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Ocular Physiology 2 > Lens > Flashcards

Lens Flashcards

1
Q

What are the 3 things that need to happen properly for the lens to function

A
  1. Be transparent
  2. Have a higher index of refraction than the medium its suspended in (AH)
  3. Have flexible refractive surfaces with the proper curvature
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2
Q

Disruption of the organization of the lens fiber cells or damage to the proteins result in

A

Cataract Formation

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3
Q

What type of cells does the anterior surface of the lens (lens epithelium) contain?

A

Cuboidal cells

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4
Q

What does the bulk of the lens consist of

A

Elongated fiber cells

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5
Q

What is secreted by the lens epithelium and elongated fiber cells

A

Lens capsule, an ECM

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6
Q

Where do most epithelial cells divide

A

In the germanative zone, near the equatorial margin of the lens epithelium

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7
Q

What are sutures

A

Junctions between the apical and basal ends of the cells from the opposite side of the lens

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8
Q

Lens increases in size and synthesis just before…

A

organelle degradation

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9
Q

Where do zonules originate

A

Non pigmented layer of the ciliary epithelium and insert into the lens capsule near the equator

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10
Q

What do tensions in the zonules cause

A

Curvature changes during accomodation

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11
Q

The refractive properties of the lens are the result of…

A

High concentrations of crystallins in the cytoplasm of the lens fiber cells and the curvature of the lens surfaces

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12
Q

Crystallins are how many times more higher than in typical cells

A

3x, which increases the refractive index of the lens

which makes up 40% of the wet weight of the lens fiber

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13
Q

In younger indivisuals, is refractive error caused by the lens?

A

NO

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14
Q

What are the 2 things that causes refractive error in younger indivisuals

A
  1. Corneal curvature

2. Length of the globe

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15
Q

What does transparency of the lens depend upon?

A

Minimizing light scattering and absorption

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16
Q

Light passes smoothly through due to:

A
  1. Regular structure of lens fibers
  2. The absence of membrane bound organelles
  3. Small/uniform extracellular space between cells
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17
Q

In young individuals the human lens is nearly…

A

Colorless

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18
Q

As we age, what wavelength of light does the lens absorb

A

Blue light which consists of short wavelengths

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19
Q

What is aphakia?

A

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

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20
Q

What is congenital aphakia

A

Rubella infection in first 4 weeks of pregnancy

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21
Q

What is pseudophakia

A

The substitution of the natural crystalline lens with a synthetic lens

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22
Q

What does lens fiber differentiation depend on

A

Synthesis and accumulation of large amounts of crystallins

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23
Q

What consist of the classical crystallins

A

Alpha crystallin family and Beta/gamma crystallin superfamily (all vertebrae lenses accumulate these)

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24
Q

What are taxon specific crystallins

A

Functional enzymes or proteins that are structurally similar to enzymes but that lack enzymatic activity

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25
Q

Do adult lenses produce taxon specific crystallins

A

NO

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26
Q

What enzyme is in the embryonic nucleus which serves as a taxon specific crystallin during the early development of the human lens

A

Betaine-homocysteine methyltransferase

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27
Q

What type of classical alpha crystallins do humans express

A

alpha A and alpha B

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28
Q

What is the role of alpha A and alpha B classical crystallins?

A

To prevent protein aggregation and precipitation

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29
Q

What did the analysis of the alpha A knockout in mice show

A

That the lens is smaller than normal but structurally similar, formation of cataracts in a few weeks, and that they contained large amounts of alpha B crystallin and smaller amounts of other proteins

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30
Q

Where else is the alpha B crystallin present besides the lens

A

Heart and skeletal muscle

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31
Q

What important function does the alpha B crystallin lens have

A

Chaperone functions in the lens

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32
Q

T/F the alpha A is more diverse than the betta gamma superfamily

A

False, the betta/gamma superfamily is more diverse

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33
Q

How many betta and how many gamma are found in the lens

A

6 betta and 3 gamma crystallins

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34
Q

T/F The classical crystallins are made up of 65% of water

A

True

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35
Q

How much organic matter is in the classical crystallins

A

35%

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36
Q

T/F the insoluble varitey of protein is greater than the soluble

A

F -The soluble variety is greater

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37
Q

Microtubles play an important role in:

A
  1. Stabilizing the fiber cell membrane
  2. May be transporting vesicles to the apical and basal ends of the elongating fiber cells
  3. Gives lens its birefringent property
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38
Q

WHat is the definition of birefringent?

