Anatomy and physiology of cornea

ANATOMY AND PHYSIOLOGY OF CORNEA

1. Anatomy and physiology of cornea DR.K.V.SUDHEER

2. The word cornea has come from “ Kerato ”. The term “ Kerato ” in greek means horn or shield like. Ancient Greek used to believe that cornea is derived from same material like that of thinly sliced horn of animal.

3. The Cornea The cornea is a transparent avascular tissue with smooth, convex outer surface and concave inner surface, which resembles a small watch-glass. To meet the diverse functional demands the cornea must be: - Transparent - Refract light - Contain the intraocular pressure - Provide a protective interface

4. Dimensions Microcornea: when HCD is less than 10mm Macrocornea when HCD is more than 13 mm  

5. Shape : - Prolate Surface area: - About 1.3 cm² (one-sixth of the globe)

6. Radius of curvature: - Anterior surface – about 7.8 mm - Post. Surface – about 6.5 mm Refractive power: +43.1 D Refractive index: 1.376

7. Composition of human 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 % Keratan sulphate: 0.7 % Condroitin / dermatan sulphate: 0.3 % Hyaluronic acid: + Salts: 1 %

8. Embryonic origin of Cornea: Structure      Derived from Corneal epithelium Surface ectoderm Stroma, Descmet’s membrane, endothelium Paraxial mesoderm

9. Structure

10. Epithelium: Stratified, squamous and nonkeratinized Three types of cells are seen a.Superficial cells b.Wing cells c.Basal cells

11. Basal cells: - Germinative layer of the epithelium -only epithelial layer with mitotic activity Superficial cells - microvilli + -helps in glycocalyx formation and stability of tear fim .

12. Adhesion is achieved by – desmosomes And hemidesmosomes Desmosomes are tight junctions between cells Hemidesmosomes are tight junctions between cell and basal lamina - Langerhans cells (cells of immune recognition system) present near periphery. They are almost absent at central cornea but aggregate in response to infection

13. Epithelial Turnover: - Early studies suggested that the epithelium replaced approximately weekly by division of basal cells and the oldest shed from the surface - It is now recognized that the germinative region lies at the limbus , the stem cells, and cells migrate at a very slower rate (123 µm/week) to the center of the cornea which may be as long as a year

14. - The XYZ hypothesis: 1. Thoft R. and Friend J. (1983) proposed that both limbal basal and corneal basal cells are the source for corneal epithelial cells, and there is a balance among division, migration & shedding.

15. Epithelial Repair: Repair of corneal epithelial injury like abration follows a distinctive sequence of events. Injury ( abration ) Cells at wound edge retract, thicken and lose attachment. Travel in an amoeboid movement to cover the defect Cells at wound edge ruffle and send out filopodia and lamellipodia towards the center of wound

16. Migration process is halted by contact inhibition They then anchor and Mitosis resumes to re-establish epithelial thickness Surface tight junctions re- establised Adhesion with Bowman’s layer within 7 days (if basal lamina intact) The healing process occurs rapidly, rate of cell migration is 60 – 80 µm/hr

17. Bowman’s layer: (Ant. Limiting lamina) - Modified region of anterior stroma - Acellular homogeneous zone - 8 – 14 µm thick - It delineates the anterior junction between cornea and limbus Compact arrangement of collagen gives it great strength and relatively resistant to trauma both mechanical and infective.

18. Stroma : ( Substantia propria ) Stroma : ( Substantia propria ) - About 500 µm thick (about 90% of corneal thickness) - Consists of regularly arranged lamellae of collagen bundles, lie in proteoglycan ground substance with – - 200 – 300 bundles – centrally - 500 bundles – peripherally - Small population of cells – keratocytes present

19. - Arrangement of lamellae – - Lamellae are arranged in layers, parallel with each other & with corneal surface - In deeper stroma the lamellae form strap-like ribbons which run approximately at right angles to those in consecutive layers - At the periphery this right-angular arrangement is slightly changed where it gets scleral fibres - At the limbus the bundles appeared to take a circular course

