Lens Final Info Flashcards
lens scatters less then ___ % of incident light
5
how are the cells in the lens packed?
cells are densely packed resulting in destructive interference of scattered light
the different percentages of phospholipids in the cell membrane change as the cells age, resulting in:
a less fluid membrane (lens easier to rupture and scatter light)
3D change of membrane phospholipid example
the MIP of the gap junctions is cleaved from 26,000 dalton post-translational form to 22,000 dalton form
how are structural proteins of the lens affected by age?
the internal cell structure or the cytoskeleton of microtubules, actin, and intermediate filaments is gradually degraded as the cell ages
what occurs when the lens structural proteins are gradually lost?
- without a cytoskeleton, the cell has difficulty maintaining its shape
- old cells have less internal water and also shrink
what occurs to the crystallin proteins as the cell ages?
the crystallin proteins begin to clump together
initially the crystallin in a young lens are strongly associated with water, binding about ___ % of the water to the cell
24
with aging, the tertiary structure of the protein changes and less water ___ % is bound by the proteins
13
what is the overall result of crystallin proteins increasing aggregation in the lens? (and what is the term for it)
clumps of protein and pools of water (called syneresis) and light scatter
how is the water content of the lens affected by aging?
water no longer bound by proteins, osmolarity of lens decreases, water flows out of lens
cortical thickening rate of the lens with age is:
0.02 mm/ year
what is the result of cortical thickening of the lens with aging?
increased thickness of lens, nuclear fibers may not be able to get nourishment that they need from diffusion
explain the cataract changes on the synthesis of glutathione
concentration of glutathione decreases and the structural damage that results from UV and metabolic insult cannot always be repaired
explain the cataract changes on ion pumps
- ion pumps become less effective
- increased Na+, decreased K+ and amino acids
- less protein synthesis
- water control of lens harder
explain the cataract changes on metabolic activity
decrease in metabolic activity
- fewer proteins made
- less ATP available for normal functions
explain the cataract changes on crystallin proteins
crystallin proteins are oxidized and glycosylated, leading to cross-linking and insolubility
explain the cataract changes on metabolic breakdown
metabolic breakdown leads to:
- osmotic imbalance
- formation of vacuoles and clefts
explain the cataract changes on optical density
increased optical density leads to:
-color changes (brunescence) especially in nucleus
what causes an increase in optical density of the lens?
decrease in the water content of the lens
the lens nucleus changes colors because of the :
breakdown of proteins
explain the cataract changes causing opacities
-membranes rupture and accumulation of debris results in opacities
_____ results in rupture of cell membranes
osmotic pressure
cell membrane rupture results in
debris accumulation and light scatter- causing cataract
when does diabetes mellitus usually cause a cataract?
usually 15-20 years after DM present unless it is very severe, uncontrolled case
how high does the glucose concentration need to be in the aqueous to cause an increase in lens glucose concentration?
high > 175 mg/100mL
describe how diabetes mellitus can cause “snowflake” opacities
- high glucose causes osmotic imbalance which attracts water -> vacuoles form
- if [glucose] goes down, vacuoles can disappear leaving snowflake opacities “punctate dot” cataracts
other than causing an osmotic balance, what else can the excess glucose in the lens cause?
- it also binds to proteins (glycosylation)
- it is converted to sorbitol which can accumulate
what does glycosylation (excess glucose binding to proteins) result in:
causes protein aggregation, cross-linking, and insoluble proteins
what is a galactosemic cataract?
from a galaxies breakdown issue- metabolic disease that has identical cataract appearance to diabetics
a UV induced cataract (cortical in humans) from radiation cataract is a result of what cellular changes:
- decreased glutathione, protein synthesis and sodium
- increased potassium
type of cataract that has increased optical density in the nuclear portion of the lens and brunescent color
nuclear cataract
type of cataract that is superficial to the nucleus
supranuclear cataract
type of cataract that has opacities of the sutures
sutural cataract (very rare)
type of cataract that is in the cortical region and can look like small dots, flecks, vacuoles, or vossius ring
cortical cataract (very common)
type of cataract that is a side effect of anti-psychotic drug therapy (thorazine, phenylthiozine) that causes proliferation of epithelial cells
anterior subcapsular (rare)
type of cataract that is a side effect of steroid use
posterior subcapsular (10% of all cataracts)
type of cataract that looks like a spider web when viewed with a slit lamp and results from cellular proliferation or swelling at the equator
posterior subcapsular
what type of migration occurs in posterior sub capsular cataract?
abnormal migration to the posterior pole from equator
this cataract results in the greatest loss of vision of any cataract
posterior subcapsular
the accommodative response requires about ___ ms to initiate
350 msec
explain the changes that occur when the ciliary muscle contracts
decreased tension on zonules, lens becomes more spherical, increase in dioptric power
ciliary muscle contraction causes what movements of the lens
- lens moves forward (decreasing depth of anterior chamber)
- lens sinks a little (gravity effects)
ciliary muscle contraction causes what IOP change
IOP rises momentarily and then decreases
ciliary muscle contraction causes what change in circumference of the lens
circumference of the equator of the lens decreases
ciliary muscle contraction causes what surface of the lens to change shape the most?
front surface of lens
even though all accommodation is generally lost by age 50, what still is present?
depth of focus (1-2 D)
4 potential causes of presbyopia
- hardening of the lens
- weakening of ciliary muscle
- enlargement of the lens
- loss of capsule elasticity
