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A&P > Lens > Flashcards

Lens Flashcards

1
Q

Where is the crystalline lens located?

A
  • within posterior chamber
  • anterior to vitreous chamber
  • posterior to iris
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2
Q

How is the posterior lens surface attached to anterior vitreous face?

A
  • by the hyaloid capsular ligament (circular ring adhesion)
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3
Q

What is the within the hyaloid capsular ligament ring ?

A
  • area of non adhesion
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4
Q

What is the anterior and posterior radius of curvature of the lens?

A

anterior radius curvature = 8 to 14μm
posterior radius curvature = 5 to 8μm

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5
Q

What is the thickness of an unaccommodated lens?

A

3.5 to 5mm

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6
Q

What is the adult lens diameter?

A

9mm

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7
Q

How much does lens thickness increase each year?

A

0.02mm

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8
Q

What is the refractive power of an unaccommodated lens?

A
  • 20 diopters
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9
Q

What does the refractive power of the lens depend on?

A
  • surface curvature
  • refractive index
  • change in index between between the lens and surrounding environment
  • length of optical path
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10
Q

Why does the lens have a gradient refractive index?

A
  • because of changes in optical density throughout the lens
  • index increases from anterior to the centre of lens
  • index decreases towards posterior surface
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11
Q

What is the refractive index a factor of?

A
  • protein concentration within the lens fibers
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12
Q

What is the power of an accommodated lens?

A
  • 14D at ages 8 to 12
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13
Q

When does accommodative power approach zero?

A
  • after 50 years
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14
Q

What is the lens capsule?

A
  • transparent envelope that surrounds entire lens
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15
Q

What kind of membrane is the lens capsule?

A
  • basement membrane
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16
Q

Where is the lens capsule the thinnest?

A
  • at the posterior pole
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17
Q

Where is the lens capsule the thickest?

A
  • anterior pole
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18
Q

Which part of the lens capsule increase in thickness with age?

A
  • anterior pole
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19
Q

What is the lens capsule composed of primarily?

A
  • collagen
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20
Q

How is the lens capsule highly elastic?

A
  • contains no elastic fibers
  • due to arrangement of lamellar fibers
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21
Q

What is the outer superficial zone of the capsule called?

A
  • zonular lamella
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22
Q

What does the zonular lamella consist of?

A
  • zonules interconnected with matrix
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23
Q

What is the anterior lens capsule produced by?

A
  • anterior epithelium
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24
Q

What do the anterior lens epithelium do?

A
  • secrete anterior capsule throughout life
  • site of metabolic transport mechanisms
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25
Q

Why is there no posterior epithelium present?

A
  • was used during embryologic development to form primary lens fibers
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26
Q

What holds the lens epithelial cells together?

A
  • small number of tight junctions
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27
Q

What is the germinal zone of the epithelium?

A
  • band of cells in the pre-equatorial region anterior to the equator
  • location of mitosis
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28
Q

What happens once a lens epithelial cells divided?

A

1) cells divide
2) daughter cell migrates posteriorly towards equator
3) stops cell cycle
4) differentiates into a lens fiber

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29
Q

How does a newly formed epithelial cell become a lens fiber?

A
  • basal part stretches towards posterior pole
  • apical part stretches’ towards anterior pole
  • cellular nuclei move with cytoplasm
  • as elongates, loses organelles and becomes lens fiber
30
Q

How are the lens fibers laid?

A
  • new fibers laid on top of older ones
  • concentric layers of secondary lens fibers
31
Q

What is the cross section shape and size of a lens fiber?

A
  • hexagonal
  • 3 by 9um
32
Q

What are crystallin’s?

A
  • proteins in the lens fiber cytoplasm
33
Q

What is the concentration of crystallins in the lens fiber?

A
  • 40% net weight
  • 15% in cortex
  • 70% in nucleus
34
Q

What is the lens fiber cytoskeleton made up of?

A
  • microtubules and filaments
35
Q

How do the lens fiber cytoskeleton provide stability?

A
  • is anchored to the plasma membrane
36
Q

What are some of the epithelium lateral membranes interdigitations shapes?

A
  • ball and socket
  • tongue and groove
37
Q

What do the interdigitation shapes of the epithelium lateral membranes allow for?

A
  • allow for sliding fibers
38
Q

What are the lens fibers joined by?

