Episclera Limbal sclera Peripheral uvea

56. Significance of limbus Demarcation between clear cornea and opaque sclera Determines site of incision in glaucoma and cataract surgeries Contains stem cells that differentiates into corneal epithelial cells Provides nourishment to peripheral cornea through limbal vasculature

57. Thank you !