Topic 2: Cornea Flashcards
State cornea properties
Total thickness : ~555 microns
(central is steeper and thinner than periphery)
(anterior has greater radius of curvature than the posterior)
State the corneal layers and its features
epithelium: anterior most, 5-7 layers, can regenerate
bowman’s membrane: epithelial basement membrane, cannot regenerate
stroma: lattice arrangement of collagen fibres, cannot regenerate
descemet’s membrane: endothelial basement membrane
endothelium: posterior most, single layer of hexagonal cells, cannot regenerate
describe the epithelium
anterior most
10% of cornea thickness: 50 microns
in contact with mucus layer of TF
superficial cells (tight, gap, desmosomes) 2 layers, non-keratinised, flattened, polygonal
wing cells (gap, desmosomes) 2-3 layers, irregularly shaped
basal cells (gap, hemi/desmosomes) 1 layer, capable of mitosis/regeneration, columnar shaped
high metabolic activity for regeneration
describe the stroma
90% of cornea: 500 microns
lattice arrangement of collagen fibres -> destructive interference, perpendicular/criss cross to keratocytes -> undeviated light
keratocytes: flattened, stellate in shape
cannot regenerate
describe the endothelium
posterior most
5% cornea thickness: 5 microns
endo cells (gap, tight) 1 layer, hexagonal
cannot regenerate
high metabolic activity for pump function
explain maurice’s theory
stroma collagen fibres are EQUIDISTANT from eo and of equal diameter
the orderly arrangement causes destructive interference which minimises light scattering
allowing maximum light transmission
state corneal function and its factors
to transmit and focus light onto retina (through transparency)
(1) stroma lattice arrangement
(2) deturgescence (78%)
(3) avascularity, acellularity, lack of pigments (10% epi: minimal cells, 90%: no BVs/pigments)
state factors for corneal deturgescence
epi/endo barrier function, metabolic pump, loss in osmotic equilibrium, intra-ocular pressure
describe barrier function of epithelium and endothelium
zonula occludens (EPI) tight junction that allows minimal tear film to enter
macula occludens (ENDO) tight junction that does not completely encircle the cell -> forming leaky barrier allowing AQH to enter stroma
water content compromised during pleomorphism/polymegathism of endothelium cells
describe metabolic pump
transportation of ions through cells, hence water follows movement
water content maintained when leak=pump (2000-3000 cells/mm^2)
water content compromised during endothelium decompensation/ leak > pump (<500 cells/mm^2)
describe loss of osmotic equilibrium
CL wearers most susceptible: sleeping with CL on, over worn, tight fitted
eye faces hypoxia -> anaerobic metabolism increase -> increased lactate ions -> reduced pH (osmotic balance disrupted) -> endo pump abnormal function -> AQH freely enters stroma -> corneal/stroma oedema -> lattice arrangement lost -> light scattering increase -> blur vision
describe intra-ocular pressure
sudden increase in IOP (>50mmHg)
abnormal endo pump function -> corneal/stroma swelling -> lattice arrangement lost -> light scattering increase -> blurred vision
describe corneal metabolism & state pathways
energy for cell renewal (epi regeneration), transport processes (endo pump), wound healing, maintaining ocular temperature
(1) anaerobic glycotic pathway
(2) aerobic kreb’s cycle
(3) aerobic hexose monophosphate shunt
explain corneal wound healing
epi: starts with migration of superficial and wing cells to abraded area, followed by mitosis (by basal cells) to renew cells (1 week process)
endo: cell enlargement, no regeneration
describe factors affecting corneal wound healing
(1) size: smaller heals faster
(2) depth: superficial wounds heal faster than deeper wounds as epithelium can regenerate whereas injuries to stroma cannot
(3) location: periphery heals faster than central because corneal limbus at the periphery has blood vessels
(4) infection: slows healing because repair mechanism has to control infection first
(5) medication: steroids speed recovery process, anaesthesia slows wound healing