A
  • desmosomes
39
Q

What does the distribution and concentration of crystallin’s contribute to?

A
  • gradient refractive index
40
Q

Why is fiber to fiber and cell to cell mechanisms of communication necessary in the lens fibers?

A
  • lens has no vascular supply
  • fibers lose their organelles as they age
41
Q

What allows for the movement of nutrients and ions within the lens?

A
  • network of gap junctions along lateral fiber membranes
42
Q

How are the gap junctions of the lateral fiber membranes distributed?

A
  • few near poles
  • more towards equator
  • fewer junctions deeper layers
43
Q

What are micropinocytic vesicles?

A
  • at basal and apical parts of the fiber membranes
  • membrane fusion allows for movement from fiber to fiber
44
Q

What is the epithelium fiber interface?

A
  • the border between the apical membrane of the anterior epithelium and the apical membrane of the elongating fiber
45
Q

What forms the very center of the lens?

A
  • primary lens fibers from the elongated posterior epithelium
46
Q

What is the very center of the lens called?

A
  • embryonic nucleus
47
Q

What are the fibers that surround the primary lens fibers?

A
  • secondary lens fibers
48
Q

Where are the secondary lens fibers formed from?

A
  • cell mitosis at pre-equatorial region of epithelium
49
Q

What is a suture?

A
  • junction formed when lens fibers reach the poles and meet the other fibers in their layer
50
Q

What are the anterior and posterior sutures formed by?

A
  • anterior suture formed by the joining of apical fibers
  • posterior suture formed by joining of basal fibers
51
Q

What forms Y sutures?

A
  • secondary fibers meet in three branches
52
Q

What is the anterior suture shape?

A
  • upright Y
53
Q

What is the posterior suture shape?

A
  • inverted Y
54
Q

What happens to the sutures as the lens grows?

A
  • become asymmetric
55
Q

What is the lens attached to the ciliary body by?

A
  • zonules (of Zinn)
56
Q

What are the Zonules of Zinn categorized as?

A
  • microfibrils
57
Q

Where do the Zonules of Zinn arise from?

A
  • basement membrane of the nonpigmented ciliary epithelium in the pars plana
  • and valleys between the ciliary processes in the pars plicata
58
Q

What happens during accommodation when the eye is viewing a distant object?

A
  • ciliary muscle is relaxed
  • ciliary ring diameter is large
  • zonules stretched exterting tension on the lens capsule, holding the lens in a unaccommodated state
59
Q

What happens during accommodation when viewing a object thats near?

A
  • ciliary muscle contracts
  • diameter of ciliary ring surround lens decreases
  • tension reduced on zonules holding lens
  • lens forms more spherical shape
60
Q

What features of the lens allow for transparency?

A
  • absence of blood vessels
  • few cellular organelles
  • orderly arrangement of fibers
  • short distance between components of different indices relative to wavelength of light
61
Q

Why do the lens cells require significant energy?

A
  • high metabolic activity occurs in anterior epithelium maintaining cell and fiber function
  • pre-equatorial region high level miotic activity
62
Q

Where does the lens get the majority of its nutrients from?

A
  • the surrounding aqueous and vitreous
63
Q

What is the energy required for cellular metabolism and cellular replication within the lens?

A
  • anaerobic glycolysis
64
Q

Why is the lens epithelium rich in transport mechanisms?

A
  • get nutrients from surrounding aqueous
  • maintain electrolyte balance
65
Q

How can ultraviolet absorption affect the lens?

A
  • produces oxidative changes within tissue causing formation of free radicals
  • free radicals cause disruption to cellular processes and cause cellular damage
66
Q

What are the purposes of the lens capsule?

A
  • mould lens
  • enclose lens contents
  • prevent large molecules such as albumin and haemoglobin entering lens
  • permeable to water, small solutes and proteins
67
Q

What is the water and protein percentage of the lens?

A
  • 65% to 70% water
  • 30% to 35% protein
68
Q

What is the refractive index of the lens cortex and nucleus?

A
  • cortex 1.38
  • nucleus 1.41
69
Q

Whats the length of an outer fiber?

A
  • 1cm
70
Q

What percentage of the proteins in the lens are water soluble crystallin’s?

A
  • 90%

A&P (7 decks)

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