Lens Flashcards
1
Q
What are the 3 things that need to happen properly for the lens to function
A
- Be transparent
- Have a higher index of refraction than the medium its suspended in (AH)
- Have flexible refractive surfaces with the proper curvature
2
Q
Disruption of the organization of the lens fiber cells or damage to the proteins result in
A
Cataract Formation
3
Q
What type of cells does the anterior surface of the lens (lens epithelium) contain?
A
Cuboidal cells
4
Q
What does the bulk of the lens consist of
A
Elongated fiber cells
5
Q
What is secreted by the lens epithelium and elongated fiber cells
A
Lens capsule, an ECM
6
Q
Where do most epithelial cells divide
A
In the germanative zone, near the equatorial margin of the lens epithelium
7
Q
What are sutures
A
Junctions between the apical and basal ends of the cells from the opposite side of the lens
8
Q
Lens increases in size and synthesis just before…
A
organelle degradation
9
Q
Where do zonules originate
A
Non pigmented layer of the ciliary epithelium and insert into the lens capsule near the equator
10
Q
What do tensions in the zonules cause
A
Curvature changes during accomodation
11
Q
The refractive properties of the lens are the result of…
A
High concentrations of crystallins in the cytoplasm of the lens fiber cells and the curvature of the lens surfaces
12
Q
Crystallins are how many times more higher than in typical cells
A
3x, which increases the refractive index of the lens
which makes up 40% of the wet weight of the lens fiber
13
Q
In younger indivisuals, is refractive error caused by the lens?
A
NO
14
Q
What are the 2 things that causes refractive error in younger indivisuals
A
- Corneal curvature
2. Length of the globe
15
Q
What does transparency of the lens depend upon?
A
Minimizing light scattering and absorption
16
Q
Light passes smoothly through due to:
A
- Regular structure of lens fibers
- The absence of membrane bound organelles
- Small/uniform extracellular space between cells
17
Q
In young individuals the human lens is nearly…
A
Colorless
18
Q
As we age, what wavelength of light does the lens absorb
A
Blue light which consists of short wavelengths
19
Q
What is aphakia?
A
The absence or removal of the natural crystalline lens from natural causes
20
Q
What is congenital aphakia
A
Rubella infection in first 4 weeks of pregnancy
21
Q
What is pseudophakia
A
The substitution of the natural crystalline lens with a synthetic lens
22
Q
What does lens fiber differentiation depend on
A
Synthesis and accumulation of large amounts of crystallins
23
Q
What consist of the classical crystallins
A
Alpha crystallin family and Beta/gamma crystallin superfamily (all vertebrae lenses accumulate these)
24
Q
What are taxon specific crystallins
A
Functional enzymes or proteins that are structurally similar to enzymes but that lack enzymatic activity
25
Do adult lenses produce taxon specific crystallins
NO
26
What enzyme is in the embryonic nucleus which serves as a taxon specific crystallin during the early development of the human lens
Betaine-homocysteine methyltransferase
27
What type of classical alpha crystallins do humans express
alpha A and alpha B
28
What is the role of alpha A and alpha B classical crystallins?
To prevent protein aggregation and precipitation
29
What did the analysis of the alpha A knockout in mice show
That the lens is smaller than normal but structurally similar, formation of cataracts in a few weeks, and that they contained large amounts of alpha B crystallin and smaller amounts of other proteins
30
Where else is the alpha B crystallin present besides the lens
Heart and skeletal muscle
31
What important function does the alpha B crystallin lens have
Chaperone functions in the lens
32
T/F the alpha A is more diverse than the betta gamma superfamily
False, the betta/gamma superfamily is more diverse
33
How many betta and how many gamma are found in the lens
6 betta and 3 gamma crystallins
34
T/F The classical crystallins are made up of 65% of water
True
35
How much organic matter is in the classical crystallins
35%
36
T/F the insoluble varitey of protein is greater than the soluble
F -The soluble variety is greater
37
Microtubles play an important role in:
1. Stabilizing the fiber cell membrane
2. May be transporting vesicles to the apical and basal ends of the elongating fiber cells
3. Gives lens its birefringent property
38
WHat is the definition of birefringent?
