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Ocular Physiology #2 > Lens > Flashcards

Lens Flashcards

2
Q

what type of tissue is the lens

A

two types of specialized epithelial tissue: cuboidal and elongated fiber cells

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3
Q

what 3 things must the lens have in order to project an image onto the retina

A

transparent, higher refractive index than the medium its suspended in, and have flexible refractive surfaces with the proper curvature

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4
Q

what happens to the lens if there is disruption of precise organization or damage to the proteins

A

destroys the transparency of the lens and cataract formation

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5
Q

what type of tissue is the lens epithelium

A

a sheet of cubodial cells

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6
Q

what type of tissue is the bulk of the lens

A

elongated fiber cells

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7
Q

what secretes the lens capsule

A

the epithelial and superficial fiber cells

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8
Q

where is the germanitive zone

A

near the equatorial margin of the lens epithelium

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9
Q

when do the fibers stop elongating

A

when they reach the Y sutures

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10
Q

what are the lens sutures

A

junctions between the apical and basal ends of the cells from opposite ends of the lens

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11
Q

where do the mature lens fibers go

A

they get buried deeper as fibers elongate and differentiate

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12
Q

when does synthesis stop

A

just before the organelle degradation

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13
Q

how do the mature fiber cells compare to those in the rest of the body

A

the components of mature fiber cells are more stable

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14
Q

what are the zonules

A

the inelastic microfibrils that suspend the lens

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15
Q

where do the zonules originate and insert

A

in the NPE of the ciliary epithelium and insert into the lens capsule near the equator

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16
Q

what is responsible for changing the lens curvature during accommodation

A

the zonules

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17
Q

what contributes to the refractive nature of the lens

A

the high concentration of crystallins in the cells and the curvature of the surfaces

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18
Q

what is the concentration of crystallins in the lens compared to typical cells

A

3 times higher concentration

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19
Q

what causes refractive error is younger individuals

A

corneal curvature or the length of the globe (rarely by the lens)

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20
Q

what does the transparency of the lens depend on

A

minimizing light scattering and absorption

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21
Q

why does light pass smoothly through the lens

A

regular structure of lens fibers, absence of membrane-bound organelles and small /uniform extracellular space between fiber cells

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22
Q

what happens to the nearly colorless lens as we age

A

becomes more yellow- lens absorbs short wavelength light (blues)

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23
Q

what is aphakia

A

the absence of the natural crystalline lens from natural causes or removal

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24
Q

what is congenital aphakia

A

caused by Rubella infection in first 4 weeks of pregnancy (mutation in PAX-6 gene)

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25
Q

what is pseudophakia

A

substitution of the natural crystalline lens with a synthetic lens

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26
Q

what does lens fiber differentiation depend on

A

synthesis and accumulation of large amounts of crystallin proteins

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27
Q

what percentage of the wet weight of the lens fiber is crystillins

A

40% (3 times the amount in a typical cell)

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28
Q

what are the classic crystallins that all vertebrae lenses accumulate

A

alpha-crystallin family and beta/gamma-crystallin superfamily

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29
Q

do adults have taxon-specific crystallins

A

no- high levels in the embryonic nucleus

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30
Q

why are taxon-specific crystallins important

A

for certain events to take place- development of the human lens

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31
Q

what taxon-specific crystallin is present at high levels in embryonic nucleus

A

betaine-homocysteine methyltransferase

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32
Q

what is a taxon-specific crystallin

A

a functional enzyme or protein- lack enzymatic activity

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33
Q

what are the two classical crystallins humans express

A

alpha A and alpha B

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34
Q

what is the role of alphaA and alphaB crystillins

A

to prevent protein aggregation and precipitation of one another

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35
Q

what did the analysis of alphaA knockout mice show

A

that protein aggregates show large amounts of alphaB crystallin and smaller amounts of other proteins

