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OPTM 5032 - Ocular Physiology > Cornea and Sclera > Flashcards

Cornea and Sclera Flashcards

0
Q

How much refractive power does the anterior portion of the cornea supply?

A

48 Diopters of plus power

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1
Q

Corneal Dimensions

A

H - 12.6mm

V - 11.7mm

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2
Q

Radius of curvature of the cornea

A

7.8mm

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3
Q

How much of the cornea is spherical/toroidal?

A

Central 1/3rd

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4
Q

Corneal Thickness?

A

Central - 0.52mm

Peripheral - 0.65mm

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5
Q

How much incident light is transmitted through the cornea?

A

> 99% of light above 400nm

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6
Q

Range of radiation that the cornea transmits?

A

310nm in the UV to 2500 in infrared

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7
Q

Corneal Layers (Anterior to Posterior)

A 
Epithelium
Bowman's Layer
Stroma
Descemet's Membrane
Endothelium
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8
Q

Define Epithelium

A

Stratified, nonkeratinized, non secretory squamous epithelium

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9
Q

3 layers of the epithelium (anterior to posterior)

A 
Surface (3-4 cells; sloughing)
Wing (1-3 cells; intermediate)
Basal Cells (1 cell thick; location of mitosis)
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10
Q

Epithelial Turnover

A

7 days

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11
Q

Where do basal cells originate from?

A

Stem cells in the basal layer of the limbal epithelium

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12
Q

Which layer of the epithelium has high metabolic activity and what does that entail?

A

Basal cell layer

Entails prominent mitochondria, ER and golgi apparatus along with tons of glycogen storage

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13
Q

How do stem cells become basal cells?

A

Migrate onto the cornea and differentiate into basal cells thus renewing the supply of basal cells

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14
Q

Mitotic rate of epithelium

A

10-15% per day

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15
Q

How is the cornea seen when viewed under scanning electron microscopy?

A

As an irregular array of polygonal cells

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16
Q

What are smaller, light cells of the cornea viewed under electron microscopy?

A

Younger cells - recently reached the corneae

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17
Q

What are larger, dark cells of the cornea when viewed under electron microscopy?

A

Mature cells - will be sloughed

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18
Q

What are exfoliation holes of the cornea when viewed under electron microscopy?

A

Breaks in the epithelium - cells in process of peeling off the surface

Superficial cells may be viewable in these holes

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19
Q

How do basal cells adhere to the basement membrane and stroma?

A

Hemidesmosomes

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20
Q

What are hemidesmosomes linked to in the basement membrane?

A

Anchoring fibrils that pass through Bowman’s layer and into the stroma

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21
Q

What should you know about anchoring fibrils?

A

Type VII collagen
Penetrate 2 um into the stroma
End in structures known as anchoring plaques

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22
Q

How does photorefractive keratectomy affect the adhesion between basal cells and the stroma?

A

The adhesions are completely destroyed but new ones are reformed post surgery

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23
Q

Zonula Occludens

A

Of greatest importance in barrier function
Completely encircle cells - anastimosis of lipid bilayer of the adjoining membranes
Only found between superficial cells of the epithelium

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24
Q

What kind of membrane makes up the surface of the cornea?

A

A highly effective semipermeable membrane

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25
Q

Gap junctions

A

Found in all layers

More numerous in the basal layers than in the superficial layers

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26
Q

Epithelial Ion Transport (Na+ and Cl-)

A

Na+ is pumped from tears to stroma
Cl- is transported from stroma to tears
Results in a net flux of 0 (balanced)

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27
Q

Epithelial Basement Membrane Dystrophy (EBMD) Definition

A

Basal cells have a decreased number of hemidesmosomes (abnormal adhesion)

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28
Q

Result of EBMD

A

Cornea is susceptible edema and infection resulting in a lot of pain due to recurrent epithelial erosion

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29
Q

When does thickening (reduplication) of basement membranes occur?

A

Aging and diabetic patients

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30
Q

How does thickening of basement membranes affect the epithelial layer of the cornea?

A

Increased risk of epithelial erosion because anchoring fibrils cannot penetrate as deep

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31
Q

Where do basal cells of the epithelium rest?

A

On the basement membrane/basal lamina

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32
Q

How thick is the basal lamina?

A

~40-60 nm thick

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33
Q

What does the basal lamina contain?

