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Ocular Biology > Physio and Biochem of Cornea > Flashcards

Physio and Biochem of Cornea Flashcards

1
Q

Functions of the cornea

A

major refractive surface of the eye

biodefense system

protect intraocular contencts

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2
Q

Functions of the cornea require what characteristics?

A

smooth anterior surface - glycocalyx and mucin layer of tears

transparency - arrangement of collagen in stroma, relative dehydration

normal thickness

intact epithelium and endothelium - cell jxn

normal corneal contour - collagen

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3
Q

epithelium cell replacement equilibrium

A

cell division, cell maturation, and cell death

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4
Q

epithelium cell maintenance

A

complete turnover 7-10 days (involution, apoptosis, desquamation)

limbal stem cells help replenish/divide/provide new basal cells for epithelium of cornea

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5
Q

limbal stem cells

A

located in basal cell layer of limbal epithelium

lifetime capacity for self-renewal

palisades of vogt - niche microenvironment that house the limbal stem cells
-less susceptible to UV damage

stem cells have asymmetric division

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6
Q

transformation of daughter cells and then differentiate to:

A

transient amplifying cells (basal epithelial cell)

  • centripetal migration
  • finite rounds of mitosis

transient amplifying cells differentiate to 1) wing cell (postmitotic) 2) superficial squamous cell (differentiated)

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7
Q

epithelium smooth surface by:

A

glycocalyx: glycoprotein, glycolipid, carbohydrate molecules, and MAM embedded in plasma membrane of superficial epithelial cells

MAMs (membrane anchored mucins): produced by superficial epithelial cells that are anchored to apical surface

optically important for smooth surface -> allows hydrophilic spreading of tear film

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8
Q

epithelium barrier

A

zona occludens - tight jxns

restrict paracellular mvmt of ion and water soluble molecules

more resistant than endothelium

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9
Q

bowman’s layer - normal maintanence and fxn

A

anchoring fibrils: type 7 collagen

stabilizes association between surface epithelium, BM and underlying stroma

anterior limiting lamina -> provides shear strength and barrier

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10
Q

stroma - normal maintenance and fxn

A

transparency

mechanical strength: arrangement of collagen bundles to tighter cohesive strength

corneal curvature: refraction - rigidity maintain corneal curvature

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11
Q

cells of the stroma

A

keratocytes: quiescent cell (fibroblasts), gap jxn - fxnl synctium, synthasize procollagen and corneal crystallins (for transparency)

also in stroma: lymphocytes, neutrophils, plasma cells, macrophages

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12
Q

Stroma - Collagen

anterior 1/3

A

thin, less regular obliquely arrange lamellae

interweave and rigidity contribute to corneal curvature

resist change to stromal hydration

shear resistance

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13
Q

stroma - collagen

posterior 2/3

A

thicker, arranged at approximate right angles

can develop folds with increased hydration

contributes to tensile trength

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14
Q

stroma collagen: role in transparency

A

fibrils in parallel orientation lamellae extend limbus to limbus

fibrils small diameter (23nm)

close packing and uniform distance between fibrils (41.4 +- 0.5 nm) -> ensures min. light scattering

lamella ensure uniform distribution of tensile strength radially in all directions

each fibril surrounded by proteoglycans -> GAGs neg charged

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15
Q

stroma - non collagenous proteins: proteoglycans

A

proteoglycans - ground substance
-core protein with GAG side chain negatively charged (chondroitin/dermatan sulfate, keratan sulfate, heparan sulfate)

  • GAGs contribute to tendency of stroma to swell
  • anionic group repel
  • colloid osmotic pressure (cation pull in water)

fxn:
1) provide tissue volume
2) surround collagen fibrils to create uniform spacing between fibrils
3) act as pressure extering polyelectrolyte gel -> contribute viscoelastic properties to cornea
- ocular response analyzer give corneal hysteresis which shows viscoelstic tendencies

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16
Q

Stroma -non collagenous proteins: keratocytes

A 
Corneal crystallins:
aldehyde dehydrogenase (ALDH3A1)

contribute to cellular transparency
-reduce light scattering in cornea

also antioxidant to prevent UV-induced damage

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17
Q

Stroma content - relative dehydration

A

water 78%
collagen 15%
other 7%

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18
Q

Endothelium normal maintenance and fxn

A

single layer

metabolically active

focal or leaky tight jxn to allow paracellular transport (of nutrients)

help maintain relatively dehydrated state and transparency

synthesize descemet’s membrain

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19
Q

endothelial pump

A

pump-leak hypothesis:

rate of leakage of water and solutes into stroma is balanced by transport of ions out of the stroma -> osmotic gradient generated

equilibrium maintained if 1) stroma remains relatively dehydrated and 2)cornea remains transparent