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36
Q

is alphaA crystallin only present in the lens

A

no- also in heart and skeletal muscle

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37
Q

what is the function of alphaB in the lens

A

important chaperone functions

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38
Q

how many beta and gamma crystallins are in the lens

A

6 beta and 3 gamma

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39
Q

which is more diverse, the beta/gamma superfamily or the alphaA crystallins

A

the beta/gamma superfamily

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40
Q

how much of the lens is water

A

65%

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41
Q

how much of the lens is organic matter

A

35%

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42
Q

how much of the lens organic matter is structural proteins

A

33%

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43
Q

where are microtubles found

A

beneath membranes of lens fiber cells

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44
Q

what are the roles of the microtubles

A

stabilizing the fiber cell membrane and may be transporting vesicles to the apical and basal ends of elongating fiber cells

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45
Q

what gives the lens its birefringent property

A

the microtubules

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46
Q

what is birefringent property

A

(uniform pattern) path of polarized light ray can be changed and split its contents into different phases

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47
Q

what does birefringent depend on

A

the number of microtubles and their thickness

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48
Q

what other parts of the eye have birefringent property

A

cornea, sclera and retinal nerve fiber layer

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49
Q

what components does the lens fiber cell membrane have

A

unusual lipid composition and high proportion of cholesterol and phingomyelin

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50
Q

what increases in the cell membrane as the cells mature

A

cholesterol

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51
Q

what causes the fiber cell membranes to be rigid

A

the presence of high concentrations of cholesterol and sphingomyelin

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52
Q

how much more cholesterol is in the nuclear fibers compared to the cortical fibers

A

3 times more

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53
Q

when is the lens growth the most rapid

A

in the embryo and 1st postnatal year

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54
Q

where do the capillaries at the anterior part of the lens arise from

A

blood vessels of the developing iris stroma

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55
Q

where do the capillaries at the posterior part of the lens arise from

A

hyaloid artery and form the tunica vasculosa lentis

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56
Q

when do the capillaries regress

A

during the 2nd trimester (caused by apoptosis)

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57
Q

if the lens is absent at an early stage of development, what will it lead to

A

absence of corneal endothelium, abnormal differentiation of the corneal stroma and absence of the iris, ciliary body and anterior chamber

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58
Q

where does the lens get its energy from

A

glycolysis

59
Q

what is the end product of glycolysis

A

lactic acid

60
Q

what happens with lactic acid accumulation

A

intracellular pH drops significantly from peripheral to deeper fiber cells

61
Q

what is the source of the most oxidative damage

A

molecular oxygen

62
Q

what protects the lens proteins and lipids from oxidative damage

A

low oxygen tension around and within the lens (15 mmHg)

63
Q

how does the lens derives a substantial proportion of its ATP

A

from oxidative phosphorylation- process that generates free radicals

64
Q

where is most of the solar irradiation absorbed

A

in the cornea

65
Q

what 5 things absorb UV light (potential sources of free radicals)

A

DNA, proteins, nucleoside-containing metabolites, flavonoids and pigments

66
Q

what 2 things absorb visible and UV light

A

flavonoids and pigments (especially shorter wavelengths)

67
Q

Glutathione is a tripeptide of what 3 things

A

glutamine, cysteine and glycine

68
Q

what provides the most protection against oxidative damage in the lens

A

Glutathione

69
Q

what happens to the lens when glutathione levels are decreased

A

cell damage and cataract formation follow rapidly

70
Q

what is actively transported from the blood to the aqueous humor by Na-dependent transporter in the ciliary epithelium

A

Ascorbic acid

71
Q

what else besides glutathione provides protection against oxidative damage

A

ascorbic acid (from aqueous humor)

72
Q

how much more ascorbic acid is in the aqueous humor compared to blood

A

20 times more

73
Q

why does the lens depend on glycolytic metabolism to produce ATP

A

the oxygen concetration within and around the lens is low (lower than other parts of the body)

74
Q

where does the glucose come from that supplies the glycolytic metabolism in the lens

A

from the aqueous humor- facilitated diffusion across ciliary epithelium

75
Q

where are mitochondria located in the lens

A

lens epithelium and superficial fiber cells

76
Q

what electrolyte should the lens have in high concentrations

A

potassium

77
Q

how do cells near the lens surface derive energy from glucose

A

via glycolytic and oxidative pathways

78
Q

what electrolytes should the lens have in low concentrations

A

Na+, Cl-, and water

79
Q

what happens if sodium moves into the lens

A

the solutes and water content will increase (osmotically)

