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Physiology of the Eye > Lens > Flashcards

Lens Flashcards

1
Q

What are some key differences between the lens and cornea regarding solubility of proteins, transparency, source of nutrition.?

A
  • lens depends on surrounding aqueous humor, where cornea has tears and aqueous humor
  • lens consists of mostly soluble proteins and low levels of glycoproteins, where the cornea consists mostly of insoluble collagen and high levels of glycoproteins
  • transparency of lens depends on arranged nature of lens fibers, but also on the solubility and physical arrangement of lens proteins, where the cornea relies mostly on structural arrangement for transparency.
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2
Q

What is the lens’ extracellular matrix, and what is it composed of?

A

anterior and posterior lens capsule (type 4 collagen and haparitin sulfate glycoprotein)

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3
Q

The lens is what % protein?

A

35% (65% water)

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4
Q

What does the lens consist of?

A

water, protein, lipids, inorganic ions, carbohydrates, ascorbic acid, glutathione, and amino acids.

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5
Q

When does protein synthesis cease in the lens?

A

When lens epithelial cells become lens fibers protein synthesis stops, and all changes to proteins beyond that point are from posttranslational modification.

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6
Q

What are the two classes of lens proteins?

A
  • soluble (crystalline)

- insoluble (albumanoid)

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7
Q

What percentage of proteins are soluble vs insoluble in the normal lens?

A

85% are soluble (crystalline), compared to 15% insoluble (albumanoid). This ration changes with age (soluble decreases) as well as by species and lens pathology.

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8
Q

In what part of the lens are most of the insoluble proteins concentrated?

A

nucleus

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9
Q

What structures are the insoluble and soluble proteins associated with within the lens?

A
  • insoluble proteins are associated with with the membrane of lens fibers.
  • soluble proteins make up the refractive fibers, and are considered the structural proteins of the lens.
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10
Q

What are the four groups of soluble lens proteins, and how are they classified?

A 
- classified based on molecular size
1 - alpha
2 - beta heavy
3 - beta light
4 - gamma crystallins
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11
Q

What group of soluble lens proteins is found in birds and reptiles, and is the major lens protein in the developing chicken?

A

delta crystallin

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12
Q

In the mammalian lens, what percentage to each group of proteins contribute overall?

A

gamm and alpha crystallins account for 50%, and beta crystallins account for the remaining 50%

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13
Q

Which group of crystallins appear to be conserved between species?

A

beta

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14
Q

In the canine lens, what happens to the proportion of lens proteins during aging?

A

alpha and beta light crystallins increase, while gamma and beta heavy crystallins decrease (*changes appear to be accelerated in age matched cataracts lenses)

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15
Q

What happens to proteinase activity in aging lenses, and how might proteinase activity also affect the lens?

A
  • increase with age

- proteinases may be responsible for spontaneous cataract resorption

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16
Q

What allows the lens fibers to act as a syncytium?

A

The interdigitations between lens fibers are also specialized gap junctions

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17
Q

Name the cytoskeletal proteins of the lens, and where do they occur.

A
  • actin
  • vimentin
  • spectrins
  • beaded filaments (lens specific intermediate filament)
    (occur in the urea-extractable fraction of the lens)
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18
Q

What is the most abundant intermediate filament in the human lens, and where does it occur?

A

Vimentin - occurs in the epithelial and cortical fiber cells

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19
Q

What are the two classes of beaded filaments, and what is their proposed function?

A
  • 90 and 48 kD filaments

- likely play a role in crystallin packing, density distribution, and attachment sites for crystallin molecules

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20
Q

Ture of False: lens proteins are immunologically organ-specific, but not species-specific?

A

true

21
Q

What type of proteins are the human lens capsule permeable to?

A

small molecular weight proteins less than 50,000 kD. (this includes low molecular weight crystallines)

22
Q

What is the potential significance of the annular pad in the avail and reptilian lens?

A

This thickened region may modify the active transport of cations across the lens capsule (sodium out and potassium in)

23
Q

In what species are ascorbic acid levels lower in the lens compared to the aqueous humor?

A

dog, rabbit, guinea pig

24
Q

What are the proposed functions of lenticular ascorbic acid?