A

Property of any tissue that (changes path of polarized light) polarized light rays are split into its different components and travel at different phases. You can measure this property for any tissue and figure out if its healthy or not.

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39
Q

T/F The membranes of mature fiber cells have an unusual lipid composition

A

True

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40
Q

The high proportion of cholestrol and sphingomyelin compared to other plasma membranes causes the cell membranes to be…

A

RIGID

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41
Q

Cholesterol is 3 x greater in nuclear fibers compared to…

A

cortical fibers

42
Q

When is the growth in lens most rapid

A

In the embryo and in first postnatal year

43
Q

When does lens growth slow down?

A

Between ages of 1-10, continues at a slower almost linear rate throughout life

44
Q

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

A

blood vessels of the iris stroma

45
Q

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

A

the hyaloid artery from the tunica vasculosa lentis

46
Q

When do capillaries regress

A

During second trimester

47
Q

Absence of the lens in early embryogenesis leads to:

A
  1. Absence of corneal endothelium
  2. Abnormal differentiation of the corneal stroma
  3. Absence of the iris, ciliary body, and anterior chamber
48
Q

Where does the lens gets its energy from?

A

Glycolysis

49
Q

What is the end product of glycolysis

A

lactic acid, intracellular pH drops significantly from peripheral to deeper fiber cells secondary to lactic acid accumulation

50
Q

What is the source of most oxidative damage?

A

Molecular oxygen

51
Q

T/F The HIGH oxygen tension around/within the lens probably protects it from oxidative damage

A

False! The low oxygen tension does

52
Q

Where does the lens derive its ATP from

A

Oxidative phosphorylation which is a process that generates free radicals

53
Q

Where is most of the solar irradiation that reaches the eye absorbed by

A

The cornea

54
Q

T/F If light is not absobed, it produces no damage

A

True

55
Q

Which structures absorb UV light

A
  1. DNA
  2. Proteins
  3. Nucleoside-metabolites
  4. Flavonoids
  5. Pigments
56
Q

What provides most of the protection against oxidative damage and what is it made out of

A

Glutathione. It is a tripeptide made up of amino acids: glutamine, cysteine, and glycine

57
Q

What also provides minor protection to the lens and where is it transported from

A

Ascorbic acid. It is actively transported from the blood to the AH by a Na dependent transporter in the ciliary epithelium.

58
Q

How much more ascorbic acid is in the AH humor than in the blood

A

20 x

59
Q

T/F Oxygen within the lens is much higher than other parts of the body

A

False, its much lower

60
Q

Where is the glucose required for glycolytic metabolism derived from

A

The aqueous humor - due to fascilitated diffusion across the ciliary epithelium

61
Q

Lens should maintain which electrolytes?

A

High K+, Low Na+ and Cl- and water

62
Q

The water electrolyte balance is maintained by the _______ which operates in the ________ which has _____

A

sodium potassium pump
epithelium
mitochondria

63
Q

What does the human lens accumulate which absorbs the short visible wavelengths

A

Chromophores

64
Q

What color pigmentation in the lens increases with age and what does this do?

A

Yellow - it absorbs shorter wavelengths of light that prevent them from getting to the retina

65
Q

What happens when there is a high concentration of chromophores

A
  • increased light absorbance
  • reduced VA
  • Brunescent/nigrescent cataract forms (more common in developing countries which suggests environment of nutritional factors are important)
66
Q

Loss of lens transparency with increased light scattering is caused by:

A
  1. Disruption of lens fiber cell structure
  2. Increased protein aggregation
  3. phase separation lens cell cytoplasm
67
Q

What is the general effect of cataract formation

A

Change in amount of soluble to insoluble lens protein.

The process of change of soluble to insoluble is by CROSS LINKING

68
Q

What are the three types of age-related cataracts

A
  1. Nuclear (Occur in older fiber cells formed during embryonic and fetal life)
  2. Cortical (Cells formed later in life)
  3. Posterior Subcapsular (result from light scattering by a plaque of swollen cells at the posterior pole of the lens.)
69
Q

Which type of cataract is associated with increased oxidative damage to lens proteins and lipids?

