Cornea 2b - Tagged PDF

The Cornea: 2 Limbus The Limbus nourishment of peripheral cornea Corneal wound healing Immunosurveillance Contains pathways of aqueous humour outflow Contains circumferential or tangential collagen fibrils Site of surgical incisions Theoretical model showing the directions of preferentially aligned,...

The Cornea: 2 Limbus The Limbus nourishment of peripheral cornea Corneal wound healing Immunosurveillance Contains pathways of aqueous humour outflow Contains circumferential or tangential collagen fibrils Site of surgical incisions Theoretical model showing the directions of preferentially aligned, reinforcing collagen fibrils in the cornea, limbus, and adjacent sclera of human eyes. The sclera Outer coat of eye Contains collagen, proteoglycans and elastic fibres Relatively avascular Thickness: 1mm (posterior) 0.4mm behind muscle tendon insertions Variable fibril diameter Corneal stroma Scleral stroma Corneal nerves Derived from ophthalmic division of trigeminal nerve Branches radiate to anterior cornea from limbus Nerves loose myelin sheaths in cornea Terminal axons very sensitive to pain and temperature Endothelium in human not innervated Corneal touch threshold (CTT) Soft (low CTT) Medium Stiff (high CTT) Sensitivity (s) = 1/CTT S S Nasal Temporal Inferior Superior Corneal Swelling Contains GAGs which create a stromal swelling pressure of about 50mmHg in normal human Donnan effect The Donnan effect - alters distribution of diffusable cations and anions across any membrane where there is a non-diffusable charged species present on one side of the membrane NB Do not need a membrane for a Donnan effect – just a change in phase Creates a swelling pressure Corneal stroma - biological polyelectrolyte gel - bears a net electrical charge at physiological pH. Charge ligands a) charged GAGs radiating from the fibrils b) a charge resulting from the transient binding of free chloride ions to This creates a Donnan-osmotic swelling ClNa+ Na+ ClCl- Cl - Na+ Na+ Cl- Cl - Na+ Cl- Cl- Transparency Data 60 65 70 75 80 85 90 95 above Corneal transparency For transparency Long range order of atoms or molecules - crystalline lattice Short range order Short range: the first- or second-nearest neighbours of an atom or molecule Several theories: Uniform Refractive Index – cornea is transparent because the refractive index (RI) of the collagen fibrils matches the refractive index of the matrix between them. Not true – RI of human corneal collagen = 1.411 and RI of interfibrillar matrix = 1.365 Diffraction Grating –collagen fibrils are arranged in a crystalline lattice with perfectly regular spacing. As fibrils all have the same diameter they act like a three-dimensional diffraction grating whose spacing (interfibrillar centre-to-centre distance) is much less than wavelength of light. Diffraction Grating Not true. X-ray diffraction (Sayers, Elliott, Meek) showed no long-range order Short Range Order–transparency would result from destructive interference of all the scattered light as long as there is sufficient short-range order in the positioning of the uniform diameter collagen fibrils. Only the straight through light would interfere constructively and thus be transmitted by the cornea Destructive interference Constructive interference Destructive interference KEY TO ANIMATIONS Plane wave - black line = peak - grey line = trough Scattered wave – red = peak - grey = trough Calculated Intensities = square of the amplitude Keratocytes: Cytoplasm contains proteins called ‘corneal crystallins’ that give them a uniform refractive index close to that of the rest of the stroma and thus render them transparent. In addition they are thin in the direction of the light path through the stroma. Lens transparency Long fibre cells – lack of nuclei or organelles Hexagonal fibres contain crystallins - liquid like short range order Lens grows continually through out life MAMMALIAN LENS CRYSTALLIN PROTEIN FAMILY ? Alpha Beta Gamma All are globular & have different isoforms COMMON STRUCTURAL PROPERTIES of sHSPs 24 su M. Jannaschii HSP 16.5 Kim et al. Nature 1998 12 su Wheat HSP 16.9 Van Monfort et al. NSB 2001 Optical Density (a. u.) 1.2  Chaperone assay 1 0.8 0.6 0.4 0.2 0 0 10 20 30 40   Time (Mins) Spectrophotometry 56oC, l = 360 nm Transparency – maintained by alpha-crystallin Age related cataract- develops in 75% of people over 65 years of age and in 95% of those over 85 years of age. Cataract symptoms include: decreased illumination, blurred vision Light from the sun or a lamp can appear too bright or glaring. Colours will not appear as bright as they did. Refractive Surgery PRK Photorefractive keratectomy (PRK) Surface flattened to reduce power of eye (to correct for myopia Surface made more curved to increase power of eye (to correct for hyperopia) Corneal flap cut and hinged backwards Central lamellae flattened using excimer laser Corneal flap replaced. Advantages over PRK: Epithelium not removed Therefore less woundhealing response Disadvantage: Lack of sufficient wound healing means flap never heals – cornea mechanically weaker Laser Assisted in Situ Keratomileusis LASIK