A 
Type IV collagen
Laminin
The proteoglycan perlecan
Fibronectin
Fibrin
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34
Q

What happens in corneal epithelial wound response?

A

Mitosis ceases and the attachment to the basement membrane is lost, cells then enlarge and migrate via ameboid movement to cover the defect, mitosis resumes as soon as the wound closes

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35
Q

How long would it take a 6mm diameter epithelial wound to close?

A

Within 48 hours

Closes at a rate of 60-80um/hr

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36
Q

Does protein synthesis by epithelial cells increase of decrease during cell migration?

A

Increases

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37
Q

Energy use in migrating cells

A

Glycolytic activity increases resulting in decreased levels of glycogen

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38
Q

Do migrating cells use aerobic or anaerobic glycolisis?

A

Anaerobic, it’s a quick and dirty process

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39
Q

What glucose transporter mRNA increases in number during epithelial wound healing?

A

GLUT1

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40
Q

How long does GLUT1 remain elevated?

A

At least 2 weeks

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41
Q

What growth factor can be found in tear fluid during healing?

A

Epidermal Growth Factor (EGF) - expressed by corneal epithelium

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42
Q

What do stromal keratocytes synthesize?

A

Keratocyte and Hepatocyte growth factor (KGF & HGF)

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43
Q

How is epithelial wound response different in patients with EBMD, Diabetes, Persistent Epithelial Defects and Severe Injuries?

A

Healing is delayed and normal epithelial adhesion is not established

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44
Q

How is wounding in the limbus and peripheral epithelium different between the central cornea and the peripheral cornea?

A

Decreased length of cell cycle as there are more nutrients in the periphery

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45
Q

How long does it take for normal adhesion complexes to come back after basement membrane is removed during experimental keratectomy?

A

Delayed for more than 12 months

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46
Q

Anatomy of Bowman’s Layer

A

~12um thick
Randomly arranged Type I collagen fibrils
Acellular
Considered a modified superficial layer of the stroma

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47
Q

Anatomy of the Stroma

A

An extracellular matrix of collagen fibrils separated by proteoglycans in a lamellar arrangement parallel to the corneal surface

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48
Q

What does the ECM of the stroma produce?

A

Keratocytes and fibroblasts which also maintain the stroma ECM

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49
Q

How many lamellas of collagen fibers can be found in the stroma? How do the run?

A

200-250; They run limbus to limbus

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50
Q

How do lamellae lie in the anterior and posterior stroma?

A

Anterior - obliquely

Posterior - orthogonally

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51
Q

How do the lamellas of collagen run in the limbus?

A

Circumferentially forming an annulus 1.5 to 2.0mm wide around the cornea

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52
Q

Types of collagen fibers in the corneal stroma?

A

Mostly Type I but also type V and VI

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53
Q

Refractive index of Collagen fibers and extrafibrillar matrix?

A

Collagen fibers - n=1.411

Extrafibrillar matrix - n=1.365

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54
Q

Percent of light that is scattered traveling through the stroma

A

10%

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55
Q

Define Maurice’s Principle

A

Transparency of the cornea is the result of a lattice arrangement that cancels light scatter via destructive interference

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56
Q

Requirements of Maurice’s Principle

A

Fibrils must be of equal diameter and all equidistant (NOT SATIFIED BY THE CORNEA)

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57
Q

Scleral collagen arrangement

A

Fibers of the sclera are large with varying diameters and are not orderly or closely spaced resulting in a great deal of light scatter and nontransparency

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58
Q

Collagen arrangement of the Stroma of the cornea

A

Sizes vary within a small range which is a small fraction of a wavelength of visible light and the distance between fibrils is less than 1/2 the wavelength of visible light.

This creates transparency and weak light scatter

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59
Q

What happens to the cornea when damage to the epithelial or endothelial barrier?

A

The cornea swells (uptakes water) resulting in a loss of corneal transparency

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60
Q

What do “lakes” of water in the stroma cause?

A

Increased divergence of refractive index
Increased distance between collagen fibers
Loss of light transmittance

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61
Q

What are “lakes” of water in the stroma?

A

Swelling of the cornea where there aren’t collagen fibers

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62
Q

How does fibril diameter of the anterior and posterior cornea compare to each other?