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20
Q

Corneal fxn summary

A

1) major refractive surface of eye
2) biodefense - intact barriers
3) protect intraocular contents

epithelium - smooth optical surface, tight jxn, cell replacement

bowman’s layer - barrier, resists shearing damage

stroma - transmits light, transparent, contour, resists shearing damage and provides tensile strength,

endothelium - regulates hydration and thickness, provides nutrients

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21
Q

Corneal transparency requirements

A

smooth optical surface

uniform and regular arrangement of epithelium -homogeneity of refractive index

arrangement of collagen fibrils (small diameter, uniform diameter and spacing)

corneal crystallins

corneal deturgescence (regulate hydration, relative dehydration)

absence of vasculature

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22
Q

Corneal transparency theory

A

Benedek’s theory

collagen fibrils are: uniformly small diameter (22.5 - 35 nm), closely spaced together (45-55 nm), quasi-random arrangement

when distance between scattering structures is small, less than 1/2 wavelength of light (200nm) -> media transparent

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23
Q

stromal hydration (swelling) and transparency

A

cornea transparent when relatively dehydrated (78% water)

GAGs surround collagen fibrils

  • adv: transparency
  • risk: swelling due to neg. charge attract Na and absorb water

can swell up to 98% water

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24
Q

factors that contribute to stromal hydration (swelling)

A 
Swelling pressure (SP) - stroma wants to swell, electrostatic repulsion between neg. charges of GAGS creates force to expand tissue and draw fluid in
- typically 55 mmHg at normal

imbibition pressure (IP) - neg. pressure drawing fluid into cornea normally ~40 mmHg

intraocular pressure (IOP) - if withing normal range, no effect

  • glaucoma w/ elevated IOP have higher incidence of corneal edema
  • endothelial dysfxn can cause swelling

damage to endothelial layer results in more swelling due to IOP than epithelium

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25
Q

Necessary prerequisites to maintain relatively dehydrated state

A

1) tear film hypertonicity
-open eyes - hypertonic tears
-closed eyes - isotonic tears
(waking up -> symptomatic)

2) intact cellular barriers

3) active metabolic pumps
- epithelial pump: electrical imbalance, osmotic gradient, water follows Cl out to tears through aquaporins

  • endothelial pump: Na/K ATPase pump, bicarbonate-dependent ATPase, aquaporin 1
  • -water follows net transport of Na and HCO3 from stroma to aqueous by active transport
  • -establish osmotic gradient
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26
Q

corneal transparency summary

A

collagen fibrils (uniformly small diameter, closely spaced, quasi-random arrangement)

crystallins - cells

relative dehydration - hypertonic tears, intact cell barriers - aquaporins, metabolic pumps -endothelium (na and HCO3) and epitheliam (Cl)

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27
Q

Clinical Edema

A

edema is swelling

minor if <5%:

  • little effect on refractive, transparency, and mechanical fxn
  • increases during sleep due to reduced oxygen levels and decreased evaporative loss

major if >5%: can cause light scattering and loss of transparency

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28
Q

corneal swelling - epithelia

A

extracellular - water vacuoles form b/n cells

intracellular - fluid in cell, scatters light

decreases VA, causes halos and pain

29
Q

corneal swelling - stroma

A

GAGs retain water - collagen fibrils do not change diameter

causes irregular spacing -> scattered light and loss of transparency

accumulates in post 2/3 vs. ant 1/3 -> swells in posterior direction
-folds in decemet’s membrane

30
Q

general metabolism

A

energy required: transparency, mitosis, cell migration

unique environment: temperature, avascularity

31
Q

source of nutrients

A

tears: oxygen to epithelium (155 open, 55 mmHg closed)
aqueous: oxygen (40 mmHg), glucose (stored as glycogen in basal epithelium), amino acids, vitamins

limbal blood supply (not much): negligible oxygen and glucose, typically only peripheral cornea

32
Q

carbohydrate metabolism

A

glycolysis:

epithelium takes most glucose from stroma (80% metabolized to pyruvate and converted to lactic acid; metabolism by PPS -> impt for cell turnover and generation of reducing agents)

endothelium metabolically active -> lactate can accumulate

33
Q

hypoxic condition

A

glycolysis leading to accumulation of lactate

34
Q

protein metabolism

A

epithelium has highest rate of protein synthesis

proteins of cornea:

1) epithelium - keratin, actin
2) stroma - collagen, proteoglycans
3) ECM - laminin, fibronectin
4) glutathione

35
Q

Vitamin A

A

maintenance of ocular surface

retinoic acid - active maintains non-kertinaized epithelium

36
Q

vitamin a deficiency

A

nyctalopia (night blindness) - initial and most common ocular manifestation of vit. A deficiency

squamous metaplasia - kertinization of epithelium

mucin formaction affected

xerophtalmia (blindness in children)

37
Q

corneal wound healing - what are important?

