Cornea

Cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye.

Cornea forms the transparent and anterior 1/6th of the external fibrous coat of the globe of the eyeball.
The cornea is the eye's most powerful structure for focusing light that provides approximately 65 to 75 percent of the focusing power of the eye.
The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid.

1. Anatomy and physiology of Cornea By Getachew Kassahun Gondar, Ethiopia 1

2. CORNEA Cornea forms the transparent and anterior 1/6th of the external fibrous coat of the globe of the eyeball. It is just like glass cover of a watch which is set on the sclera. It is oval from front and circular from behind. It is the main refracting medium of the eye. 2

3. 3

4. Cornea consists of six layers Epithelium Bowman's membrane Stroma Descemet's membrane Dua's layer Endothelium 4

5. 5

6. 1. Stratified Squamous Epithelium It is composed of 5–6 layers of non keratinised mounted on a basement membrane and continuous with the bulbar conjunctiva at the limbus . i. Superficial cells (or Squamous cells): It is 2–3 cell layered and the cells are polyhedral. Outer surfaces of these cells have projections called microvilli and microplicae . These projections extend into the mucin layer of precorneal tearfilm and help in retaining the tearfilm . ii. Wing cells (or Umbrella cells ): It consists of also 2–3 layered polyhedral cells with their concave base fitting over the apex of the basal cells. These cells send wings or process between the basal cells. 6

7. iii. Basal cells : It is a single layer of columnar cells with flat bases and rounded apices (or head). These are germinal cells and show mitosis. New cells are gradually pushed superficially. iv. Basement membrane : It is formed by secretions from the basal cells. Despite lack of vascularity corneal epithelium is capable of active regeneration after injury or abrasion. 7

8. 2. Bowman’s Membrane It is the modified condensed anterior layers of the corneal stroma,approximately 8 to 14 μm thick. It is acellular and composed of a dense, fibrous sheet of interwoven collagen fibrils randomly arranged in a mucoprotein ground substance. Bowman’s layer is produced prenatally by the epithelium and is not believed to regenerate. Therefore, if injured, the layer usually is replaced by epithelial cells or stromal scar tissue. However , it exhibits strong resistance against infection and injury. It is well-demarcated from the corneal epithelium. 8

9. 3.Stroma ( Substantia propria ) It is the forward continuation of sclera. 500 μm thick, or about 90% of the total corneal thickness. It consists of approximately 200 layers of parallel collagen fibrils, parallel to the surface of the cornea. Two types of cells are found in the stroma . Keratocytes (or Corneal Corpuscles) : They secrete collagen and the ground substance and lie within the collagen lamellae. Wandering leucocytes : They are derived from the limbal vessels. Cornea is the most sensitive structure in the body due to presence of plenty non medullated nerve fibres in the stroma . 9

10. 4.Descemet’s Membrane It is formed by secretion of corneal endothelium. Hence, it is a modified basement membrane of the endothelium. It is produced continually and therefore thickens throughout life, such that it has doubled by age 40 years. It has wart-like elevations at the periphery termed as Hassall- Henle bodies. It terminates peripherally at the Schwalbe’s line. It is strong and capable of regeneration after injury. 10

11. 5.Dua’s layer Dua’s layer is well-defined, acellular and strong, consisting of five to eight lamellae of type-1 collagen bundles total about six to 15 microns thickness. The bundles are coarse and arranged in transverse, longitudinal and oblique directions. Bundle spacing is similar to that in stromal tissue, but Dua’s layer is entirely free of keratocytes in the zone that forms the posterior wall of the bubble. These histological features also distinguish Dua’s layer from the adjacent Descemet’s membrane, which consists of finer, closer spaced, parallel collagen bundles in banded and non- banded layers with endothelial cells. 3/27/2021 11

12. 12

13. 6.Endothelium It is the deepest layer consisting of a mosaic of single layer of hexagonal cells, bound together and continuous with the endothelium of the anterior surface of the iris. Endothelial cells are responsible for maintaining relative dehydration ( deturgence ) of corneal stroma and transparency. Endothelial cells of the cornea can be seen by specular reflection with the slit-lamp biomicroscope . Once damaged, the endothelial cells do not regenerate . At birth the endothelial cell count is 4500 cells/mm2. In the first year of life 25% reduction in cell count occurs. 13

14. COMPOSITION Water 75–80% Electrolytes—Electrolyte level varies in different layers Collagen—It is destroyed by the enzyme collagenase Soluble protein—Albumin glycoprotein Immunoglobulins — IgG , IgA and IgM Glycosaminoglycans (GAG)—It is highly concentrated in the stroma than in the epithelium and endothelium. Glycoprotein . 14

15. CORNEAL INNERVATION The cornea is densely innervated with sensory fibers. Seventy to 80 large nerves, branches of the long and short ciliary nerves, enter the peripheral stroma . Approximately 2 to 3 mm after they pass into the cornea, the nerves lose their myelin sheath, but the covering from the Schwann cell remains. 15