A

Nuclear cataract

70
Q

What is the onset of nuclear cataract formation associated with

A

Increased refractive power of the lens

71
Q

What is second sight

A

A temporary improvement in near vision for hyperopoic patients that undergo a myopic shift

72
Q

What procedure can cause a nuclear cataract to form and why

A

Vitrectomy - With a removed vitreous the lens is exposed to increased oxygen –> oxidative stress

73
Q

T/F During nuclear cataract formation, changes in protein organization leading to cataract are subtle

A

TRUE

74
Q

T/F a small change in protein aggregation leads to a small amount of light scattering

A

False; leads to a large amount of light scattering

75
Q

What is the senile cataract hypothesis

A

HM fractions grow into the cell cytoplasm

76
Q

Which type of cataract occurs in the mature fiber cells lying close to the lens surface

A

Cortical cataract

77
Q

Where do cortical cataracts most often occur

A

in the inferior nasal quadrant

78
Q

T/F a person can have cortical cataract for years without it affecting visual function

A

True, because it early on, the opacity is limited to the periphery

79
Q

What are cortical spokes

A

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

80
Q

What type of cataract is caused by light scattering in a cluster of swollen cells at the posterior pole of the lens, just beneath the capsule

A

Posterior Subcapsular Cataract

81
Q

With a PSC, where is there increased disruption

A

In the optical axis, near vision is affected more when compared to distance vision

82
Q

T/F Patients have a combination of nuclear, corticol & PS cataracts

A

True

83
Q

What can a cataract develop into if it is not removed

A

morgagnian (Effects the entire lens)

84
Q

What is Extracapsular Cataract Extraction

A

Remove a portion of the anterior lens epithelium and capsule to expose the underlying fiber mass; extract the nuclear and cortical fiber cells, and implant a plastic IOL in the capsular bag.

85
Q

What is a common complication of Extracapsular Cataract Extraction

A

Formation of secondary cataracts that is posterior capsular opacification (PCO)

86
Q

What is Intracapsular Cataract Extraction

A

When the entire lens is removed to remove the cataract; still common in developing countries, rare in most places

87
Q

What are Elschnig’s pearls

A

When epithelial cells migrate further onto the posterior caspsule after surgery that may differentiate into small lentoid bodies

88
Q

What are congenital cataracts

A

Cataracts present at birth or that appear soon after birth (can be caused by Rubella)

89
Q

What are anterior polar cataracts

A

Another early onset cataract that usually involve the formation of an opaque plaque near the center of the lens epithelium

90
Q

What are some hereditary syndromes associated with cataract formation

A

Galactokinase deficiency and galactosemia

91
Q

In galactosemia and galactokinase deficiency, what accumulates to high levels in the body

A

Sugar galactose

92
Q

Conversion of glucose or galactose to sugar alcohols is via which pathway

A

Sorbitol

93
Q

What has been linked to the formation of cataracts

A
  • A high lifetime exposure to UV light leads to corticol cataracts
  • Long term exposure to infrared light and focused microwaves also can cause cataracts
  • Occupation (Glass blowers cataract)
  • Long term exposure to high-dose steroids inc. the risk of PSC cataract
94
Q

What does increased levels of calcium result in

A
  1. Affects glucose metabolism
  2. inhibition of protein synthesis
  3. Induction of HM aggregates
  4. Direct loss of transparency
  5. Inhibit sodium pump
95
Q

What does hypocalcemia (low calcium) result in

A

Related to dependency of the membrane permeability to levels of calcium
Marked electrolyte and water imbalance in low calcium environment

96
Q

People with diabetes are at increased risk for…

A

early onset cataracts

97
Q

What are the sequence of events for diabetes and lens- osmotic hypothesis

A
  1. Glucose is at toxic level in the lens.
  2. Alodose reductase activated and converts glucose to sorbitol
  3. Sorbitol gets trapped and lens and cannot escape lens…which causes influx of water into the lens
  4. Polyl dehydrogenase tries to get rid of sorbitol by converting it to fructose (slow)
  5. High intracellular osmotic pressure
  6. Cells burst debris..becomes manifestation of cataract
98
Q

If someone complains about fluctuating vision

A

Ask if they have diabetes and if it is under control

99
Q

What are some other risk factors for cataracts

A
  1. Age
  2. Low socioeconomic status
  3. Females
  4. Smoking and high alcohol consumption (dose dependent)
  5. Dark iris color (higher levels of corticol cataracts in african americans)
100
Q

What research plays a role regarding cataract formation

A
  1. Studying genetics
  2. Moderate caloric restriction reduces cataract incidence and progression
  3. Nutrition

Ocular Physiology 2 (8 decks)

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