20. Fig.: Arrangement of stromal lamellae

21. Ultrastructural features: - Each lamellae comprises of a band of collgen fibrils arranged in parallel with each other - There is a unique uniformity of fibril diameter, it is 22 (±1) nm from ant. to post. - There is remarkable regularity of seperation both within and between lamella

22. - The keratocytes occupy 2.5 – 5 % of total stromal volume and is responsible for synthesis and maintaining of collagen & proteoglycan substance of stroma . - There are stellate processes extened for great distance and frequent contacts are made with other keratocytes in same horizontal plane forming gap junctions

23. Stromal repair: - Repair of stroma after small injuries involves: - Keratocytes activation Migration & transformation into fibroblasts Production of scar tissue Initial fibrils are large & irregular

24. Remodelling of scar tissue occurs, it ensues – 1. Thinning of fibrils 2. Reformation of lamellae over months 3. Increase in transparency - Larger wounds provoke rapid vascular response and leaving vascularised scar .

25. Descemet’s membrane: (Post. Limiting layer) - It is the basal lamina of corneal endothelium - First appears at 2nd month of gestation and synthesis continue throughout adult life Thickness – at birth :- 3 – 4 µm at childhood :- about5µm at adult :- 10 – 12 µm

26. - It is a strong resistant sheet - It thickens with age and in some corneal degenerative conditions - Major protein of DM is Type IV collagen

27. Hassal-Henle bodies: - It is the peripheral posterior excrescence produced by focal overproduction of basal lamina like material in aging cornea - No clinical abnormality in corneal function

28. Schwalbe’s line : • The peripheral rim of DM is the internal landmark of corneal limbus and also it is the anterior limit of drainage angle, is called Schwalbe ’ line Posterior embryotoxon : Schwalbe’s line may hypertrophied in congenital anomalies and appears as visible shelf on gonioscopy , is called posterior embryotoxon

29. Repair of Descemet’s layer: After traumatic interuption of DM (Path./Mech.) Endothelium spread its cells to resurface the defect Synthesis of fresh basal lamina which is structurally identical to normal descemet’s layer

30. Endothelium: - It is a single layer of hexagonal, cuboidal cells attached posterior aspect of DM - It is mesenchymal in origin - Corneal endothelial cells production is relatively fixed, it is about 500000

31. Endothelial cells density - If cells density falls below 500 cells/mm² corneal oedema devlops and transparency reduced

32. Endothelium is rich in subcellular organeles – - large number of mitochondria, both rough and smooth endoplasmic reticulum, free ribozomes , these reflects that endothelium is extremely active metabolically Nutrition to endothelium: - Endothelium gets its nutrition & O₂ from aqueous - Essential nutrients (such as glucose & amino acids) pass across its surface to supply the cellular needs of all the corneal layers

33. Endothelial Repair: - Physical & chemical damage to endothelium results in loss of cells - Neighboring cells move over to fill the gap by sliding process and enlargement of cells occur ( polymegathism ) - Thus, after injury, the endothelial cell density falls, the cell area increases and the cell height decreases

34. Limbal Stem Cells: Only 5% to 15% of the cells in the limbus are stem cells The basal cells of limbal epithelium comprises the limbal stem cells. They are the precursor for all other cells of the tissue They have a self maintaining population They accounts for only a small portion of total cells of the tissue In vivo,they show slow cycling,but when placed in cell culture,they demonstrate high potential to proliferate. They can not be differentiated with rest of the cells of tissue.

35. Blood supply to cornea: In normal condition, cornea does not contain any blood vessels Anterior ciliary artery , a branch ophthalmic artery forms a vascular arcade in the limbal region and helps in corneal metabolism and wound repair by providing nourishment.  Absence of blood vessel in cornea is one of the contributing factors for its transparency.

36. Nerve supply of Cornea: Density of the nerve ending in cornea is about 300 times of that of skin. The ophthalmic division of the trigeminal nerve has three parts: the frontal nerve, the lacrimal nerve, and the nasociliary nerve. The nasociliary nerve provides sensory innervation to cornea.

