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Ocular Physiology Exam #2 (part 2) > Lens > Flashcards

Lens Flashcards

1
Q

Describe the lens and its function.

A

Specialized epithelial tissue

Responsible for fine-tuning an image that is projected onto the retina.

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2
Q

State the requirements of the lens to perform necessary functions.

A

In order to do this, the lens must:
Be transparent
Have a higher refractive index than the medium its suspended in
Have flexible refractive surfaces with the proper curvature.

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3
Q

How does a cataract form?

A

Disruption of precise organization of the lens fiber cells or damage to the proteins – cataract formation

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4
Q

What is the lens epithelium made of and where is it located?

A

a sheet of cubodial cells.

located at the anterior of the lens closest to the cornea.

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5
Q

The bulk of the lens consists of what types of cells?

A

elongated fiber cells

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6
Q

what is the lens capsule? where is it secreted?

A

ECM (elastic extracellular matrix) that surrounds the lens. Secreted by epithelial and superficial cells.

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7
Q

Most epithelial cells in the lens DO NOT divide. T or F

A

True.

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8
Q

where is the exception in the lens epithelium that cells do germinate or divide?

A

at the equatorial margin of the lens. Called the germinative zone.
These new fibers as they grow in and squeeze the older fibers in. occupy a larger and larger part of anterior chamber.
space is limited in the eyeball.

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9
Q

what is the germinative zone?

A

the one place, at the equatorial margin of the lens, where cells divide in the lens epithelium.

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10
Q

where are the sutures of the lens located and what do they look like?

A

junctions between apical and basal ends of the cells from the opposite side of the lens.
Y shaped both inverted and upright.

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11
Q

when do fibers cells stop elongating?

A

when they reach sutures. and the basal ends detach from capsule.

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12
Q

how is a cataract named?

A

according to its location.
for example:
sutural cataract
posterior cortical cataract

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13
Q

Describe the process of cell differentiation in the lens.

A

Mature fiber cells are gradually buried deeper in the lens as successive generations of fibers elongate and differentiate. In this way the lens continues to increase in size and synthesis ceases just before organelle degradation, the components of mature fiber cells must be much more stable than those in cells found in other parts of the body.

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14
Q

The components of mature fiber cells must be much more stable than those in cells found in other parts of the body.
T or F

A

True

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15
Q

what are zonules?

A

The inelastic microfibrils that suspend the lens

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16
Q

What are zonules originated and where do they insert in the lens?

A

Originate in the non-pigmented layer of the ciliary epithelium and insert into the lens capsule near the equator.

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17
Q

lens are suspended in its place by what?

A

zonules.

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18
Q

what causes lens curvature changes during accommodation?

A

Changes in zonule tension cause lens curvature changes during accommodation.

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19
Q

what is the refractive nature or properties of the lens?

A

Due to the high concentration of crystallins in the cells and the curvature of the surfaces
Crystallins are 3x higher than in typical cells
This increases the refractive index of the lens

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20
Q

Younger people refractive error is caused by….?

A

1) corneal curvature or the
2) length of the globe-
3) rarely by defects of curvature or refractive index of the lens

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21
Q

person with extremely steep cornea, longer eyeball…..

A

myopia

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22
Q

person with flat cornea, shorter eyeball……

A

hyperopia

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23
Q

transparency of the lens depends on what two things?

A

1) Depends on minimizing light scattering and absorption
2) Light passes smoothly through due to:
Regular structure of lens fibers
The absence of membrane-bound organelles
Small/uniform extracellular space between fiber cells
3) Young individuals
human lens is nearly colorless –
4) Aging process
Lens absorbs short wavelength light

Absorbs blue light

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24
Q

If water gets into the lens this can cause what?

A

rainbow halo

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25
Q

what type of light does the lens absorb?

A

short wavelengths of light. Blue light.

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26
Q

what happens to the lens as it ages?

A

yellowing of the lens. This happens as a protective mechanism for the lens. Trying to prevent blue light from getting to retina.

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27
Q

what do you lose when the eye begins to yellow with age?

A

Color perception goes away. Also vision is loss. There is a decline in vision, specifically what distinguishes contrast is lost.

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28
Q

Is blue light good for the eye to absorb?

