Lens Flashcards
what type of tissue is the lens
two types of specialized epithelial tissue: cuboidal and elongated fiber cells
what 3 things must the lens have in order to project an image onto the retina
transparent, higher refractive index than the medium its suspended in, and have flexible refractive surfaces with the proper curvature
what happens to the lens if there is disruption of precise organization or damage to the proteins
destroys the transparency of the lens and cataract formation
what type of tissue is the lens epithelium
a sheet of cubodial cells
what type of tissue is the bulk of the lens
elongated fiber cells
what secretes the lens capsule
the epithelial and superficial fiber cells
where is the germanitive zone
near the equatorial margin of the lens epithelium
when do the fibers stop elongating
when they reach the Y sutures
what are the lens sutures
junctions between the apical and basal ends of the cells from opposite ends of the lens
where do the mature lens fibers go
they get buried deeper as fibers elongate and differentiate
when does synthesis stop
just before the organelle degradation
how do the mature fiber cells compare to those in the rest of the body
the components of mature fiber cells are more stable
what are the zonules
the inelastic microfibrils that suspend the lens
where do the zonules originate and insert
in the NPE of the ciliary epithelium and insert into the lens capsule near the equator
what is responsible for changing the lens curvature during accommodation
the zonules
what contributes to the refractive nature of the lens
the high concentration of crystallins in the cells and the curvature of the surfaces
what is the concentration of crystallins in the lens compared to typical cells
3 times higher concentration
what causes refractive error is younger individuals
corneal curvature or the length of the globe (rarely by the lens)
what does the transparency of the lens depend on
minimizing light scattering and absorption
why does light pass smoothly through the lens
regular structure of lens fibers, absence of membrane-bound organelles and small /uniform extracellular space between fiber cells
what happens to the nearly colorless lens as we age
becomes more yellow- lens absorbs short wavelength light (blues)
what is aphakia
the absence of the natural crystalline lens from natural causes or removal
what is congenital aphakia
caused by Rubella infection in first 4 weeks of pregnancy (mutation in PAX-6 gene)
what is pseudophakia
substitution of the natural crystalline lens with a synthetic lens
what does lens fiber differentiation depend on
synthesis and accumulation of large amounts of crystallin proteins
what percentage of the wet weight of the lens fiber is crystillins
40% (3 times the amount in a typical cell)
what are the classic crystallins that all vertebrae lenses accumulate
alpha-crystallin family and beta/gamma-crystallin superfamily
do adults have taxon-specific crystallins
no- high levels in the embryonic nucleus
why are taxon-specific crystallins important
for certain events to take place- development of the human lens
what taxon-specific crystallin is present at high levels in embryonic nucleus
betaine-homocysteine methyltransferase
what is a taxon-specific crystallin
a functional enzyme or protein- lack enzymatic activity
what are the two classical crystallins humans express
alpha A and alpha B
what is the role of alphaA and alphaB crystillins
to prevent protein aggregation and precipitation of one another
what did the analysis of alphaA knockout mice show
that protein aggregates show large amounts of alphaB crystallin and smaller amounts of other proteins
is alphaA crystallin only present in the lens
no- also in heart and skeletal muscle
what is the function of alphaB in the lens
important chaperone functions
how many beta and gamma crystallins are in the lens
6 beta and 3 gamma
which is more diverse, the beta/gamma superfamily or the alphaA crystallins
the beta/gamma superfamily
how much of the lens is water
65%
how much of the lens is organic matter
35%
how much of the lens organic matter is structural proteins
33%
where are microtubles found
beneath membranes of lens fiber cells
what are the roles of the microtubles
stabilizing the fiber cell membrane and may be transporting vesicles to the apical and basal ends of elongating fiber cells
what gives the lens its birefringent property
the microtubules
what is birefringent property
(uniform pattern) path of polarized light ray can be changed and split its contents into different phases
what does birefringent depend on
the number of microtubles and their thickness
what other parts of the eye have birefringent property
cornea, sclera and retinal nerve fiber layer
what components does the lens fiber cell membrane have
unusual lipid composition and high proportion of cholesterol and phingomyelin
what increases in the cell membrane as the cells mature
cholesterol
what causes the fiber cell membranes to be rigid
the presence of high concentrations of cholesterol and sphingomyelin
how much more cholesterol is in the nuclear fibers compared to the cortical fibers
3 times more
when is the lens growth the most rapid
in the embryo and 1st postnatal year
where do the capillaries at the anterior part of the lens arise from
blood vessels of the developing iris stroma
where do the capillaries at the posterior part of the lens arise from
