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OD1 - Integrated Ophthalmic Sciences & Preclinical Optometry > BS - Lens Structure & Metabolism 1 - Week 5 > Flashcards

BS - Lens Structure & Metabolism 1 - Week 5 Flashcards

1
Q

Describe the percentage composition of the human lens.

A

61% water
33% protein
6% other

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2
Q

Does the lens have any neurovascular supply?

A

No, it is aneurovascular.

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3
Q

Are lens cells shed?

A

No, they divide, but are not shed.

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4
Q

What are two age related changes to the lens?

A

Cataract

Presbyopia

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5
Q

Define the following on a lens:
Anterior/posterior pole
Equator

A

Anterior pole - centre of the anterior surface
Posterior pole - centre of the posterior pole
Equator - midway between the two poles, where zonules attach

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6
Q

Describe the lens capsule’s structure.

A

Elastic basement membrane that envelops the lens.

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7
Q

Where on the lens is the capsule thickest?

A

At the anterior and equatorial surfaces.

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8
Q

What is the dominant collagen type in the lens capsule?

A

Type IV

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9
Q

Where are the epithelial cells of the lens located?

A

Beneath the capsule only on the anterior surface.

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10
Q

The lens epithelium is divided into what three regions? Describe the regional variations between them.

A 
Central zone
-No division occurs
Intermediate zone
-Smaller zone, occasional mitosis
Germinative zone
-Mitosis, elongation, and differentiation occurs here
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11
Q

How do the nuclei appear histologically on a cross section of the lens? Describe why this formation occurs.

A

It has a hairpin formation.
As the cells migrate to the equator, they elongate once they reach it, and the nucleus is drawn anteriorly, resulting in a hairpin look.

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12
Q

Concerning the epithelium, what is vital to maintaining lens transparency?

A

Gap junctions between cells.

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13
Q

What are the lateral membranes between lens epithelial cells like?

A

They are highly folded, contributing to the tight connection between them.

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14
Q

What forms the main bulk of the lens?

A

Lens fibres

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15
Q

What happens to lens fibres as they enter the lens cortex?

A

They lose their nucleus and organelles, and increase protein synthesis.

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16
Q

What shape do lens fibre cross sections have?

A

They are hexagonal, and form rows.

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17
Q

Compare the lens diamter anteriorly vs posteriorly. What is the purpose of this?

A

They are thinner posteriorly, giving the lens its shape.

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18
Q

Where do the tips of the lens fibres meet?

A

At the sutures.

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19
Q

What structure do the lens sutures have in newborns? What is it like in adults?

A

Its forms a Y anteriorly, and a ⅄ anteriorly.

In adults, it is highly branched, and the number of arms increases.

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20
Q

Where do zonules of ciliary bodies attach?

A

The superficial 8-10 layers of the anterior cortex of the lens.

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21
Q

What kind of joint is formed by the zonules and the lens? How do the joints change in deeper layers?

A

It is a ball and socket joint.

In deeper layers, the joints are smaller.

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22
Q

What are zonules composed of? What is the major collagen type?

A

Has fibrillar and non-fibrillar components.

Major collagen type is IV

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23
Q

Where do anterior zonules begin and insert to? What diameter do they have?

A

Runs from the pars plana to the pre-equatorial lens, with large diameters.

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24
Q

Where do posterior zonules begin and insert to? What diameter do they have?

A

Runs from the pars plicata to the post-equatorial lens, with large diameters.

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25
Q

Where do equatorial zonules begin and insert to? What diameter do they have?

A

Runs from the pars plicata to the ens equator, with small diameters.

26
Q

What 3 features of the lens contribute to its transparency?

A

No organelles/nuclei
No blood supply
No cellular turnover

27
Q

Does the lens play any role in filtering light?

A

Yes, it filters out smaller wavelengths that degrade the image.

28
Q

Where does the lens get its nutrient supply from?

A

The aqueous humour, providing glucose, ions and lactate.

29
Q

Are fibre cells alive?

A

No, they are just sacks of protein

30
Q

What is the major source of energy for the lens, and what happens in its absence?

A

Glucose.

Opacity results from its absence.

31
Q

How does the lens uptake glucose?