80
Q

is the lens more positive or negative inside

A

electronegative inside

81
Q

is aqueous humor higher in Na+ or K+

A

higher Na+ and low K+

82
Q

where is the Na+K+ATPase pump located in the lens

A

in the epithelium (also has mitochondria)

83
Q

what is removed and what is allowed in with the Na+K+ATPase pump

A

3 Na+ removed and 2 K+ allowed in

84
Q

which part of the lens, inner or outer part, requires more energy

A

the outer part of the lens

85
Q

what type of chromophores does the lens accumulate

A

ones that absorb the short visible wavelengths

86
Q

what pigmentation increases as we age

A

the amount of yellow pigmentation in the lens increases

87
Q

what can happen with high concentrations of chromophores in the lens

A

increase in light absorbance and reduced visual acuity

88
Q

what is a brunescent or nigrescent cataracts

A

a black cataract (cataract nigra)

89
Q

when does a brunescent cataract form

A

common in developing countries (environmental or nutritional factors may contribute)

90
Q

what is the textbook definition of a cataract

A

an opacification of the lens

91
Q

what is the clinical definition of a cataract

A

it interferes with visual funciton

92
Q

what 3 things cause loss of transparency with increased light scattering

A

disruption of lens fiber cell structure, increased protein aggregation, and phase separation lens cell cytoplasm

93
Q

what is phase separation

A

formation of opacities without disruption of cell

94
Q

at what visual acuity does medicare pay for cataract surgery

A

20/50 or worse

95
Q

what is the general effect of a cataract formation

A

a change in amount of soluble and insoluble lens protein

96
Q

why does the protein content in the lens increase with age

A

because the lens grow with age

97
Q

what changes the soluble protein to insoluble

A

caused by cross-linking (why UV light is blamed for changes on lens)

98
Q

what are the 3 most common types of cataracts

A

nuclear, cortical, and posterior subcapsular cataracts

99
Q

where do nuclear cataracts occur in the lens

A

in the oldest fiber cells (those formed during embryonic and fetal life)

100
Q

what changes in the lens are nuclear cataracts associated with

A

increased oxidative damage to lens proteins and lipids

101
Q

are nuclear cataracts age dependent

A

yes- more susceptible to oxidative damage

102
Q

what happens to refractive power of lens with the onset of a nuclear cataract

A

there is an increase in refractive power

103
Q

what is second sight

A

in nuclear cataracts: a temporary improvement in near vision for hyperopic patients that undergo a myopic shift

104
Q

what can occur following a vitrectomy (within 6 months to 3 years after)

A

a nuclear cataract in older patients

105
Q

what causes a nuclear cataract following a vitrectomy

A

changes the environment around the lens- increase in the oxygen tension around the lens

106
Q

what is the oxygen tension in the anterior vitreous

A

normally low- about 16mmHg

107
Q

what is the oxygen tension in the posterior vitreous (near the retinal vessels)

A

high- decreasing in a sharp gradient within the first 1mm of the vitreous body

108
Q

what happens to the oxygen levels when the vitreous is removed

A

the fluid in the eye can circulate freely and the lens is exposed to increased oxygen from the retina

109
Q

are the changes in the protein organization leading to cataracts rapid or subtle

A

subtle

110
Q

do a large or small fraction of proteins in the lens need to have protein changes to cause a large amount of light scattering

A

only a small fraction

111
Q

where do higher molecule fractions grow into in senile cataracts

A

into the cell cytoplasm

112
Q

where do cortical cataracts occur in the lens

A

in the mature fiber cells lying close to the lens surface- most often in the inferior nasal quadrant

113
Q

are the affected regions in cortical cataracts partially or completely disrupted

A

completely disrupted

114
Q

where does the opacity begin and end in cortical cataracts

A

in the periphery and spreads toward the visual axis interfering with vision

115
Q

what are cortical spokes

A

extensions of the opacity on a small cluster of the fiber cells

116
Q

what type of refractive error can you expect with cortical cataracts

A

a hyperoptic shift (hypermetropia) stretching the lens flatter or changing the curvature