Property of any tissue that (changes path of polarized light) polarized light rays are split into its different components and travel at different phases. You can measure this property for any tissue and figure out if its healthy or not.
39
T/F The membranes of mature fiber cells have an unusual lipid composition
True
40
The high proportion of cholestrol and sphingomyelin compared to other plasma membranes causes the cell membranes to be...
RIGID
41
Cholesterol is 3 x greater in nuclear fibers compared to...
cortical fibers
42
When is the growth in lens most rapid
In the embryo and in first postnatal year
43
When does lens growth slow down?
Between ages of 1-10, continues at a slower almost linear rate throughout life
44
Where do capillaries at the anterior part of the lens arise from
blood vessels of the iris stroma
45
Where do capillaries at the posterior part of the lens arise from
the hyaloid artery from the tunica vasculosa lentis
46
When do capillaries regress
During second trimester
47
Absence of the lens in early embryogenesis leads to:
1. Absence of corneal endothelium
2. Abnormal differentiation of the corneal stroma
3. Absence of the iris, ciliary body, and anterior chamber
48
Where does the lens gets its energy from?
Glycolysis
49
What is the end product of glycolysis
lactic acid, intracellular pH drops significantly from peripheral to deeper fiber cells secondary to lactic acid accumulation
50
What is the source of most oxidative damage?
Molecular oxygen
51
T/F The HIGH oxygen tension around/within the lens probably protects it from oxidative damage
False! The low oxygen tension does
52
Where does the lens derive its ATP from
Oxidative phosphorylation which is a process that generates free radicals
53
Where is most of the solar irradiation that reaches the eye absorbed by
The cornea
54
T/F If light is not absobed, it produces no damage
True
55
Which structures absorb UV light
1. DNA
2. Proteins
3. Nucleoside-metabolites
4. Flavonoids
5. Pigments
56
What provides most of the protection against oxidative damage and what is it made out of
Glutathione. It is a tripeptide made up of amino acids: glutamine, cysteine, and glycine
57
What also provides minor protection to the lens and where is it transported from
Ascorbic acid. It is actively transported from the blood to the AH by a Na dependent transporter in the ciliary epithelium.
58
How much more ascorbic acid is in the AH humor than in the blood
20 x
59
T/F Oxygen within the lens is much higher than other parts of the body
False, its much lower
60
Where is the glucose required for glycolytic metabolism derived from
The aqueous humor - due to fascilitated diffusion across the ciliary epithelium
61
Lens should maintain which electrolytes?
High K+, Low Na+ and Cl- and water
62
The water electrolyte balance is maintained by the _______ which operates in the ________ which has _____
sodium potassium pump
epithelium
mitochondria
63
What does the human lens accumulate which absorbs the short visible wavelengths
Chromophores
64
What color pigmentation in the lens increases with age and what does this do?
Yellow - it absorbs shorter wavelengths of light that prevent them from getting to the retina
65
What happens when there is a high concentration of chromophores
- increased light absorbance
- reduced VA
- Brunescent/nigrescent cataract forms (more common in developing countries which suggests environment of nutritional factors are important)
66
Loss of lens transparency with increased light scattering is caused by:
1. Disruption of lens fiber cell structure
2. Increased protein aggregation
3. phase separation lens cell cytoplasm
67
What is the general effect of cataract formation
Change in amount of soluble to insoluble lens protein.
| The process of change of soluble to insoluble is by CROSS LINKING
68
What are the three types of age-related cataracts
1. Nuclear (Occur in older fiber cells formed during embryonic and fetal life)
2. Cortical (Cells formed later in life)
3. Posterior Subcapsular (result from light scattering by a plaque of swollen cells at the posterior pole of the lens.)
69
Which type of cataract is associated with increased oxidative damage to lens proteins and lipids?