A

Facilitated diffusion

32
Q

What are the three major metabolic pathways for glucose in the lens?

A

Glycolysis and TCA cycle
Pentose pathway
Polyol pathway

33
Q

What percentage of glucose is used in the pentose pathway?

A

10-20%

34
Q

What pathway is a major source of lens CO2?

A

Pentose pathway.

35
Q

Which pathway is an important source of NADPH?

A

Pentose pathway.

36
Q

What is they key purpose of NADPH?

A

Resisting oxidative stress.

37
Q

Too much glucose increases activity of which pathway?

A

Polyol pathway

38
Q

What happens to excess glucose via what pathway?

A

Converted to fructose via the polyol pathway.

39
Q

What can increased activity of the polyol pathway lead to an accumulation of, and what is a consequence of this? Which population is this of particular importance in?

A

Accumulation of sorbitol, leading to increased osmotic pressure, leading to sugar cataract.
Important pathway in diabetics.

40
Q

How does diabetic/sugar cataract appear on a lens?

A

Snowflake appearance, as it begins primarily at the sutures.

41
Q

What happens when there isnt enough glucose for the lens?

A

Cataracts.

42
Q

What protein channel controls lens ion balance?

What is the movement of ions like?

A

Na+K+ATPase

Na+ transported into the epithelium from the cortex, and actively exchanged for K+ by ATPase

43
Q

Consider the balance of ions in the lens. What is the levels of Na+ and K+ like in the lens, what is the charge of the lens like, and what is the net flow of K+ and Na+?

A
  • Low Na+ and high K+ in the lens
  • Interior of the lens is more negative than outside
  • Na+ passively diffuses through the posterior capsule, actively pumped anteriorly
  • K+ diffuses into the vitreous humour
44
Q

What is the circulation pathway of the lens like?

A

Consider a cross section.
Divided into four quadrants, the flow would run out of the equator, out and around the lens to the pole, and back out the equator.

45
Q

What does an ionic imbalance result in?

A

Leads to an influx of water, resulting in cell lysis.

This is called osmotic cataract.

46
Q

Where does the major protein synthesis in the lens occur?

A

Equatorial region

47
Q

What are the three main lens proteins, and what is their purpose?

A

Needed for the refractive index of the lens.
α-crystallin
β-crystallin
γ-crystallin

48
Q

How is lens transparency maintained despite the high protein content?

A

The interactions between the proteins are uniformly distributed, allowing light transmission.

49
Q

What is the major lens protein? Name its two subunits and their functions.

A

α-crystallin
αA subunit acts as a chaperone preventing agglutination
αB subunit acts as a heat-shock protein

50
Q

What is the largest lens protein?

A

α-crystallin

51
Q

What percentage of lens protein does the β-crystallin form?`

A

30%

52
Q

Where is the β-crystallin formed?

A

Outer cortex

53
Q

Which lens protein has the lowest molecular weight?

A

γ-crystallin

54
Q

Where are γ-crystallins made, and what does a high γ-crystallin content indicate?

A

Made prenatally, found in the lens nucleus.

High γ-crystallin indicates a hard lens

55
Q

What 4 molecules are important for antioxidation in the lens?

A

Ascorbate
Vitamin C
Vitamin E
Glutathione

56
Q

Where is glutathione found in high concentrations, and in what form?

A

Found in the epithelium and the cortex as GSH, its reduced form.

57
Q

How are glutathione and the pentose phosphate pathway related?

A

NADPH produced by the pentose pathway reduces the oxidised form of glutathione, GSSG, to its reduced form, GSH.

58
Q

During natal development, the epithelium of the lens forms on the anterior surface. What happens if the lens is flipped during development?

A

The epithelium migrates from the posterior surface ,to the anterior surface.

59
Q

What factor is critical to the lens development, and what 3 processes is it known to cause?

A 
Fibroblast growth factor 2.
Known to cause
Proliferation
Migration
Differentiation
60
Q

Describe the FGF-2 gradient hypothesis.

A

The concentration increases from anterior to posterior lens, allowing the epithelium to eventually proliferate, migrate and differentiate as it moves posteriorly.

OD1 - Integrated Ophthalmic Sciences & Preclinical Optometry (123 decks)

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