Lens Flashcards
(70 cards)
1
Q
Where is the crystalline lens located?
A
- within posterior chamber
- anterior to vitreous chamber
- posterior to iris
2
Q
How is the posterior lens surface attached to anterior vitreous face?
A
- by the hyaloid capsular ligament (circular ring adhesion)
3
Q
What is the within the hyaloid capsular ligament ring ?
A
- area of non adhesion
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
5
Q
What is the thickness of an unaccommodated lens?
A
3.5 to 5mm
6
Q
What is the adult lens diameter?
A
9mm
7
Q
How much does lens thickness increase each year?
A
0.02mm
8
Q
What is the refractive power of an unaccommodated lens?
A
- 20 diopters
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
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
11
Q
What is the refractive index a factor of?
A
- protein concentration within the lens fibers
12
Q
What is the power of an accommodated lens?
A
- 14D at ages 8 to 12
13
Q
When does accommodative power approach zero?
A
- after 50 years
14
Q
What is the lens capsule?
A
- transparent envelope that surrounds entire lens
15
Q
What kind of membrane is the lens capsule?
A
- basement membrane
16
Q
Where is the lens capsule the thinnest?
A
- at the posterior pole
17
Q
Where is the lens capsule the thickest?
A
- anterior pole
18
Q
Which part of the lens capsule increase in thickness with age?
A
- anterior pole
19
Q
What is the lens capsule composed of primarily?
A
- collagen
20
Q
How is the lens capsule highly elastic?
A
- contains no elastic fibers
- due to arrangement of lamellar fibers
21
Q
What is the outer superficial zone of the capsule called?
A
- zonular lamella
22
Q
What does the zonular lamella consist of?
A
- zonules interconnected with matrix
23
Q
What is the anterior lens capsule produced by?
A
- anterior epithelium
24
Q
What do the anterior lens epithelium do?
A
- secrete anterior capsule throughout life
- site of metabolic transport mechanisms
25
Why is there no posterior epithelium present?
- was used during embryologic development to form primary lens fibers
26
What holds the lens epithelial cells together?
- small number of tight junctions
27
What is the germinal zone of the epithelium?
- band of cells in the pre-equatorial region anterior to the equator
- location of mitosis
28
What happens once a lens epithelial cells divided?
1) cells divide
2) daughter cell migrates posteriorly towards equator
3) stops cell cycle
4) differentiates into a lens fiber
29
How does a newly formed epithelial cell become a lens fiber?
- 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
How are the lens fibers laid?
- new fibers laid on top of older ones
- concentric layers of secondary lens fibers
31
What is the cross section shape and size of a lens fiber?
- hexagonal
- 3 by 9um
32
What are crystallin's?
- proteins in the lens fiber cytoplasm
33
What is the concentration of crystallins in the lens fiber?
- 40% net weight
- 15% in cortex
- 70% in nucleus
34
What is the lens fiber cytoskeleton made up of?
- microtubules and filaments
35
How do the lens fiber cytoskeleton provide stability?
- is anchored to the plasma membrane
36
What are some of the epithelium lateral membranes interdigitations shapes?
- ball and socket
- tongue and groove
37
What do the interdigitation shapes of the epithelium lateral membranes allow for?
- allow for sliding fibers
38
What are the lens fibers joined by?
- desmosomes
39
What does the distribution and concentration of crystallin's contribute to?
- gradient refractive index
40
Why is fiber to fiber and cell to cell mechanisms of communication necessary in the lens fibers?
- lens has no vascular supply
- fibers lose their organelles as they age
41
What allows for the movement of nutrients and ions within the lens?
- network of gap junctions along lateral fiber membranes
42
How are the gap junctions of the lateral fiber membranes distributed?
- few near poles
- more towards equator
- fewer junctions deeper layers
43
What are micropinocytic vesicles?
- at basal and apical parts of the fiber membranes
- membrane fusion allows for movement from fiber to fiber
44
What is the epithelium fiber interface?
- the border between the apical membrane of the anterior epithelium and the apical membrane of the elongating fiber
45
What forms the very center of the lens?
- primary lens fibers from the elongated posterior epithelium
46
What is the very center of the lens called?
- embryonic nucleus
47
What are the fibers that surround the primary lens fibers?
- secondary lens fibers
48
Where are the secondary lens fibers formed from?
- cell mitosis at pre-equatorial region of epithelium
49
What is a suture?
- junction formed when lens fibers reach the poles and meet the other fibers in their layer
50
What are the anterior and posterior sutures formed by?
- anterior suture formed by the joining of apical fibers
- posterior suture formed by joining of basal fibers
51
What forms Y sutures?
- secondary fibers meet in three branches
52
What is the anterior suture shape?
- upright Y
53
What is the posterior suture shape?
- inverted Y
54
What happens to the sutures as the lens grows?
- become asymmetric
55
What is the lens attached to the ciliary body by?
- zonules (of Zinn)
56
What are the Zonules of Zinn categorized as?
- microfibrils
57
Where do the Zonules of Zinn arise from?
- basement membrane of the nonpigmented ciliary epithelium in the pars plana
- and valleys between the ciliary processes in the pars plicata
58
What happens during accommodation when the eye is viewing a distant object?
- 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
What happens during accommodation when viewing a object thats near?
- ciliary muscle contracts
- diameter of ciliary ring surround lens decreases
- tension reduced on zonules holding lens
- lens forms more spherical shape
60
What features of the lens allow for transparency?
- absence of blood vessels
- few cellular organelles
- orderly arrangement of fibers
- short distance between components of different indices relative to wavelength of light
61
Why do the lens cells require significant energy?
- high metabolic activity occurs in anterior epithelium maintaining cell and fiber function
- pre-equatorial region high level miotic activity
62
Where does the lens get the majority of its nutrients from?
- the surrounding aqueous and vitreous
63
What is the energy required for cellular metabolism and cellular replication within the lens?
- anaerobic glycolysis
64
Why is the lens epithelium rich in transport mechanisms?
- get nutrients from surrounding aqueous
- maintain electrolyte balance
65
How can ultraviolet absorption affect the lens?
- produces oxidative changes within tissue causing formation of free radicals
- free radicals cause disruption to cellular processes and cause cellular damage
66
What are the purposes of the lens capsule?
- mould lens
- enclose lens contents
- prevent large molecules such as albumin and haemoglobin entering lens
- permeable to water, small solutes and proteins
67
What is the water and protein percentage of the lens?
- 65% to 70% water
- 30% to 35% protein
68
What is the refractive index of the lens cortex and nucleus?
- cortex 1.38
- nucleus 1.41
69
Whats the length of an outer fiber?
- 1cm
70
What percentage of the proteins in the lens are water soluble crystallin's?
- 90%
