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Ocular Biology > Physio and Biochem of Cornea > Flashcards

Physio and Biochem of Cornea Flashcards

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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
Necessary prerequisites to maintain relatively dehydrated state
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
26
corneal transparency summary
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)
27
Clinical Edema
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
28
corneal swelling - epithelia
extracellular - water vacuoles form b/n cells intracellular - fluid in cell, scatters light decreases VA, causes halos and pain
29
corneal swelling - stroma
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
general metabolism
energy required: transparency, mitosis, cell migration unique environment: temperature, avascularity
31
source of nutrients
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
carbohydrate metabolism
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
hypoxic condition
glycolysis leading to accumulation of lactate
34
protein metabolism
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
Vitamin A
maintenance of ocular surface retinoic acid - active maintains non-kertinaized epithelium
36
vitamin a deficiency
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
corneal wound healing - what are important?
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
Wound healing - epithelium
equilibrium of cell division, maturation, and death
39
wound healing - epithelium phase 1
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
wound healing - epithelium phase 2
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
wound healing - epithlium phase 3
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
wound healing - epithelium phase 4
attachment -reestablishes firm attachment of basal cells to BM and stroma if BM not damaged, can occur in matter of days
43
limbal stem cell deficency
if destruction of stem cells -> conjunctivalization of cornea => vascularization and goblet cells
44
wound healing - epithelium summary
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
damage to epithelium and basement membrane
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
wound healing - bowman's layer
acellular no regeneration collagen scar (collage from stromal keratocytes filled in)
47
Wound healing - epithelial stroma interaction
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
wound healing - stroma | destructive phase
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
wound healing - stroma syntehtic phase
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
wound healing - stroma with excessive myofibroblast activity
contraction of corneal wound may cause irregular corneal astigmatism repair tissue opacity
51
wound healing - stroma remodeling phase
ECM slowly restructured - weeks to years regulated by TGf-beta and proteases myofibroblast disappear keratocytes become quiescent
52
wound healing stroma | summary
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
wound healing - corneal nerves
nerves regenerate from uninjured peripheral nerve trunks rate of recovery = slow af denervated cornea risk for epithelial defects
54
wound healing - descemet's membrane
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
wound healing - endothelium
no mitosis in adult defect covered by enlargement and spreading of neighboring cells - polymegathism, pleomorphism
56
Cornea wound healing summary
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
Photodamage
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
antioxidant defense mechanisms
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
photodamage clinical effects
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
acute photodamage
snow blindness, welding, tanning sun lamps -symptoms: discomfort followed by pain and photophobia
61
chronic photodamage
pinguecula and pterygium
62
characteristics of corneal degenerations
deterioation and decrease in function unliateral or bilateral -> asymmetric no inheritance pattern many degeneration considered aging changes progression variable often begin peripheral
63
general aging changes
1) decrease corneal thickness 2) increase descemet's membrane thickness 3) decrease endothelial cell count 4) decrease corneal luster
64
arcus senilis
high cholesterol deposit at periphery of cornea shrp peripheral border ending at edge of bowman's layer with clear zone to the limbus
65
pterygium
proposed reactive fibrovascular response of conj strom to chronic irritants (especially UV) invades superficial cornea, pinguecula does not
66
limbal girdle (of vogt)
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
guttata (not tested)
"droplet like" beaten-metal appearnce of descemet's membrane with abnormal basement mebrane brownish pigmentation associated with fuch's endothelial dystrophy
68
hassal-henle bodies (not tested)
focal thickenings of descemet's membrane in peripheral cornea common agin changes
69
crocodile shagreen (not tested)
asymptomatic degeneration in elderly mosaic pattern usually bilateral

Ocular Biology (9 decks)

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