hyaloid artery and form the tunica vasculosa lentis
when do the capillaries regress
during the 2nd trimester (caused by apoptosis)
if the lens is absent at an early stage of development, what will it lead to
absence of corneal endothelium, abnormal differentiation of the corneal stroma and absence of the iris, ciliary body and anterior chamber
where does the lens get its energy from
glycolysis
what is the end product of glycolysis
lactic acid
what happens with lactic acid accumulation
intracellular pH drops significantly from peripheral to deeper fiber cells
what is the source of the most oxidative damage
molecular oxygen
what protects the lens proteins and lipids from oxidative damage
low oxygen tension around and within the lens (15 mmHg)
how does the lens derives a substantial proportion of its ATP
from oxidative phosphorylation- process that generates free radicals
where is most of the solar irradiation absorbed
in the cornea
what 5 things absorb UV light (potential sources of free radicals)
DNA, proteins, nucleoside-containing metabolites, flavonoids and pigments
what 2 things absorb visible and UV light
flavonoids and pigments (especially shorter wavelengths)
Glutathione is a tripeptide of what 3 things
glutamine, cysteine and glycine
what provides the most protection against oxidative damage in the lens
Glutathione
what happens to the lens when glutathione levels are decreased
cell damage and cataract formation follow rapidly
what is actively transported from the blood to the aqueous humor by Na-dependent transporter in the ciliary epithelium
Ascorbic acid
what else besides glutathione provides protection against oxidative damage
ascorbic acid (from aqueous humor)
how much more ascorbic acid is in the aqueous humor compared to blood
20 times more
why does the lens depend on glycolytic metabolism to produce ATP
the oxygen concetration within and around the lens is low (lower than other parts of the body)
where does the glucose come from that supplies the glycolytic metabolism in the lens
from the aqueous humor- facilitated diffusion across ciliary epithelium
where are mitochondria located in the lens
lens epithelium and superficial fiber cells
what electrolyte should the lens have in high concentrations
potassium
how do cells near the lens surface derive energy from glucose
via glycolytic and oxidative pathways
what electrolytes should the lens have in low concentrations
Na+, Cl-, and water
what happens if sodium moves into the lens
the solutes and water content will increase (osmotically)
is the lens more positive or negative inside
electronegative inside
is aqueous humor higher in Na+ or K+
higher Na+ and low K+
where is the Na+K+ATPase pump located in the lens
in the epithelium (also has mitochondria)
what is removed and what is allowed in with the Na+K+ATPase pump
3 Na+ removed and 2 K+ allowed in
which part of the lens, inner or outer part, requires more energy
the outer part of the lens
what type of chromophores does the lens accumulate
ones that absorb the short visible wavelengths
what pigmentation increases as we age
the amount of yellow pigmentation in the lens increases
what can happen with high concentrations of chromophores in the lens
increase in light absorbance and reduced visual acuity
what is a brunescent or nigrescent cataracts
a black cataract (cataract nigra)
when does a brunescent cataract form
common in developing countries (environmental or nutritional factors may contribute)
what is the textbook definition of a cataract
an opacification of the lens
what is the clinical definition of a cataract
it interferes with visual funciton
what 3 things cause loss of transparency with increased light scattering
disruption of lens fiber cell structure, increased protein aggregation, and phase separation lens cell cytoplasm
what is phase separation
formation of opacities without disruption of cell
at what visual acuity does medicare pay for cataract surgery
20/50 or worse
what is the general effect of a cataract formation
a change in amount of soluble and insoluble lens protein
why does the protein content in the lens increase with age
because the lens grow with age
what changes the soluble protein to insoluble
caused by cross-linking (why UV light is blamed for changes on lens)
what are the 3 most common types of cataracts
nuclear, cortical, and posterior subcapsular cataracts
where do nuclear cataracts occur in the lens
in the oldest fiber cells (those formed during embryonic and fetal life)
what changes in the lens are nuclear cataracts associated with
increased oxidative damage to lens proteins and lipids
are nuclear cataracts age dependent
yes- more susceptible to oxidative damage
what happens to refractive power of lens with the onset of a nuclear cataract
there is an increase in refractive power
what is second sight
in nuclear cataracts: a temporary improvement in near vision for hyperopic patients that undergo a myopic shift
what can occur following a vitrectomy (within 6 months to 3 years after)
a nuclear cataract in older patients
what causes a nuclear cataract following a vitrectomy
changes the environment around the lens- increase in the oxygen tension around the lens
what is the oxygen tension in the anterior vitreous
normally low- about 16mmHg
what is the oxygen tension in the posterior vitreous (near the retinal vessels)