A

Anterior > Posterior

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63
Q

How does the density of fibrils compare between the anterior and posterior cornea?

A

Anterior < Posterior

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64
Q

Does the anterior or posterior cornea scatter more light and by how much?

A

The anterior has a two-fold increase in light scatter when compared to the posterior

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65
Q

Rate of tear film evaporation of the cornea

A

2.5 uL/cm^2/hr

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66
Q

By how much does the cornea thin during the day in comparison to the night?

A

5%

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67
Q

Patients with comprised endothelial metabolic pump function (Fuch’s endothelial dystrophy) experience what in the morning?

A

Increased edema due to lack of evaporation of tear film at night when eyes are closed

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68
Q

Define Dellen

A

Localized area of corneal drying and evaporation

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69
Q

What happens when Dellen persists?

A

Decreased stromal fluid flow when stroma hydration is abnormal
in addition to
Minimal lateral flow of water in the cornea

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70
Q

What is normal IOP?

A

Less than 50 mmHg

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71
Q

What happens if IOP rises above 50 mmHg?

A

Epithelial edema occurs as well as increased stromal thickness

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72
Q

How is stromal swelling pressure affected by increased corneal thickness?

A

Pressure of stromal swelling decreases

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73
Q

What can mild corneal edema with elevated pressure lead to?

A

High imbibition (fluid replacement) pressure and subsequent epithelial edema and bullae (fluid filled blisters on the surface of the cornea)

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74
Q

How to restore tensile strength to the stroma after damage?

A

Resynthsize and cross-link collagen
Alter proteoglycan synthesis
Gradual wound remodeling

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75
Q

What happens around areas of cellular necrosis in the stroma?

A

Polymorphonuclear (granulocytes) cells appear which are then followed by monocytes (WBC)

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76
Q

How long after an operation does tensile strength take to fully return?

A

Gradually up to the 4th year

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77
Q

Descemet’s Membrane Anatomy

A

Basement membrane the corneal endothelium rests on
10-15 um thick (increases with age)
Composed of Type IV collagen, laminin and fibronectin

78
Q

Descementocele

A

Descemet’s membrane remains intact in some cases following severe corneal ulceration; shows that the membrane is resistant to proteolytic enzymes

Bulges out because no longer supported by stroma

79
Q

Fuch’s Dystrophy

A

Disease of the endothelial cells in which descemet’s membrane is abnormally secreted

80
Q

What happen’s as Fuch’s Dystrophy progresses?

A

Decreased endothelial cell function resulting in cell loss and corneal decompensation

81
Q

Define guttata

A

Wartlike excrescences of collagenous material on the posterior surface of descemet’s membrane

82
Q

What do guttata cause?

A

Enlargment of overlying endothelial cells

83
Q

Endothelial Anatomy

A

A single layer of polygonal cells covering the posterior surface of the cornea
Cell density increases with age

84
Q

Newborn endothelial cell density

A

> 5500 cells/mm^2

85
Q

Adult endothelial cell density

A

2500 - 3000 cells/mm^2

86
Q

Minimum endothelial cell density

A

400-700 cells/mm^2

87
Q

Function of endothelial cells

A

Metabollically active in transport, synthesis and secretory functions

88
Q

What do endothelial cells contain?

A

Large nucleus
Numerous mitochondria
Prominent ER
Golgi apparatuss

89
Q

Endothelial Tight Junctions (Macula Occludens)

A

Create a leaky barrier between aqueous and stroma

90
Q

Gap junctions in the endothelium

A

Mainly in lateral membranes for intercellular communication

Do not contribute to the endothelial barrier

91
Q

How much water is maintained in the stroma?

A

78%

92
Q

If the endothelium is disrupted at what rate does the cornea swell?

A

127 um/hr

93
Q

At what rate does the cornea swell when just the metabolic pump is disrupted?

A

33 um/hr

94
Q

What does normal leakage in the corneal stroma supply?

A

glucose and amino acids

95
Q

What happens to the cornea if it is cooled? And what is this process called?

A

The cornea swells; this is called temperature reversal

The cornea will return to normal thickness if the temperature returns to normal

96
Q

Where are the Na+K+ATPase pumps located in the endothelium?

A

In the basolateral membrane

97
Q

Approximately how many Na+K+ATPase pumps are there per cell?