37. Corneal Nutrition & Metabolism Cornea requires energy for normal metabolic activities as well as for maintaining transparency and dehydration Energy is generated by the breakdown of glucose in the form of ATP Most actively metabolizing layers are epithelium & endothelium

38. Sources of Nutrients: - Oxygen – mainly from atmosphere through tear film, with minor amounts supplied by the aqueous and limbal vasculature - Normal Po₂ in tears is 155 mm Hg - In aqueous is about 40 mm Hg - Minimum 25 mm Hg Po₂ is needed for maintaining deturgescent state and transparency

39. - Glucose, amino acid, vitamins, and other nutrients supplied to cornea by aqueous humor , a lesser amounts from tears or limbal vessels. - Glucose also derived from glycogen stores in corneal epithelium. - Epithelium consumes O₂ 10 times faster than stroma .

40. Metabolic pathways: Three processes or pathways – 1. Pentose shunt ( Hexose monophosphate shunt) – occurs both In hypoxic and normoxic condition forms NADPH and Pentose (Ribose 5-P) from glucose which are used in nucleic acid synthesis 2. Glycolysis ( Embden meyerhof pathway) – Glucose/glycogen converted to pyruvate yelding 2 ATPs 3. TCA or Krebs or citric acid cycle – in aerobic conditions pyruvate is oxidized to yield 36 ATP, water, CO₂.

41. Corneal Transparency The cornea transmits nearly 100% of the light that enters it. Transparency achieved by – 1. Arrangement of stromal lamellae Two theories – i ) Maurice (1957): The transparency of the stroma is due to the lattice arrangement of collagen fibrils. He explained, because of their small diameter and regularity of separation, back scattered light would be almost completely suppressed by destructive interference

42. ii) Goldman et al. (1968): Proposed that lattice arrangement is not a necessary factor for for stromal transparency . Cornea is transparent because fibrils are small in relation to light and do not interfere with light transmission unless they are larger than one half of a wavelenght of light(2000 A). Both theories failed to explain why there is corneal clouding occurs with raised IOP and why there is corneal clearing occurs on reduction of IOP.

43. Other factors of corneal transparency – 2. Corneal epithelium & tear film • Epithelial non- keratinization • Regular & uniform arrangement of corneal epithelium • Junctions between cells & its compactness and also tear film maintain a homogenicity of its refractive index 3. Relative deturgescence state of normal cornea. 4. Corneal avascularity 5. Non myelenated nerve fibres

44. Factors affecting corneal Hydration: Stromal swelling pressure exerted by GAGs IP = IOP – SP or IP = 17 -60 = -43 Barrier function of epithelium and endothelium Hydration controled by active pump mechanisms of the corneal endothelium The enzyme pump systems are – • Na⁺/K⁺ ATPase pump system • Bicarbnate dependent ATPase • Carbonic anhydrase enzyme • Na⁺/H⁺ pump Evaporation of water from corneal surface Intraocular pressure

45. Drug permeability across the Cornea Factors affecting drug penetration through the cornea are – 1. Lipid and water solubility of the drug 2. Molecular size, weight and concentration of drug 3. Ionic form of the drug 4. pH of the solution 5. Tonicity of the solution 6. Surface active agents 7. Pro-drug form

46. 1. Lipid and water solubility of the drug Drug should be amphipathic,should have both lipid and wter soluble properties. Epithelium – lipophilic Stroma - hydrophilic

47. 2. Molecular size, weight and concentration of the drug lipid soluble molecules can cross the corneal epithelium irrespective of their molecular size,while water molecules with size less than 4A only canfilter through pores in cell membrane. Substances with molecular weight less than 100 can pass &more than 500 can not.

48. 3. Ionic form of the drug Capacity to exist in both ionic and non-ionic forms,becaus only non-ionised drugs can penetrate epithelium and ionised drugs can pass through stroma . Florescein dye test

49. 4. PH of the solution Normal range – 4 to 10 Any solution outside this range increases permeability.

50. 5. Tonicity of the solution Hypotonic solutions increase permeability. Those below 0.9%NaCl.

51. 6. Surface active agents Agents which reduce surface tension,increase corneal wetting and ,therefore ,present more drug for absorption. Eg.Benzalkonium chloride

52. 7. Pro-drug form Pro-drug forms are lipophilic which after absorption through epithelium converted into proper drug which can easily pass through stroma . Eg . dipivefrine - epinephrine

53. Thank you