16. Cont… The density of sensory nerve endings in the epithelium is approximately 400 times that of the epidermis of the skin, with approximately 7000 nociceptors per square millimeter in the cornea. Stimulation of the cornea, even just touch, is recognized as pain because of the density of nociceptors . The cornea also recognizes changes in temperature. Contact lens wear over time and aging cause a decrease in sensitivity 16

17. Assessing Corneal Sensitivity can be “measured” clinically by touching the cornea gently with a wisp of cotton from a swab and initiating a blink response. It can be measured quantitatively by using a measuring device, an esthesiometer . A small, fine filament is introduced from the side to just touch the cornea. Because rigidity depends on the length of the filament, the longer the filament (the more flexible the filament.) that initiates a blink, the more sensitive the cornea. 17

18. NUTRITION SOURCE Oxygen from the air via diffusion across tear film Glucose and amino acids from Aqueous humour Perilimbal capillaries Tearfilm . 18

19. METABOLISM Cornea requires energy for renewal of tissues and maintenance of transparency. Energy is derived in the form of ATP Anaerobic glycolysis: This is essentially an anaerobic pathway and accounts for majority of corneal metabolism. Tricarboxylic acid (or TCA/ Kreb’s cycle)— This is essentially an aerobic pathway and results in a net yield of 36 molecules of ATP (greater energy yield). Hexose monophosphate shunt (or Pentose phosphate shunt) plays a significant role in both the epithelium and the endothelium. It gives a net yield of 6 molecules of ATP 19

20. CORNEAL TRANSPARENCY Cornea maintains high level of transparency to transmit more than 90% of incident light. Several factors contribute to the corneal transparency . Epithelial Uniform regular arrangement of epithelium Homogenous refractive index throughout the epithelium Presence of precorneal tearfilm Tight junctions between superficial epithelial cells . 20

21. Stromal Regular crystalline lattice arrangement of corneal collagen fibrils in a mucopolysaccharide ground substance, separated by less than the wavelength of light (Maurice Theory) Absence of blood vessels The diameter of the corneal collagen fibrils are smaller than the wavelength of light and therefore, do not interfere with the light transmission Demyelination of corneal nerves. 21

22. Deturgence (Relative Dehydration) Relative dehydration of stroma which is responsible for maintenance of normal corneal thickness is maintained by following structures: Epithelium offers high resistance to flow of ions and water Endothelium Water imbibes into corneal stroma , but it is driven out by metabolically driven pump, i.e. Na–K ATPase pump of the endothelium. Zonulae occludentes , i.e. focal tight junctions of the adjacent endothelial cells maintain barrier to intercellular fluid traffic. Stromal deturgence Water is driven out of the stroma by negative imbibition pressure of the stroma Glycosaminoglycans (GAG) present in the corneal stroma act as a sponge to suck in water. However, in a normally dehydrated cornea they offer high resistance to inflow of water. 22

23. CORNEAL WOUND HEALING Corneal wound healing is very essential to maintain transparency of cornea for clear vision. Corneal healing process is quite different from other tissues due to avascularity . It also varies according to site, suture material, nature of injury and topical ophthalmic preparations. Epithelial level—Corneal epithelial repair is a complex interaction between the cells and the extracellular matrix. 23

24. Bowman’s membrane level—Bowman’s membrane doesn’t regenerate. Stromal level—Central avascular stromal wound takes longer time to heal than the peripheral wound Descemet’s membrane level— Descemet’s membrane easily regenerate from endothelial cells. It is essentially the basement membrane of the endothelial cells. Endothelial level—Proliferation of endothelial cells by mitosis is restricted only to younger group of peoples. In adults the endothelial cells simply undergo enlargement and sliding to cover the defects. CORNEAL WOUND HEALING Cont… 24

25. CORNEAL BLOOD SUPPLY The cornea is avascular and obtains its nourishment by diffusion from the aqueous humor and from the conjunctival and episcleral capillary networks located in the limbus . Absence of blood vessels is an important factor in corneal transparency and although it is surrounded by conjunctival capillary loops a balance between angiogenic and antiangiogenic factors maintains its avascular state. 25

26. CORNEAL FUNCTION The cornea has two primary functions: to refract light and to transmit light. The cornea transmits light with wavelengths between 310 and 2500 nm. Wave lengths below 300 nm are absorbed by the epithelium and Bowman’s layer and do not penetrate deeper; those between 300 to 320 nm are absorbed by corneal stroma 26

27. Refraction of light Factors that affect the amount of corneal refraction include (1) the curvature of the anterior corneal surface, (2) the change in refractive index from air to cornea (actually the tear film), (3) corneal thickness, (4) the curvature of the posterior corneal surface, and (5) the change in refractive index from cornea to aqueous humor. 27

28. The total refractive power of the eye focused at infinity is between 60D, with 43 D attributable to the cornea. 28

29. References Clinical Anatomy and Physiology of the visual system 3 rd edition Textbook of Visual Science and Clinical Optometry by Bikas bhttacharyya Adler’s physiology of the eye 29

30. Thank you አመሰግናለሁ 30