A

cytokines, growth factors, and proteases together regulate the healing process involving cell death, migration, proliferation, differentiation, and ECM remodeling

basement membrane controls bidirectional movement of cytokines and growth factors b/n epithelium and stroma

sensory nerve endings release neuropeptides -> essential for corneal homeostasis and wound healing

38
Q

Wound healing - epithelium

A

equilibrium of cell division, maturation, and death

39
Q

wound healing - epithelium phase 1

A

latency (4-6) hours

transformation to facilitate migration:

  • metabolic activity increased and cell structure reorganized
  • damaged epithelial shed after apoptosis -> release cytokines (IL-1 and TNF-alpha) causing keratocyte apoptosis in superficial stroma and immune cell infiltration to remove debris

mitosis CEASED for 24 hours

cell jxn lost at edge of wound -> initated by release of MMPs

basal cell at wound margin flattin and lose microvilli

growth factors stimulate production:

1) fibronectin -> laid down on denuded area (red carpet), acts as provisional matrix
2) integrins -> receptors for ECM proteins

40
Q

wound healing - epithelium phase 2

A

Migration (24-36 hrs)
-triggered by cytokines and growth factors

  • flattened basal cells move to cover wound with monolayer of cells -> bind to fibronectin matrix
  • > pulls sheet of cells along fibronectin carpet step by step
  • cell movement is energy dependent requiring protein and glycogen synthesis and Ca++
  • single layer of cells cover wound -fill in
  • no cell mitosis in or around the wound area until wound covered
41
Q

wound healing - epithlium phase 3

A

proliferation (24-30 hrs)

limbal stem cells proliferation increasingly

basal cells differentiate into wing and squamous cells

  • zonula occludens reform in superficial squamous layer
  • epithelium regains normal thicknesss

BM remodeled by secretion of laminin that can attach to integrins

42
Q

wound healing - epithelium phase 4

A

attachment

-reestablishes firm attachment of basal cells to BM and stroma

if BM not damaged, can occur in matter of days

43
Q

limbal stem cell deficency

A

if destruction of stem cells -> conjunctivalization of cornea => vascularization and goblet cells

44
Q

wound healing - epithelium summary

A

cell migration - sliding
cell proliferation - basal layer
cell differentation - normal structure restored

regulated by interaction b/n epithelial cells and extracellular matrix (BM)

  • Type 4 collagen, laminin and fibronectin: regulate adhesion and migration
  • growth factors and cytokines: stimulate growth, proliferation, migration, differentiation, adhesions, ECM despotision
45
Q

damage to epithelium and basement membrane

A

regenerate basement membrane

form adhesion complexes: 4-6 weeks to months

can be susceptible to recurrent corneal erosion

basement membrane important to sequeseter cytokine and form stabel adhesion complex

46
Q

wound healing - bowman’s layer

A

acellular

no regeneration

collagen scar (collage from stromal keratocytes filled in)

47
Q

Wound healing - epithelial stroma interaction

A

epithelium interacts chemically with keratocytes via cytokines to mediate stromal healing

IL-1: master regulator of corneal wound healing

  • released injured epithelial cells => keratocyte apoptosis
  • modulate MMPs
  • modulate growth factors
  • prompt infammatory response
48
Q

wound healing - stroma

destructive phase

A

removal of abnormal tissue

release chemokins that attract immune cells from limbal blood supply and tears

neutrophils as part of innate inflammatory response
-> release proteolytic enzymes and MMP to digest necrotic tissue

macrophage come later

platelet necessary for re-epithelialization

49
Q

wound healing - stroma syntehtic phase

A

residual quiescent keratocytes adjacent to wound are activated

activated keratocytes repopulate wound site
-proliferation and migration

keratocyte migration is proposed to occur when epithelium has resurfaced the defect

subpopulation of activated keratocytes transformed by TGF-beta and PDGF into myofibroblasts

myofibroblasts:

