outer coat 2 Flashcards
what are the metabolic requirements for cornea ?
- source of oxygen
- source of glucose
- source of essential amino acids, vitamins and minerals
- elimination of the waste products of metabolism ( e.g. lactic acid )
what is the oxygen level in anterior eye ?
- when the eyes are open the oxygen tension at the anterior corneal surface is equivalent to that of the atmosphere ( 155 mmHg at sea level )
what is oxygen level at back of eye ?
oxygen tension in the aqueous humour lie in range 20-80 mmHg
what happens to oxygen tension during eye closure ?
- during eye closure the oxygen tension at the corneal surface is equal to that of the palpebral conjunctiva
- the palpebral conjunctiva contains a rich blood supply as vessels lie in close proximity to the epithelium
explain oxygen tension profile across the cornea ?
- when our eyes are open , there is a very high oxygen concentration , oxygen diffuses through cornea and concentration gets progressively less and gets very low at the interface between anterior stroma and endothelium
- goes from 155mmHg at front to 55mmHg at back
- when eyes are closed oxygen level drop significantly
- we only need about 10% oxygen for cornea to maintain its transparency
how oxygen is needed by cornea to avoid corneal oedema ?
- oxygen concentrations of at least 10% are required to avoid oedema
- large inter-subject variability in oxygen requirements exits
what happens if oxygen levels in cornea drop below 10%?
cornea begins to loose transparency
how does oxygen get into contact lens ?
oxygen diffuses through plastic of the lens
what is the difference in corneal thickness during waking hours compared to during sleep hours ?
the cornea is 5% thinner during waking hours than during sleep
what is overnight swelling ?
cornea swells 5% during the night then starts to thin during the day
what is the cause of overnight swelling?
- overnight oedema may be the result of reduced oxygen availability or the result of changes in tear tonicity due to reduced tear evaporation
what is tear tonicity ?
- once our eyes are closed the osmolarity of the tears is going to be slightly higher than it is during the day
what happens to concentration of tears as our eyes open ?
- during the open eye tears evaporate , creating a slight tear hypertonicity and osmolarity of tears increase and subsequent corneal dehydration
what is glucose concentration in the tears?
- low glucose levels compared to the back
- the glucose concentration of the tears is low and insufficient to meet corneal needs .
what is glucose concentration in the aqueous ?
- higher levels of glucose compared to front
- the bulk of glucose required by the cornea is derived from the aqueous ( which also acts as a source of amino acids , vitamins and other metabolites
what serves as glucose source during periods of metabolic stress ?
- intracellular glucose reserves ( in the form of glycogen ) serves as a glucose during periods of metabolic stress
where does inferior part of cornea gets it glucose from ?
- glycogen reserves
- tears
where does posterior part of cornea get its glucose from ?
- aqueous humour
what is the role of limbal vasculature ?
- it is likely that limbal vascular arcades provide oxygen and nutrients to the peripheral cornea only
why does cornea need glucose ?
- glucose is the main energy source for the cornea
what are the three principal pathways that exist for glucose metabolism ?
- anaerobic glycolysis ( releases part of ATP )
- hexose monophosphate shunt ( pentose phosphate pathway )
- aerobic glycolysis ( and krebs cycle ) ( releases all ATP )
explain corneal metabolism ?
- we can get glucose coming in from aqueous humour
- we can get glycogen being broken down from the reserves that are sitting within epithelium
- glucose gets fed into glycolysis then broken down into pyruvate
- the fate of pyruvate depends on whether oxygen is present or oxygen is absent
- absence of oxygen pyruvate is broken down into lactic acid
- presence of oxygen pyruvate enters mitochondria and broken by krebs cycle and thus generating ATP and co2 as bi product
what is pentose phosphate pathway or HMP pathway ?
- takes glucose - 6 - phosphate out of krebs cycle then it passes through a series of five carbon sugar intermediates to then get fed back into the krebs cycle
why does the HMP pathway operate strongly in the cornea ?
- it does two things , it generates NADPH molecule which acts an an electron carrier
- also generates pentose sugars that are needed for nucleic acid synthesis
how does epithelium break down its glucose ?
- epithelium is largely breaking its glucose using anaerobic metabolism as it has no mitochondria
- epithelium is this producing a lot of lactic acid
how does endothelium break down its glucose ?
