-- card: 4671 from stack: in -- bmap block id: 4870 -- flags: 0000 -- background id: 2278 -- name: Cornea -- part 1 (button) -- low flags: 00 -- high flags: 2000 -- rect: left=330 top=324 right=336 bottom=342 -- title width / last selected line: 0 -- icon id / first selected line: 0 / 0 -- text alignment: 1 -- font id: 0 -- text size: 12 -- style flags: 0 -- line height: 16 -- part name: 1 ----- HyperTalk script ----- on mouseUp visual iris close go to card id 13434 end mouseUp -- part contents for background part 1 ----- text ----- CORNEA: I. Dimensions: Partial sphere or ellipsoid. A. Radius = 7.8 mm. (flatter at periphery, more curved at apex) B. Horizontal = 12 mm. Vertical = 11 mm. 1. Megalocornea is a vertical measurement of > 12 mm. 2. Microcornea is a vertical measurement of < 10 mm. 3. Astigmatism is when the different meridians have a different refractive power, a shorter radius means a steeper curvature. a. With the rule astigmatism: Greater refraction in the vertical, (axis 90). b. No astigmatism: vertical refraction = horizontal. c. Against the rule astigmatism: Greater refraction in the horizontal, (axis 180). d. Oblique astigmatism: Greatest refraction in a meridian other than the vertical or horizontal. C. Thickness: .52 - .57 mm. 1. Edema: Thicker due to water absorption. 2. Keratoconus: Thinner due to uneven endothelial cells. II. Functions of the cornea: A. Refracts light: About 43 diopters, lens refracts about 15 - 20 diopters, totaling about 60 diopters for whole eye. B. Transmits light: 1. Factors in transparency: a. Regular arrangement of stromal collagen fibers. If disturbed a scar or opacity is formed. b. The degree of hydration is 75% - 80% water. c. No blood vessels are present. d. Regularity and smoothness of epithelium. III. Types of intercellular junctions: A. Zonula Occludens (ZO): Occluded intercellular space, cell membranes are touching, tight junctions, water and small molecules must move through cells, cannot move between. B. Zonula Adherens (ZA): Cellular space and cell membranes maintained. C. Macula Adherens, Desmosomal type (MA-DESM): Look like spot welds, intercellular cement, tonofilaments from each side attached to electron dense plate between cell membranes. D. Macula Adherens (MA): Fine fibrils, button-like, intercellular space maintained. E. Macula Occludens (MO): Adjacent cell membranes fused, tight junctions F. Hemi-Desmosome (HEMI-DESM): Fibrils attach to basement membrane, one sided. G. Basement membrane: Filaments continue into underlying connective tissue, has electron dense layer of attachment of fibrils from HEMI-DESM. IV. Histology of the Cornea (6 layers): A. Corneal epithelium: 1. Continuous at limbus with conjunctival epithelium. 2. Must be smooth and aligned for best vision. 3. Stratified squamous 5-6 cells thick. a. Basal: Most posterior, single layer, columnar, constant miosis, push off surface cells, connected to basement membrane by HEMI-DESM junctions. b. Wing cells: 2-3 layers, polyhedral. c. Surface cells: 1-2 layers, unkeratinized squamous, constantly slough off into tears, joined by ZO junctions. 1. They contain microvillae and microplicae (fingers projecting out into tears), covered with mucus which acts as a wetting agent. 4. Regeneration of corneal epithelium: a. Takes 24-48 hours. b. Utilizes migration and mitosis. B. Basement membrane: Very thin. 1. Is secreted by basal epithelial cells. 2. Has fine fibrils in muco-protein matrix. 3. Repair of damage takes 2-3 months and may not align properly. 4. HEMI-DESM from epithelium go through basement membrane into Bowman's. 5. Poor attachment of HEMI-DESM results in corneal erosion. C. Bowman's layer: 1. Acellular modified stroma, gradual transition into stroma. 2. Irregularly dispersed collagen fibers. 3. Acellular, therefore, it will not regenerate, damaged will be filled in with stroma or epithelium. D. Stroma (Substansia propria): 1. Constitutes 90% of corneal thickness. 2. 200+ lamellae parallel to the surface. a. Each lamellae is a band of uniformly straight collagen fibers. b. Adjacent lamellae are at angles to each other. c. The lamellae run from limbus to limbus. d. Each fiber has an MPS sheath. 1) MPS: chondroitin, chondroitin sulfate, keratin sulfate. 2) MPS controls arrangement of fibers. 3) MPS controls hydration, acts as an an-ion (-), which combines water and cat-ions (+). 3. Cells of the stroma: a. Fibroblast (keratocyte): 1) Produces collagen. 2) Have many processes which are sandwiched between lamellae. b. Wandering WBC: Are between lamellae. 4. Repair of damage: a. Fibroblasts will divide if collagen demand becomes great. b. Fibroblasts will also produce new collagen, usually irregular and forms a scar or opacity. E. Descemet's layer: 1. Acellular. 2. Secreted by the endothelial cells. 3. Acts as a basement membrane for endothelium, it is not bonded well at either side and separates easily. 4. Collagen fiber layers are parallel to the surface. 5. Schwalbe's line: Collagen ring at limbus, keeps Descemet's from curling up. 6. Thickens with age. a. At birth it is the same thickness as endothelium. b. Adult thickness is 2-3 times the endothelium. 7. It can be regenerated if damaged. 8. Hassal Henle warts: Areas of Descemet's that has thickened, in the periphery. F. Endothelium: 1. A single layer of hexagonal squamous cells. 2. Microvillae project into anterior chamber. 3. ZO junctions: a. Not perfect, but keep water out. b. Cells contain metabolic pumps that remove water and waste. 4. Absorbs metabolants for aqueous. 5. If damaged the adjacent cells spread out to cover, but this leaves a thin are which is more susceptible to further damage. (See diagram 1) V. Corneal metabolism: A. Oxygen: 1. 90% of the corneal oxygen is used in the epithelium, which makes sense, because 90% of all corneal cells are found here. 2. Oxygen source for the epithelium is the tears. 3. Oxygen source for the endothelium is the aqueous. B. Glucose: Obtained from the aqueous, diffuses through the stroma to the epithelium. C. Metabolic pathways: 1. Anaerobic glycolysis: Lactic acid. 2. Kreb's cycle: Preferred route. 3. Hexose monophosphate shunt: Produces 5C sugars for DNA/RNA. VI. Corneal innervation: A. Fifth (V) cranial nerve (Trigeminal): Sensory for face and eyes. B. Nerve path: Trigeminal-long ciliary nerves-they enter stroma at limbus, where myelin sheath is lost- they branch and turn 90 degrees through Bowman's- they lose their Schwann cell sheath and end in posterior 3 layers of epithelium (basal and wing cell layers).