A

~1.5 x 10^6

98
Q

What does ouabain do to the Na+K+ATPase pump

A

It stops sodium transport resulting in corneal swelling and preventing temperature reversal. It eliminates the transendothelial potential difference.

99
Q

How do guttata affect Na+K+ATPase pumps?

A

Increased pump site density allowing for greater capacity for the pump to counteract leaks

100
Q

What does Inflamed, edematous cornea do to the sodium pump?

A

Decreased pump site density despite increased permeability

101
Q

Function of Sodium Potassium-ATPase

A

Enzyme located on the plasma membrane of cells
Control corneal hydration
Involved in production of aqueous humor

102
Q

How do the Na+ ions being pushed out result in decreased pressure?

A

They generate osmotic pressure that water then follows to reduce the created osmotic pressure

103
Q

What does the ideal irrigating solution contain?

A 
Energy source (glucose)
Adequate buffer (bicarbonate)
Substrate (calcium)
104
Q

How do poorly buffered irrigating solutions affect the cornea?

A

They may expose the cornea to pH extremes and cause corneal edema

105
Q

What does the irrigating solution consisting of 0.9% saline and lactated Ringer’s colution do?

A

Overcomes the buffering capacity of the aqueous humor (moderate corneal thickening)

106
Q

What does 0.9% NaCl do to the cornea?

A

Causes corneal swelling (loss of pump barrier)

High corneal swelling

107
Q

What is GBR?

A

Glutathione bicarbonate ringers solution

Low corneal swelling

108
Q

How are defects in the endothelium healed?

A

Endothelial cells do not regenerate so cells spread out to cover adjacent areas

109
Q

How does then endothelium heal after keratoplasty?

A

Migration of endothelial cells over the wound edge to the periphery and tight junctions are developed to establish the endothelial barrier

110
Q

Define Polymegathism

A

A change in cell size

111
Q

Define pleomorphism

A

A change in cell shape

112
Q

Do polymegathism and pleomorphism increase or decreasen with age?

A

They increase significantly with age

113
Q

Define coefficient of variation (CV)

A

Standard deviation of mean cell area / mean cell area

114
Q

What is normal endothelium CV?

A

~0.25

115
Q

What does CV > 0.25 mean?

A

The cell size is variable = polymegathism

116
Q

What percent of a healthy cornea has hexagonal cells?

A

70-80%

117
Q

A decrease in hexagonal cells with an increase in cells with less than 6 sides shows what?

A

Endothelial stress = pleomorphism

118
Q

What range of myopia can LASIK treat?

A

2.25 to 14.5 Diopters

119
Q

Does LASIK have any effect on endothelial cells density of percent of hexagonal cells?

A

No it does not

120
Q

How much residual stroma must be left above the corneal endothelium after LASIK in order to maintain and protect the corneal endothelial structure and barrier function?

A

200 um

121
Q

Does the CV of cell size improve or diminish 3 years LASIK and why?

A

Improves significantly but is most likely due to the removal of contact lenses

122
Q

How does a PK (pentrating keratoplasty) corneal transplant affect cell counts and cell loss rate?

A

Cell Counts: 575 - 1243 cells/mm^2 with long term follow up

Cell Loss Rate: 7.8% per year (3-5 years after PK)

123
Q

How much greater is the cell loss rate in a PK cornea than that of a normal cornea?

A

13X the normal rate

124
Q

How does IOL surgery affect the endothelium?

A

Mean overall endothelial cell loss rate = 8.5% after 12 months

125
Q

How does Keratoconus (KCN) and Type 2 Diabetes affect endothelium?

A

Increased CV of cell size and decreased percent of hexagonal cells (50%)

126
Q

After 40 years of age how does Glaucoma with increased IOP affect endothelial cell density? What must one do if the Glaucoma patient also has cataracts?

A

Decreased; must take special care when performing cataract surgery on a glaucoma patient

127
Q

Contact Lens Use Endotheliopathy Syndrome (CLUE Syndrome)

A

Polymegathism reported with long-term use of rigid polymethylmethacrylate lenses which is probably secondary to contact lens induced hypoxia

128
Q

What is the cornea innervated by?

A

The ophthalmic division of the trigeminal nerve via ciliary nerves

129
Q

Which parts of the cornea are not innervated in humans?