high- decreasing in a sharp gradient within the first 1mm of the vitreous body
what happens to the oxygen levels when the vitreous is removed
the fluid in the eye can circulate freely and the lens is exposed to increased oxygen from the retina
are the changes in the protein organization leading to cataracts rapid or subtle
subtle
do a large or small fraction of proteins in the lens need to have protein changes to cause a large amount of light scattering
only a small fraction
where do higher molecule fractions grow into in senile cataracts
into the cell cytoplasm
where do cortical cataracts occur in the lens
in the mature fiber cells lying close to the lens surface- most often in the inferior nasal quadrant
are the affected regions in cortical cataracts partially or completely disrupted
completely disrupted
where does the opacity begin and end in cortical cataracts
in the periphery and spreads toward the visual axis interfering with vision
what are cortical spokes
extensions of the opacity on a small cluster of the fiber cells
what type of refractive error can you expect with cortical cataracts
a hyperoptic shift (hypermetropia) stretching the lens flatter or changing the curvature
What happens to the lens epithelial cells in a PSC
There is abnormal migration of the cells and aberrant differentiation
what causes a posterior subcapsular cataract
by light scattering in a cluster of swollen cells at the posterior pole, just beneath the capsule
do most patients have a “pure” posterior subcapsular cataract
only less than 10% have a pure form- usually occur in conjunction with nuclear or cortical cataracts
what happens if a cataract is not removed
it will become a total or “morgagnian” cataract
what happens to the lens in a total or morgagnian cataract
the capsule gets weaker and starts wrinkling and the nucleus will sink down- eventually bursting and releasing the lens protein into the aqueous humor
what are the steps of an extracapsular cataract extraction (most common approach to remove cataracts)
removal of a portion of the anterior lens epithelium and capsule, extract the nuclear and cortical fibers, and implant a IOL in a capsular bag
is intracapsular cataract extraction technique still used
still common in developing countries, older technique and not the surgery of choice
what is a secondary cataract
a common complication of cataract extraction- a posterior capsular opacification (PCO) and it needs a YAG laser to clear the opacity
what happens in intracapsular cataract extraction
the entire lens is removed when removing the cataract or if the lens has burst
what type of refractive error occurs after intracapsular cataract extraction
the patient will be aphakic- very hypermetropic and need thick lenses
what are “lentoid bodies” or Elschnig’s pearls
differentiated lens epithelial cells near the equator that persist after surgery can migrate beneath the IOL onto the posterior capsule
what are congenital cataracts
present at birth or appear soon after (can be cause by Rubella)
What are 7 hereditary syndromes associated with cataract formation
- oculocerebrorenal syndrome of Lowe
- neurofibromatosis type 2
- hyperferritinemia
- Werner syndrome
- Myotonic dystrophy
- Galactokinase deficiency and galactosemia
- Fabrys disease
what are anterior polar cataracts
early-onset cataracts or type of congenital: an opaque plaque is formed near the center of the lens epithelium
What type(s) of occupational exposure led to cataract formation
infrared light and focused microwaves (ie. glass-blowers cataract)
what is the suggested mechanism of how UV light causes cataracts
UV-generated free radicals can damage the components of the lens
what type of cataract can long-term exposure to high-dose steroids cause
increased risk of a posterior subcapsular cataract
what 5 things can happen with increased intracellular calcium in the lens
affects glucose metabolism, inhibition of protein synthesis, induction of high molecular aggregates, direct loss of transparency, and inhibit sodium pump
What are the steps in the osmotic hypothesis of diabetic cataracts
- lens toxic levels of glucose
- aldose reductase activated
- glucose converted to sorbitol
- sorbitol cannot escape the lens
- polyol dehydrogenase tries to get rid of sorbitol by converting it to fructose
- high intracellular osmotic pressure
- cells burst debris and become a cataract
what can occur in the lens with hypocalcemia
related to the dependency of the membrane permeability to levels of calcium and marked electrolyte and water imbalance
what causes early-onset of cataract in diabetic patients
they can have damage to the lens as a result of high glucose levels (the aqueous humor has too much glucose-more than the lens and cornea need)
what is the most common risk factor of developing a catarct
age
why is low socioeconomic status a cataract risk factor
it may predispose patients to nutritional deficiencies, increased exposed to diseases, poor general health
what are 5 risk factors for developing a cataract
age, low socioeconomic status, females, smoking and high alcohol consumption, and a dark iris color
what type of cataracts are linked to smoking and high alcohol consumption
nuclear and cortical cataracts
what type of cataract is more common in African-American patients
cortical cataracts
what 2 things are being looked at in cataract research
genetics of cataracts and nutrition (calorie restriction)