- endothelium needs to generate a lot of energy utilises aerobic metabolism
- packed with mitochondria
how much lights does the cornea transmit ?
- the cornea is highly transparent and transmits >90% of incident light
what does collagen fibrils act as ?
- acts as scatter source
smaller collagen fibril = less scattered light - if light was coming through and hits collagen fibrils that light would scatter
what is scattered light eliminated by ?
- scattered light reduces clarity
- scattered light is eliminated by the process of destructive interference
what is the structure of collagen fibrils in the cornea ?
- regular arrangment
- small diameter
what is the main requirement for destructive interference ?
- the main requirement for destructive interference is a short-range order of collagen fibrils overt two to three near neighbours
how does destructive interference happens ?
- cornea needed to have regular spacing
- spacing between single fibril and each of its immediate neighbour needs to be similar
what is the concept of transparency of cornea based on ?
- based on fibrils being small diameter and regularly small
2. regularity of spacing between fibrils
what is difference between sclera and cornea ?
- sclera lacks regularity , variable collagen fibres compared to cornea
- the diameter of fibrils in cornea are uniformly small ( 50 nm )
- sclera fibrils have variable diameter which are bigger
what is the significance of the state of corneal hydration ?
- the state of corneal hydration is an important determinant of corneal transparency
- physiological hydration is maintained at 78%
- transparency is maintained at hydration levels +/- 5% of this value
what are the hydrophilic properties of cornea determined by ?
- hydrophilic properties of the cornea determined largely by stromal proteoglycans and monovalent cations ( e.g. chloride )
what is function of corneal proteoglycans ?
- corneal proteoglycans maintain transparency
structure of glycoprotein ?
- core protein
- repeating disaccharide unit
- the sugars are hydrophilic
what are the 4 corneal proteoglycans?
- keratan sulphate proteoglycans
- lumican keratocan
- mimican
- dermatan sulphate
what maintains dehydration ?
- both surfaces of cornea epithelium at front and endothelium at back operate as barriers to prevent water getting in
- endothelium has pumping mechanism which is able to take out excess water
what is pump - leak hypothesis ?
- stromal swelling pressure is the driving force for water to leak across the epithelial and endothelial barrier layers
- the leak is counterbalanced by pump mechanisms which reside in the epithelium and endothelium
- the endothelium accounts for 90% of the pumping activity of the cornea
explain water leakage through tight junctions ?
- although there is tight junctions between adjacent cells , there is still a leak of water able to get through this means that the barrier is slightly leaky
- water moves from aqueous humour side and leaking through the gap between adjacent cells
- if we didn’t have counter mechanism all the water will accumulate and we would lose transparency
- endothelium pumps ions from stromal side into aqueous side and by pumping ions against the concentration gradient thus establishing an osmotic gradient along which the water flows
what is endothelial pump ?
- endothelium actively pumps ions from the stroma to the anterior chamber
- water then flows along the resulting osmotic gradient
- bicarbonate ions is the major substrate for the pump
explain ions pumps involved in regulating the flow of water ?
- bicarbonate is generated through combination f carbon dioxide and water and catalysed by CA
- bicarbonate can also be brought in cells along side sodium ions
what is corneal oedema?
- cornea swells in an anterior - posterior direction
- structural changes in the stroma occur which correlate with the level of oedema
what is corneal oedema?
- cornea swells in an anterior - posterior direction
- structural changes in the stroma occur which correlate with the level of oedema
what causes corneal oedema ?
- retardation of carbon dioxide efflux
- stromal lactate accumulation
- breach of epithelial or endothelial barriers
- inhibition of ion pumps
- influx of water increases the separation between collagen fibrils causing increased scattered light
how does corneal respond to damage ?
- regeneration of the epithelium is dependant on the integrity of the limbus
- proportion of limbal cells act as stem cells and are ultimately responsible for corneal epithelial replacement
where are the corneal epithelial cells ?
- they are in the limbal conjunctiva
- this mens that if you damage stem cells at the limbus or if you have trauma which impacts on the limbus cornea loses its ability to repair it self
what is the function of palisades of vogt ?
palisades of vogt may act as repositories for corneal stem cells
what are the palisade of vogt ?
linear streaks within the limbal conjunctiva
- connective tissue ridges