A

Descemet’s Membrane and the Endothelium

130
Q

Which corneal receptors are the most prominent?

A

Nociceptors (pain) - why abrasions, ulcers and bullous keratopathy are all extremely painful

131
Q

What can cause sensory denervation of the Cornea?

A

Stroke, diabetic neuropathy and herpes simplex

132
Q

What does Sensory Denervation cause?

A

High incidence of epithelial erosions and neurotrophic ulcers
Loss of foreign body sensations resulting in mechanical corneal damage

133
Q

How does LASIK affect corneal and conjunctival sensitivity?

A

Decreased for up to 16 months

134
Q

How do CL’s affect corneal sensitivity?

A

Decrease sensitivity in both soft and rigid

135
Q

Where does the cornea get its oxygen from?

A

Atmosphere (most)
Aqueous Humor
Limbal Vasculature

136
Q

How does aqueous humor O2 levels in aqueous humor compare to that of tears?

A

Lower

137
Q

How much more oxygen does the epithelium consume than the stroma?

A

10X

138
Q

How is oxygen delivered to the cornea during sleep?

A

Via vascularized superior palpebral conjunctiva

139
Q

What percent of oxygen is delivered to the cornea when the eyes are opened and closed?

A 
Opened = 21%
Closed = 8%
140
Q

What is aphakia?

A

Cataract removal

141
Q

How does aphakia affect the O2 demands of the cornea?

A

There may be an increase in aqueous humor oxygen thus giving greater tolerance to hypoxic stress (CL use)

142
Q

Where do nutrients for the cornea come from?

A

Mostly the aqueous humor
Lesser amounts in tears and limbal vessels
Glucose from glycogen stores in corneal epithelium

143
Q

How much of the cornea’s oxygen reserve is used up after 8 hours of wearing hard contact lenses? What are the results of this?

A

80% is used up
Results in a 20% increase in corneal thickness (edema)

Not much better in soft lenses

144
Q

Embden-Meyerhof Pathway

A

How glucose stores from the aqueous humor or glycogen stores are converted to pyruvate in the cornea

145
Q

Symptoms of epithelial edema

A

Halo and rainbow formation
Increased glare sensitivity
Decreased contrast sensitivity

146
Q

In which direction does stromal edema manifest?

A

Posteriorly - buckling of the stroma and descemet’s membrane causes vertical striae

147
Q

At what oxygen level can detergesence (decrease in swelling) be maintained?

A

As low as 25 mmHg

Any lower results in corneal hardening

148
Q

Oxygen exchange and small diameter hard lenses

A

Impermeable to oxygen

Good movement needed for tear exchange

149
Q

Oxygen exchange in large diameter soft lenses

A

Oxygen permeability needed for diffusion through the lens itself
Tear pump exchange (lesser)

150
Q

Underlying mechanism of Polymegathism

A

Byproducts of long term hypoxia and stress (12HETE) inhibit the Na+K+ATPase of the endothelial metabolic pump

151
Q

When does polymegathism occur?

A

Only after long term volume regulation stress (CL wear) and in cornea’s of diabetic patients

152
Q

Effects of extended wear CL’s on metabolism

A

Decreased rate of mitosis
Decreased oxygen uptake and glucose utilization
Decreased intercellular desmosomes

153
Q

Consequences of EW CL effects on metabolism

A

Compromised epithlial barrier function

Increased likelyhood of ulcerative microbial keratitis

154
Q

What is Xerophthalmia and what causes it?

A

Keraitnization of the epithelium; caused by a lack of vitamin A which is required for mucin production (usually found in developing countries)

155
Q

What are Bitot’s Spots?

A

Triangular foamy lesions seen in the conjunctiva

156
Q

What is the initial barrier to drug penetration and what does it allow through?

A

Epithelium (tight junctions)
Favors lipid-soluble hydrophobic compounds
Limits adsorption of hydrophilic, ionized substances

157
Q

How does the stroma affect drug penetration?

A

Barrier to lipid-based drugs

158
Q

How does the endothelium affect drug penetration?

A

Mostly depends on size of the molecule

159
Q

Ways to increase drug penetration

A 
Press on lacrimal sac
Use viscous drops, suspensions and ointments
Slow release delivery systems
Contact lenses
Porcine collagen corneal shields
160
Q

What is the action of the preservative BAK?

