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Neural Biology 1 > outer coat 2 > Flashcards

outer coat 2 Flashcards

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1
Q

what are the metabolic requirements for cornea ?

A
  • 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 )
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2
Q

what is the oxygen level in anterior eye ?

A
  • 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 )
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3
Q

what is oxygen level at back of eye ?

A

oxygen tension in the aqueous humour lie in range 20-80 mmHg

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4
Q

what happens to oxygen tension during eye closure ?

A
  • 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
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5
Q

explain oxygen tension profile across the cornea ?

A
  • 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
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6
Q

how oxygen is needed by cornea to avoid corneal oedema ?

A
  • oxygen concentrations of at least 10% are required to avoid oedema
  • large inter-subject variability in oxygen requirements exits
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7
Q

what happens if oxygen levels in cornea drop below 10%?

A

cornea begins to loose transparency

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8
Q

how does oxygen get into contact lens ?

A

oxygen diffuses through plastic of the lens

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9
Q

what is the difference in corneal thickness during waking hours compared to during sleep hours ?

A

the cornea is 5% thinner during waking hours than during sleep

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10
Q

what is overnight swelling ?

A

cornea swells 5% during the night then starts to thin during the day

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11
Q

what is the cause of overnight swelling?

A
  • overnight oedema may be the result of reduced oxygen availability or the result of changes in tear tonicity due to reduced tear evaporation
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12
Q

what is tear tonicity ?

A
  • once our eyes are closed the osmolarity of the tears is going to be slightly higher than it is during the day
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13
Q

what happens to concentration of tears as our eyes open ?

A
  • during the open eye tears evaporate , creating a slight tear hypertonicity and osmolarity of tears increase and subsequent corneal dehydration
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14
Q

what is glucose concentration in the tears?

A
  • low glucose levels compared to the back

- the glucose concentration of the tears is low and insufficient to meet corneal needs .

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15
Q

what is glucose concentration in the aqueous ?

A
  • 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
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16
Q

what serves as glucose source during periods of metabolic stress ?

A
  • intracellular glucose reserves ( in the form of glycogen ) serves as a glucose during periods of metabolic stress
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17
Q

where does inferior part of cornea gets it glucose from ?

A
  • glycogen reserves

- tears

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18
Q

where does posterior part of cornea get its glucose from ?

A
  • aqueous humour
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19
Q

what is the role of limbal vasculature ?

A
  • it is likely that limbal vascular arcades provide oxygen and nutrients to the peripheral cornea only
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20
Q

why does cornea need glucose ?

A
  • glucose is the main energy source for the cornea
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21
Q

what are the three principal pathways that exist for glucose metabolism ?

A
  • anaerobic glycolysis ( releases part of ATP )
  • hexose monophosphate shunt ( pentose phosphate pathway )
  • aerobic glycolysis ( and krebs cycle ) ( releases all ATP )
22
Q

explain corneal metabolism ?

A
  • 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
23
Q

what is pentose phosphate pathway or HMP pathway ?

A
  • 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
24
Q

why does the HMP pathway operate strongly in the cornea ?

A
  • 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
25
Q

how does epithelium break down its glucose ?

A
  • epithelium is largely breaking its glucose using anaerobic metabolism as it has no mitochondria
  • epithelium is this producing a lot of lactic acid
26
Q

how does endothelium break down its glucose ?

A
  • endothelium needs to generate a lot of energy utilises aerobic metabolism
  • packed with mitochondria
27
Q

how much lights does the cornea transmit ?

A
  • the cornea is highly transparent and transmits >90% of incident light
28
Q

what does collagen fibrils act as ?

A
  • acts as scatter source
    smaller collagen fibril = less scattered light
  • if light was coming through and hits collagen fibrils that light would scatter
29
Q

what is scattered light eliminated by ?

A
  • scattered light reduces clarity

- scattered light is eliminated by the process of destructive interference

30
Q

what is the structure of collagen fibrils in the cornea ?

A
  • regular arrangment

- small diameter

31
Q

what is the main requirement for destructive interference ?

A
  • the main requirement for destructive interference is a short-range order of collagen fibrils overt two to three near neighbours
32
Q

how does destructive interference happens ?

A
  • cornea needed to have regular spacing

- spacing between single fibril and each of its immediate neighbour needs to be similar

33
Q

what is the concept of transparency of cornea based on ?

A
  1. based on fibrils being small diameter and regularly small

2. regularity of spacing between fibrils

34
Q

what is difference between sclera and cornea ?

A
  • 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
35
Q

what is the significance of the state of corneal hydration ?

A
  • 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
36
Q

what are the hydrophilic properties of cornea determined by ?

A
  • hydrophilic properties of the cornea determined largely by stromal proteoglycans and monovalent cations ( e.g. chloride )
37
Q

what is function of corneal proteoglycans ?

A
  • corneal proteoglycans maintain transparency
38
Q

structure of glycoprotein ?

A
  • core protein
  • repeating disaccharide unit
  • the sugars are hydrophilic
39
Q

what are the 4 corneal proteoglycans?

A
  • keratan sulphate proteoglycans
  • lumican keratocan
  • mimican
  • dermatan sulphate
40
Q

what maintains dehydration ?

A
  • 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
41
Q

what is pump - leak hypothesis ?

A
  • 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
42
Q

explain water leakage through tight junctions ?

A
  • 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
43
Q

what is endothelial pump ?

A
  • 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
44
Q

explain ions pumps involved in regulating the flow of water ?

A
  • 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
45
Q

what is corneal oedema?

A
  • cornea swells in an anterior - posterior direction

- structural changes in the stroma occur which correlate with the level of oedema

46
Q

what is corneal oedema?

A
  • cornea swells in an anterior - posterior direction

- structural changes in the stroma occur which correlate with the level of oedema

47
Q

what causes corneal oedema ?

A
  • 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
48
Q

how does corneal respond to damage ?

A
  • 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
49
Q

where are the corneal epithelial cells ?

A
  • 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
50
Q

what is the function of palisades of vogt ?

A

palisades of vogt may act as repositories for corneal stem cells

51
Q

what are the palisade of vogt ?

A

linear streaks within the limbal conjunctiva

- connective tissue ridges

Neural Biology 1 (58 decks)

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