  • motile contractile cells
  • deposit provisional ECM
  • generate contractile forces to close wound
  • remodel wound ECM (actin synthesis for motility, new collagen synthesis after debris cleared)
50
Q

wound healing - stroma with excessive myofibroblast activity

A

contraction of corneal wound may cause irregular corneal astigmatism

repair tissue opacity

51
Q

wound healing - stroma remodeling phase

A

ECM slowly restructured - weeks to years

regulated by TGf-beta and proteases

myofibroblast disappear

keratocytes become quiescent

52
Q

wound healing stroma

summary

A

cellular interactions during corneal repair

upon corneal epithelial injury, IL-1 is released from injured epithelium into stroma

IL-1 induces some of the underlying stromal keratocytes to undergo cell death, while others induced to proliferate, secrete MMPs, and transition form quiescent to an activated phenotype

due to absence of basement membrane, corneal epithelial cells also secrete TGF-beta2 into underlying stroma inducing a subpopulation of keratocytes to undergo transformation into myofibroblasts that secrete ECM

the return of the basement membrane inhibits the release of TGF-beta2 into stroma and myofibroblast phenotyp no longer observed

the acitvated keratocyte continue to secrete IL1 and remodel the ECM

53
Q

wound healing - corneal nerves

A

nerves regenerate from uninjured peripheral nerve trunks

rate of recovery = slow af

denervated cornea risk for epithelial defects

54
Q

wound healing - descemet’s membrane

A

limited elastic properties

anterior banded portion - at birth

posterior portion - thorughout life

syntehsized collagen with less regular arrangement of collagen

appears as a scar/opacity

55
Q

wound healing - endothelium

A

no mitosis in adult

defect covered by enlargement and spreading of neighboring cells - polymegathism, pleomorphism

56
Q

Cornea wound healing summary

A

Epithelial cells: respond immediately to reestablish barrier fxn and protect underlying tissue from microbial organisms and dehydration

bowman’s membrane: no mechanism

stroma: corneal keratocytes (fibroblast) produce new reparative collagen and proteoglycans. repair tissue not transparent

descemet’s membrane: eventually resecreted by endothelial cells

endothelial cells: migrated and spread to cover defects - no mitosis

57
Q

Photodamage

A

can occur with UV-a, UV-b, UV-c

mostly filtered by cornea

UV, violet and blue light trigger can trigger ROS by oxidative stress

58
Q

antioxidant defense mechanisms

A

nonenzymatic antioxidants: ascorbic acid (primary antioxidant in cornea), glutathione, alpha-tocopherol

enzymatic antioxydant: corneal crystallins (ALDH3A1), catalase, glutathione peroxidase, superoxide dismutase, aldehyde dehydrogenase superfam

59
Q

photodamage clinical effects

A

overexposure to UV light => epithelial injury

  • inhibition of mitosis, loosening of epithelial layer
  • nuclear and cytoplasmic damage - apoptosis with sloughing
  • SPK - superficial punctact keratitis (dry eye)
60
Q

acute photodamage

A

snow blindness, welding, tanning sun lamps

-symptoms: discomfort followed by pain and photophobia

61
Q

chronic photodamage

A

pinguecula and pterygium

62
Q

characteristics of corneal degenerations

A

deterioation and decrease in function

unliateral or bilateral -> asymmetric

no inheritance pattern

many degeneration considered aging changes

progression variable

often begin peripheral

63
Q

general aging changes

A

1) decrease corneal thickness
2) increase descemet’s membrane thickness
3) decrease endothelial cell count
4) decrease corneal luster

64
Q

arcus senilis

A

high cholesterol deposit at periphery of cornea

shrp peripheral border ending at edge of bowman’s layer with clear zone to the limbus

65
Q

pterygium

A

proposed reactive fibrovascular response of conj strom to chronic irritants (especially UV)

invades superficial cornea, pinguecula does not

66
Q

limbal girdle (of vogt)

A

symmetric yellow to white band in intrapalpebral limbals

2 presentations

1) white band with holes and narrow clear zone approaching limbus
2) chalky band with no holes or clear interval to limbals

increase with age

seen at 3 oclock and 9 oclock

67
Q

guttata (not tested)

A

“droplet like” beaten-metal appearnce of descemet’s membrane with abnormal basement mebrane

brownish pigmentation

associated with fuch’s endothelial dystrophy

68
Q

hassal-henle bodies (not tested)

A

focal thickenings of descemet’s membrane in peripheral cornea

common agin changes

69
Q

crocodile shagreen (not tested)

A

asymptomatic degeneration in elderly

mosaic pattern

usually bilateral

Ocular Biology (9 decks)

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