A
  • facilitate in oxidation-reduction actions

- coupled to glutathione metabolism

25
Q

True or false, glutathione levels are typically present at low levels in the lens?

A

false (typically present at high levels)

26
Q

What is glutathione?

A

antioxidant tripeptide synthesized in the lens which is composed of l-glutamate, l-cysteine, and glycine

27
Q

What are some of the biologic functions of glutathione?

A
  • furnished sulfhydryl groups to help preserve lens solubility, and for the Na+, K+, ATPase reaction (keep the pump working and water out of the lens)
  • participation in amino acid transport
  • substrate for glutathione peroxidase (destroys cytotoxic lipid hydroperoxidase)
28
Q

In a normal lens, what form does glutathione typically exist in?

A 
reduced form (GSH)
(level of oxidized for is typically 2.1-2.6% that of the reduced form)
29
Q

What happens to the levels of both reduced and oxidized glutathione in the cataracts lens?

A

decreased, except in advanced cataracts where oxidized for is 9% of reduced form

30
Q

What are the mechanical functions of the lens capsule?

A

maintains the shape of the lens in association with accommodation, and is the attachment point for the zonules

31
Q

What appear to exist in the lens epithelial and cortical cells that may aid in stabilizing the shape of the lens?

A

contractile system

32
Q

What is the source of energy for the lens?

A

glucose from the aqueous humor

33
Q

From what type of metabolic process is energy derived in the lens, and what are the two main uses fro the derived energy?

A
  • anaerobic glycolysis

- active cation transport and protein synthesis

34
Q

Is oxygen necessary for normal lens metabolism?

A

No (though a small percentage of glucose is metabolized through the Kreb’s Cycle)

35
Q

What are two other pathways of glucose metabolism in the lens

A
  • hexose monophosphate (pentose)

- sorbitol pathway

36
Q

What is the major end product of glucose metabolism in the lens?

A
  • lactic acid (diffuses into the aqueous humor)
37
Q

What two factors control the rate of glycolysis in the lens?

A
  • rate that glucose diffuses into the lens

- amount of hexokinase enzyme

38
Q

What happens to glucose metabolism at high levels of glucose within the lens, and how might this be protective for the lens?

A
  • at levels greater than 175 mg/dL, glucose-6-phosphate is increased, which inhibits the hexokinase pathway limiting the rate of glycolysis, and glucose gets shunted to other metabolic systems.
  • this likely protects the lens by limiting the amount of lactic acid build up, which can activate proteases within the lens.
39
Q

Which 4 factors play a role in cataract formation from oxidative damage?

A
  • oxygen radicals
  • peroxide
  • hydroxyl groups
  • ultraviolet radiation
40
Q

What are some key antioxidants in the lens?

A
  • glutathione
  • catalase
  • superoxide dismutase
  • ascorbate
41
Q

Transport molecules for what antioxidant are found in the human lens that assures adequate metabolism and helps protect agains damage by free radicles?

A

ascorbate

deficiencies in this transport molecule may lead to cataracts changes in the lens

42
Q

Lens epithelial cells in a cataractous lens may undergo what process, leading to plaques and ectopic membrane of fibrous tissue to form on the lens capsule?

A

metaplasia (epithelial to mesenchymal transformation)

43
Q

What happens to the image in an aphasic eye?

A

the image is out of focus, right-side up, reversed and enlarged

44
Q

Does the loss of a lens appear to be clinically significant in domestic species?

A

No

45
Q

What are the differences in growth rate between the human lens and the lens of lower mammals?

A
  • humans: growth rate slowly decreases with age

- lower mammal: fast in early stages of life, little to no growth in later stages

46
Q

What are the differences in hydration status between the human lens and the lens of lower mammals?

A
  • humans: maintained at 65% water throughout life

- lower mammals: dehydrates with age (35-50% water)

47
Q

What are the differences in accommodation between the human lens and the lens of lower mammals?

A
  • humans: present

- lower mammals: little to none

48
Q

What are the differences in color between the human lens and the lens of lower mammals?

A
  • human: pale yellow

- lower mammals: colorless

49
Q

What are the differences in fluorescent polypeptides between the human lens and the lens of lower mammals?

A
  • human: present

- lower mammals: absent

Physiology of the Eye (10 decks)

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