117
Q

What happens to the lens epithelial cells in a PSC

A

There is abnormal migration of the cells and aberrant differentiation

118
Q

what causes a posterior subcapsular cataract

A

by light scattering in a cluster of swollen cells at the posterior pole, just beneath the capsule

119
Q

do most patients have a “pure” posterior subcapsular cataract

A

only less than 10% have a pure form- usually occur in conjunction with nuclear or cortical cataracts

120
Q

what happens if a cataract is not removed

A

it will become a total or “morgagnian” cataract

121
Q

what happens to the lens in a total or morgagnian cataract

A

the capsule gets weaker and starts wrinkling and the nucleus will sink down- eventually bursting and releasing the lens protein into the aqueous humor

122
Q

what are the steps of an extracapsular cataract extraction (most common approach to remove cataracts)

A

removal of a portion of the anterior lens epithelium and capsule, extract the nuclear and cortical fibers, and implant a IOL in a capsular bag

123
Q

is intracapsular cataract extraction technique still used

A

still common in developing countries, older technique and not the surgery of choice

124
Q

what is a secondary cataract

A

a common complication of cataract extraction- a posterior capsular opacification (PCO) and it needs a YAG laser to clear the opacity

125
Q

what happens in intracapsular cataract extraction

A

the entire lens is removed when removing the cataract or if the lens has burst

126
Q

what type of refractive error occurs after intracapsular cataract extraction

A

the patient will be aphakic- very hypermetropic and need thick lenses

127
Q

what are “lentoid bodies” or Elschnig’s pearls

A

differentiated lens epithelial cells near the equator that persist after surgery can migrate beneath the IOL onto the posterior capsule

128
Q

what are congenital cataracts

A

present at birth or appear soon after (can be cause by Rubella)

129
Q

What are 7 hereditary syndromes associated with cataract formation

A
  1. oculocerebrorenal syndrome of Lowe
  2. neurofibromatosis type 2
  3. hyperferritinemia
  4. Werner syndrome
  5. Myotonic dystrophy
  6. Galactokinase deficiency and galactosemia
  7. Fabrys disease
130
Q

what are anterior polar cataracts

A

early-onset cataracts or type of congenital: an opaque plaque is formed near the center of the lens epithelium

131
Q

What type(s) of occupational exposure led to cataract formation

A

infrared light and focused microwaves (ie. glass-blowers cataract)

132
Q

what is the suggested mechanism of how UV light causes cataracts

A

UV-generated free radicals can damage the components of the lens

133
Q

what type of cataract can long-term exposure to high-dose steroids cause

A

increased risk of a posterior subcapsular cataract

134
Q

what 5 things can happen with increased intracellular calcium in the lens

A

affects glucose metabolism, inhibition of protein synthesis, induction of high molecular aggregates, direct loss of transparency, and inhibit sodium pump

135
Q

What are the steps in the osmotic hypothesis of diabetic cataracts

A
  1. lens toxic levels of glucose
  2. aldose reductase activated
  3. glucose converted to sorbitol
  4. sorbitol cannot escape the lens
  5. polyol dehydrogenase tries to get rid of sorbitol by converting it to fructose
  6. high intracellular osmotic pressure
  7. cells burst debris and become a cataract
136
Q

what can occur in the lens with hypocalcemia

A

related to the dependency of the membrane permeability to levels of calcium and marked electrolyte and water imbalance

137
Q

what causes early-onset of cataract in diabetic patients

A

they can have damage to the lens as a result of high glucose levels (the aqueous humor has too much glucose-more than the lens and cornea need)

138
Q

what is the most common risk factor of developing a catarct

A

age

139
Q

why is low socioeconomic status a cataract risk factor

A

it may predispose patients to nutritional deficiencies, increased exposed to diseases, poor general health

140
Q

what are 5 risk factors for developing a cataract

A

age, low socioeconomic status, females, smoking and high alcohol consumption, and a dark iris color

141
Q

what type of cataracts are linked to smoking and high alcohol consumption

A

nuclear and cortical cataracts

142
Q

what type of cataract is more common in African-American patients

A

cortical cataracts

143
Q

what 2 things are being looked at in cataract research

A

genetics of cataracts and nutrition (calorie restriction)

Ocular Physiology #2 (7 decks)

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