corneal physiology ‫‬

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  • 1.Corneal physiologyOPTO : Ihsan Hmaid

2. Cornea :Cornea characteristics 1. 2. 3. 4. 5. 6. 7.Forms the anterior 1/6 of the outer tunic . Completely transparent. Avascular. Ellipsoid and its vertical diameter is 11.5mm,and Its horizontal diameter is 12mm. It is more thick in periphery 0.67mm than in center 0.52mm It has smooth anterior and posterior surfaces . Its radius of curvature is 7.8 mm in anterior surface but 7mm in posterior surface. 3. Functions of the cornea It is considered the main-1 .refractive surface of the eye .Its power about 42 diopters Protection of the intraocular-2 .structures of the eyes Passage of images of objects to-3 .the retina 4. Factors affecting the corneal function . A - Transparency .B - Regularity of curvature of its surface .C - Smoothness of corneal surfaces 5. The optical properties of the human cornea Two major developments were made because current measurement techniques need improvement First, the VU topographer, which uses a color coded pattern, was validated with real eye data showing better performance compare to commercial ring topographers particularly in 6. For example, ring topographers underestimate astigmatism of the anterior corneal surface by 4%. This underestimation increases with complexity of the surface. The astigmatism underestimation was found to be 13% for a post radially-keratotomized cornea. Second, the aberration contribution of the posterior surface was revealed using Scheimpflug photography. Results show that the contribution of the posterior surface to corneal coma aberration is 7. On the other hand, on average the posterior surface decreases corneal astigmatism by 31%. Also the contribution of the posterior surface to the spherical aberration of the cornea increase with age reaching up to 15% at age 65. Thus, measurement of the posterior surface is necessary to specify corneal astigmatism and spherical aberration accurately. The methods introduced in this study are useful for applications in laser refractive surgery, contact lens fitting and studies on wave aberration of the eye because it reveals the optical properties of .the cornea more accurately 8. Corneal stroma 9. stromal hydration and its regulation Stromal hydration quantifies the water component of the stroma. Its regularization depends on different factors which were detailed as; swelling pressure, corneal endothelial and epithelial metabolic pumps and barriers, tear film evaporation and intraocular pressure. Finally, the authors present different clinical procedures for evaluating stromal hydration, such as, the fluorophotometry and hypoxic-stress. This study shows the 10. Corneal keratocytes (corneal (fibroblasts are specialized fibroblasts residing in the stroma. This corneal layer, representing about 85-90% of corneal thickness, is built up from highly regular collagenous lamellae and extracellular matrix components. Keratocytes play the major role in keeping it transparent, healing its wounds, and synthesizing its components. In the unperturbed cornea keratocytes stay dormant, coming into action after any . kind of injury or inflammation 11. Some keratocytes underlying the site of injury, even a light one, undergo apoptosis immediately after the injury. Any glitch in the precisely orchestrated process of healing may cloud the cornea, while excessive keratocyte apoptosis may be a part of the pathological process in the degenerative corneal disorders such as keratoconus, and these considerations prompt the ongoing research into the .function of these cells 12. Origin and functions Keratocytes are developmentally derived from the cranial population of neural crest cells, from whence they migrate to settle in the mesenchyme. In some species the migration from neural crest comes in two waves, with the first giving birth to the corneal epithelium and the second invading the epithelium-secreted stromal ;anlage devoid of cells 13. in other species both populations come from a single wave of migration. Once settled in the stroma, keratocytes start synthesizing collagen molecules of different types (I, V, VI) and keratan sulfate. By the moment of eye opening after birth the proliferation of keratocytes is all but finished and most of them are in the quiescent . state 14. By the end of eye development an interconnected keratocyte network is established in the cornea, with dendrites of neighbouring cells contacting each over. Quiescent keratocytes synthesize the so-called crystallins, known primarily for their role in the lens. Corneal crystallins, like the lens ones, are thought to help maintain the transparency and 15. They are also part of corneal antioxidant defense. Crystallins expressed by human keratocytes are ALDH1A1, ALDH3A1, ALDH2 TKT. Different sets of crystallins are typical to distinct species. Keratan sulfate produced by keratocytes is thought to help maintain optimal corneal hydration; genetic disruption of its synthesis