Nuclear cataract
70
What is the onset of nuclear cataract formation associated with
Increased refractive power of the lens
71
What is second sight
A temporary improvement in near vision for hyperopoic patients that undergo a myopic shift
72
What procedure can cause a nuclear cataract to form and why
Vitrectomy - With a removed vitreous the lens is exposed to increased oxygen --> oxidative stress
73
T/F During nuclear cataract formation, changes in protein organization leading to cataract are subtle
TRUE
74
T/F a small change in protein aggregation leads to a small amount of light scattering
False; leads to a large amount of light scattering
75
What is the senile cataract hypothesis
HM fractions grow into the cell cytoplasm
76
Which type of cataract occurs in the mature fiber cells lying close to the lens surface
Cortical cataract
77
Where do cortical cataracts most often occur
in the inferior nasal quadrant
78
T/F a person can have cortical cataract for years without it affecting visual function
True, because it early on, the opacity is limited to the periphery
79
What are cortical spokes
Extension of the opacity on a small cluster of the fiber cells
80
What type of cataract is caused by light scattering in a cluster of swollen cells at the posterior pole of the lens, just beneath the capsule
Posterior Subcapsular Cataract
81
With a PSC, where is there increased disruption
In the optical axis, near vision is affected more when compared to distance vision
82
T/F Patients have a combination of nuclear, corticol & PS cataracts
True
83
What can a cataract develop into if it is not removed
morgagnian (Effects the entire lens)
84
What is Extracapsular Cataract Extraction
Remove a portion of the anterior lens epithelium and capsule to expose the underlying fiber mass; extract the nuclear and cortical fiber cells, and implant a plastic IOL in the capsular bag.
85
What is a common complication of Extracapsular Cataract Extraction
Formation of secondary cataracts that is posterior capsular opacification (PCO)
86
What is Intracapsular Cataract Extraction
When the entire lens is removed to remove the cataract; still common in developing countries, rare in most places
87
What are Elschnig's pearls
When epithelial cells migrate further onto the posterior caspsule after surgery that may differentiate into small lentoid bodies
88
What are congenital cataracts
Cataracts present at birth or that appear soon after birth (can be caused by Rubella)
89
What are anterior polar cataracts
Another early onset cataract that usually involve the formation of an opaque plaque near the center of the lens epithelium
90
What are some hereditary syndromes associated with cataract formation
Galactokinase deficiency and galactosemia
91
In galactosemia and galactokinase deficiency, what accumulates to high levels in the body
Sugar galactose
92
Conversion of glucose or galactose to sugar alcohols is via which pathway
Sorbitol
93
What has been linked to the formation of cataracts
- A high lifetime exposure to UV light leads to corticol cataracts
- Long term exposure to infrared light and focused microwaves also can cause cataracts
- Occupation (Glass blowers cataract)
- Long term exposure to high-dose steroids inc. the risk of PSC cataract
94
What does increased levels of calcium result in
1. Affects glucose metabolism
2. inhibition of protein synthesis
3. Induction of HM aggregates
4. Direct loss of transparency
5. Inhibit sodium pump
95
What does hypocalcemia (low calcium) result in
Related to dependency of the membrane permeability to levels of calcium
Marked electrolyte and water imbalance in low calcium environment
96
People with diabetes are at increased risk for...
early onset cataracts
97
What are the sequence of events for diabetes and lens- osmotic hypothesis
1. Glucose is at toxic level in the lens.
2. Alodose reductase activated and converts glucose to sorbitol
3. Sorbitol gets trapped and lens and cannot escape lens...which causes influx of water into the lens
4. Polyl dehydrogenase tries to get rid of sorbitol by converting it to fructose (slow)
5. High intracellular osmotic pressure
6. Cells burst debris..becomes manifestation of cataract
98
If someone complains about fluctuating vision
Ask if they have diabetes and if it is under control
99
What are some other risk factors for cataracts
1. Age
2. Low socioeconomic status
3. Females
4. Smoking and high alcohol consumption (dose dependent)
5. Dark iris color (higher levels of corticol cataracts in african americans)
100
What research plays a role regarding cataract formation
1. Studying genetics
2. Moderate caloric restriction reduces cataract incidence and progression
3. Nutrition