A

short wavelength = high energy = no good for eye.

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29
Q

what is aphakia?

A

absence of crystalline lens from either natural causes or removal.

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30
Q

what is congenital aphakia?

A

(born like this)- Rubella infection in first 4 weeks of pregnancy (cataract formed in new born infant) mother may have had infection during pregnancy. Maturation in PAX 6 gene.

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31
Q

what is pseudophakia?

A

substitution of natural crystalline lens with a synthetic lens.

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32
Q

Lens fiber differentiation depends on synthesis and accumulation of large amounts of crystallins. T or F

A

True

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33
Q

Crystallines make up how much wet weight of the lens fiber?

A

As much as 40% of the wet weight of the lens fiber cell can be accounted for by crystallins, a protein concentration that is approximately three times higher than that in the cytoplasm of typical cells.

34
Q

what are the components of a “classical crystalline lens”?

A

α-crystallin family and β/γ-crystallin superfamily

All vertebrae lenses accumulate these

35
Q

Describe Taxon-specific crystallins. Who are they found in?

A

Taxon-specific crystallins are functional enzymes or proteins that are structurally similar to enzymes but that lack enzymatic activity.
Adult human lenses do not produce taxon-specific crystallins, although the enzyme

betaine-homocysteine methyltransferase

is present at high levels in the embryonic nucleus, indicating that this enzyme serves as a taxon-specific crystallin during the early development of the human lens.

36
Q

Taxon specific crystallins are found in adults. T or F

A

False. Adult human lenses do not produce taxon-specific crystallins, although the enzyme

betaine-homocysteine methyltransferase

is present at high levels in the embryonic nucleus, indicating that this enzyme serves as a taxon-specific crystallin during the early development of the human lens.

37
Q

Humans express what two Alpha crystallines? What is their role?

A

Humans express αA and αB

Their role is to prevent protein aggregation and precipitation.

38
Q

Analysis of αA knockout mice: protein aggregates show large amounts of αB-crystallin and smaller amounts of other proteins
Lens smaller than normal-structurally similar though
Formation of cataract in first few weeks.
T or F

A

True.

39
Q

Alpha A is not needed to control Alpha B. T or F

A

False.

40
Q

If Alpha A is being seen then Alpha B can be detected. True or False.

A

True

41
Q

where is alpha B crystalline present? what if its function?

A

αB-crystallin is present in a variety of cells throughout the body, not just the lens
Found in Heart and skeletal muscle
αB-crystallin has important chaperone functions in the lens

42
Q

The β/γ superfamily is more diverse
Function is not known
T or F

A

True

43
Q

How many β- and γ-crystallins are found in the lens?

A

6 β- and 3 γ-crystallins are found in the lens

44
Q

the crystalline lens is made up of how much water and organic matter?

A

65% water

35% organic matter

45
Q

how much of the organic matter is made up of structural proteins?

A

33%

46
Q

Insoluble proteins in lens change with age as they develop cataracts. T or F

A

True.

47
Q

The lens is made up of insoluble and soluble proteins. T or F. Which is the majority of the lens?

A

True. soluble makes up the majority 84.3%

insoluble 15.7%

48
Q

where are the microtubules in the lens located and what is there function?

A

Abundant beneath membranes of lens fiber cells
Important roles in:
1) stabilizing the fiber cell membrane
2) May be transporting vesicles to the apical and basal ends of elongating fiber cells
3) Also gives lens its birefringent property

49
Q

what does birefringent mean?

A

Property by which polarized light rays is split and travel at different phases= birefringent.

property of any tissue that changes polarized light, they travel at different phases and speeds. May have delay in their speed.
The structures are cornea, crystalline lens, retina, and sclera are all birefringent all these line up one another.

50
Q

why is birefringence important?

A

you can measure retinal thickness based on how much birefringence there is.

51
Q

Describe the lens fiber cell membrane.

A

1) Have an unusual lipid composition
2) High proportion of cholesterol and sphingomyelin compared to other plasma membranes-highest proportion of cholesterol.
Causes the cell membranes to be RIGID

3) Cholesterol increases as the cells mature.
3x greater in nuclear fibers compared to the cortical fibers.

Cause cells to be rigid and gives it rigidity.
Nucleus is harder and cortex more pliable.