A

It is a detergent (breaks down bacterial cell walls)

May also make the endo and epithelium susceptible to damage

161
Q

Effects of BAK

A

One drop of 0.01% BAK increases corneal permeability to fluorescein
Inhibits epithelial wound healing

162
Q

How may BAK affect severe dry eye patients?

A

May risk further damage as they are already more permeably to fluorescein due to their compromised epithelial barrier

163
Q

EDTA

A

Used to stabilize BAK containing formulations by chelating calcium (breaks up tight junctions)
Try not to use especially in dry eye patients

164
Q

Scleral Anatomy

A

Connective tissue consisting of fibroblasts, collagen (Type I) and proteoglycans
70% water
30% dry weight; 75% of which is collagen fiber

165
Q

What overlies the sclera?

A

Episclera

166
Q

Is the sclera vascular?

A

No, it is mostly avascular with exceptions including superficial vessels of the episclera and the intrascleral vascular plexus

167
Q

Features of the posterior sclera

A

Contains scleral canal and lamina cribrosa (optic nerve passage)
Perforation site for arteries, veins and nerves
Tendon’s of recti muscles insert into superficial sclera collagen

168
Q

How much of the total surface area of the globe is sclera?

A

95%; thickness decreases at the equator and increases near the optic nerve

169
Q

Arrangement and diameter of scleral collagen fibers

A

Diameter ranges from 25 - 230nm

Arranged in bundles

170
Q

Connections between the sclera and myopia

A

Form vision deprivation results in axial elongation

Changes in rate of proteoglycan synthesis and accumulation within the sclera also results in axial elongation

171
Q

Aging effects on sclera

A

Degeneration of collagen and elastic fibers
Loss of GAG’s*
Scleral Dehydration*
Accumulation of lipids and calcium salts (yellow sclera)

172
Q

Aging effects on sclera cause?

A

Increased tissue density
Scleral thinning and yellowing
Decreased scleral elasticity
Sclera and Cornea get tougher

173
Q

Is the sclera innervated?

A

The stroma of the sclera is devoid of innervation but does allow passage of nerves
Axons of parasympathetic origin present at scleral spur

174
Q

Primary Mechanism of Transscleral Delivery of Drugs

A

Diffusion through aqueous pathway

Possibly treat posterior segment disease

175
Q

Factors controlling diffusion rates in the sclera.

A

Tissue Hydration
Tissue Thickness
Size and Volume fraction of proteoglycans present

176
Q

At what pH does the sclera swell the least?

A

pH 4

177
Q

What is the only treatment that can alter scleral permeability?

A

Surgical thinning which increases permeability

178
Q

How does increased pressure affect scleral permeability?

A

Decreases permeability to small molecules by half

179
Q

What is a strong function of permeability?

A

Molecular weight

180
Q

How do prostaglandins affect permeability? And what does this result cause?

A

They increase permeability which causes and increased expression of matrix metalloproteinases

181
Q

How to increase Transscleral delivery for large molecules?

A

Take advantage of thinner regions of tissue
Increase scleral hydration
Modification of scleral ECM

183
Q

The horizontal and vertical diameters of the cornea?

A

H - 12.6 mmV - 11.7 mm

184
Q

Three layers of cells in the epithelium of the cornea

A
  1. Surface cells (3-4 cells thick)2. Wing cells (1-3 cells thick)3. Basal cells (1 cell thick)
185
Q

What is the turnover rate of the epithelium?

A

7 days

186
Q

Basal cells have a ____ metabolilic activity

A

high

187
Q

What is the mitotic rate of epithelium?

A

10-15 % per day

188
Q

Anchoring fibrils are a type ____ collagen and penetrate _____ into the stroma

A

Type VII collagen2 um

189
Q

The adhesion complex is ______ _______ after surgery

A

completely destroyed

190
Q

Where are zonula occludens found?

A

Only found between the superdicial cells of the epithelium

191
Q

How thick is Descemet’s Membrane?

A

10-15 um

192
Q

What is deturgescence?

A

relative dehydration maintained by the normal cornea that is necessary for transparancy

193
Q

What is the lowest oxygen level that must be sustained to maintain corneal transparency (deturgescence)?

A

25 mm Hg

OPTM 5032 - Ocular Physiology (9 decks)

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