leads to the macular 16. Corneal Transparency The cornea is highly transparent tissue with less 1% of light being scattered within it. The cornea transparency is maintained by two essential factors the physical characteristics of the cornea and controlled hydration also there are other factors are important .in corneal transparency 17. Stromal structure The stroma is consist on based of lattice theory which postulated the stroma consist from collagen fibrils with small diameter and equal in diameter and the proteoglycans occupy the space between the collagen and keep the collagen at constant distance from each other, the separation between collagen fibrils is lees than one half on wavelength of light so that the scatter light will elimination by destructive interference in all direction of light except the .one direction (the direction of incident light) 18. Controlled hydration It is the second factors important in .determined the corneal transparency The corneal hydration is controlled by to layers corneal epithelium and endothelium, both of these layers possesses barrier function prosperity and .metabolic function pumping Not: hydration propriety of stromal is determined by proteoglycans which contributes fixed negative charge of 19. The physiological hydration of cornea is maintained almost 78% if the cornea allowed to _+ 5% swell of this value it is being to scatter significant quantities of light. The endothelium barrier to free passage of molecule from aqueous is formed focal tight junction between the adjacent endothelium cells, however in contrast to barrier of endothelium the endothelium is lower resistance to electronic ions and small molecules, this leaky is offset by metabolic pumping of ions out the 20. Also the epithelium contributes corneal hydration control; it is act as barrier effect on ions such as NA, CL however in contrast to barrier of endothelium is lower resistance to water diffusion. When the cornea swells by water the light scattered increase with ensued transparency loss due to disruption of regular .collagen fibrils 21. The other factors maintain corneal :transparency.Corneal an avascular-1 Unmyelination of corneal nerve-2 .fibers .Degeneration of epithelium-3 integrity of this layer and all layers of ((cornea Higher difference between refractive-4 .index of cornea and air 22. Corneal Metabolism It is the series of chemical reaction that place in living tissue. Where the constant metabolic activity in cornea is necessary to maintain on transparency, temperature ((and hydration of cornea 78% The metabolic occur in epithelium and endothelium, the main substance for this metabolic is glucose, oxygen, amino acid . and vitamin 23. Due to an avascular of cornea promotes to alternative routes of metabolic supply there are three possibilities one from aqueous second from atmosphere via tear .film third from perilimbal blood vessels The oxygen is mainly derived from atmosphere via tear film, in under steadystate the assume tear are saturated with oxygen and therefore the tension of oxygen crosspending to atmosphere is 155 mm hg at sea level in the open eye and when eye closed the tension is about 55 mm hg in this state the oxygen supply 24. The consumption rates of oxygen for layers of the cornea are not equals is as follog 40, 39, 21 epithelium, .stroma, endothelium respectively The glucose is derived from aqueous; the cornea derives the energy from the oxidative breakdown of carbohydrates and the glucose is primary substrate for generation of adenosine triphosphate is catabolized .by two metabolic pathways as follog 25. Glycilytic ( Embden Meyerhofpathway) followed by Krebs .tricarboxylic or citric acid cycle Hexose monophosphate (pentose)- . phosphate The first step in glucose is phosphorlyation into glucose-6:phosphate 26. :E mbden M eyerhof pathway - 1 This the major one which account for about 85%, in this stage the enzymes called dehydrogenises act as catalysts for each in this process, in this process the glucose molecule is split into two molecules of pyruvic acid, in third of four stages of glycolytic process liberated energy is used to form two molecules of ATP from ADP and inorganic phosphate. If occur under aerobic condition six additional 27. while under anaerobic condition two molecules are only produce, in this state the pyruvic acid is convert into lactic acid without any significant energy where the lactic acid is build up in stroma and sufficient process is created to allow to water to drawn into stroma faster than endothelium pumping so that can occur stromal edema, this because the little energy 28. In aerobic condition of glycolytic doesnt stop in stage of produce lactic acid but continues until the final products are carbon dioxide and water this called Krebs tricarboxylic acid cycle or citric acid during this cycle the carbon dioxide and hydrogen atoms are released the hydrogen atom at length become oxidative to form water and total oxidative process synthesize further .30 ATP molecules 29. :H e