52
Q

Growth of the lens is rapid during when?

A

embryo and 1st post natal year.

53
Q

Growth of the lens slows down during what age range?

A

age 1-10 years old. Growth continues at a slower linear rate throughout life.

54
Q

Explain the lens development vasculature.

A

Soon after the lens is formed, it becomes invested with a meshwork of capillaries. In the posterior of the lens, this network, the tunica vasculosa lentis, arises from the hyaloid artery. The capillaries at the anterior of the lens arise from blood vessels of the developing iris stroma to form the anterior papillary membrane.
During the second trimester of human development, the capillaries of the tunica vasculosa lentis and the anterior papillary membrane regress.

55
Q

As eye is forming the presence of one structure triggers another. The presence of the lens is a trigger for other parts of the eye to develop to completion.
T or F

A

True.

56
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

57
Q

The lens gets most of its energy from where?

A

glycolysis

58
Q

The end product of glycolysis is what?

A

lactic acid

59
Q

Intracellular pH drops significantly from peripheral to deeper fiber cells secondary to lactic acid accumulation. T or F

A

True

60
Q

what drops pH significantly?

A

Lactic acid

61
Q

Molecular oxygen is the source of most oxidative damage
The low oxygen tension around/within the lens probably protects it from oxidative damage.
T or F

A

True

62
Q

Lots of oxygen exposure is good for the lens. T or F

A

False.

63
Q

Despite the low oxygen level, the lens derives a substantial proportion of its ATP form oxidative phosphorylation- a process that generates what?

A

free radicals, which are produced to decrease oxygen in lens.

64
Q

Much of the solar irradiation that reaches the eye is absorbed by what part of the eye? Where is the remaining UV light going to have a harmful effect?

A

The cornea.
UV light have harmful effect on the lens.
If light is not absorbed, it produces no damage.

65
Q

DNA, proteins, nucleoside-containing metabolites, flavonoids, and pigments all absorb UV light
Thus potential sources of free radicals
T or F

A

True.

66
Q

what is glutathione? and what is its function?

A

Glutathione a tripeptide of the amino acids glutamine, cysteine, and glycine, provides most of the protection against oxidative damage in the lens. When glutathione levels have been lowered in lens epithelial cells or whole lenses, cell damage and cataract formation follow rapidly.

67
Q

what is ascorbic acid?

A

provides oxdative protection to the lens.

68
Q

where is ascorbic acid actively transported from?

A

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

69
Q

How much more ascorbic acid is there in the aqueous humor than in the blood?

A

20 times more

70
Q

[O2] within/around the lens is much higher than other parts of the body. T or F

A

False. lower.

71
Q

The lens depends on what to produce ATP?

A

Glycolytic metabolism to produce ATP.

72
Q

The glucose required comes from what part of the eye?

A

aqueous humor via facilitated diffusion across ciliary epithelium.

73
Q

Lens epithelial and superficial fiber cells also have numerous mitochondria (to generate energy). T or F

A

True.

74
Q

cells near the surface of the lens may use what two pathways to derive energy from glucose?

A

oxidative and glycolytic pathways

75
Q

The lens should maintain a high and/ or low amount of K+, Na+, Cl-, and water?

A

Lens should maintain:
High K+,
low Na+, Cl- and water

76
Q

What would happen if sodium moved into the lens?

A

If sodium moved into lens that is increase in solutes, water content will increase (osmotically)

77
Q

Describe how much Na+ and K+ is in the concentration gradient. which one is high and which one is low?

A

Concentration gradient

Aqueous has more Na+, low K+

78
Q

Describe the electrical gradient.

A

Lens is electronegative inside.

79
Q

The water electrolyte balance is maintained by what?

A

sodium potassium pump

80
Q

You want the aqueous humor to bring in glucose.
If more than what’s needed amount of glucose gets in, you can have cataract formation.
T or F

A

True

81
Q

What does the sodium potassium pump do? Where does it operate? For every Na+ that is pumped in how many K+ removed?

A

Operates in the epithelium- which has mitochondria
Every 3 Na+ removed 2K+ is allowed.
Requires energy!

Ocular Physiology Exam #2 (part 2) (8 decks)

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