Lens Flashcards
T/F: The cells near the surface of the lens may use both glycolytic and oxidative pathways to derive energy from glucose.
True
What protects the lens from oxidative damage?
Glutathione (provides most of the protection) Absorbic acid (also provides protection)
T/F: The lens gets much of its energy from glycolysis?
True
T/F: The deeper fiber cells have lower PH than peripheral fibers.
True
This is due to the end product of glycolysis–> Lactic acid–> lactic acid accumulation
What is the composition of the crystalline lens?
65% Water, 35% organic matter (33% protein)
What percent of the wet weight of lens fibers is crystallins?
Crystallins make up 40% of the wet weight of the lens fibers. (3X the amount in typical cell)
What contributes to the transparency of the lens?
Regular structure of the lens fibers
The absence of membrane-bound organelles
Small/uniform extra cellular space between fiber cells
What causes an increase in refractive index of the lens?
Higher concentration of crystallins and
the curvature of the surfaces
Name the 3 types of cataracts.
Nuclear cataracts
Cortical Cataracts
Posterior subcapsular cataracts
What kind of cells are present on the surface of the lens closest to the cornea?
Cuboidal epithelial cells.
T/F: the epithelial cells of the lens located in the germanitive zone near the equatorial undergo mitosis.
True
Which cells secrete the lens capsule?
Epithelial cells of the lens secrete the lens capsule. The lens capsule is basically just an elastic extracellular matrix.
T/F: Fiber cells stop elongating when they reach the sutures
True
What are sutures?
Junctions between the apical and basal ends of the cells from the opposite side of the lens
T/F: Newer fiber cells get buried deeper into the lens.
False! Mature fiber cells get buried deeper as fibers elongate and differentiate
T/F: the components of mature fiber cells must be much more stable than those in cells found in other parts of the body.
True
Where do zonules originate from?
zonules originate in the non-pigmented layer of the ciliary epithelium
T/F: In young individuals, refractive errors are rarely caused by the lens
True: in young people refractive errors are caused by corneal curvature or the
length of the globe-
and rarely by lens
T/F: With aging, the human lens absorbs more blue light/short wavelength light.
True! In young individuals the lens is colorless but as we age it gets more yellow bc it is absorbing more blue light.
What is Aphakia?
Aphakia is the absence of the natural crystalline lens, from natural causes or removal
What causes Congenital aphakia?
Rubella infection in first 4 weeks of pregnancy
What is Pseudophakia?
Pseudophakia is the substitution of the natural crystalline lens with a synthetic lens
What is betaine-homocysteine methyltransferase?
an enzyme that serves as a taxon-specific crystallin during the early development of the human lens.
What type of crystallins make up Classical crystallins?
Alpha, beta and gamma crystallins. (these are found in all vertebrae)
What is the role of αA and αB crystallins that are found in humans?
to prevent protein aggregation and precipitation
αB-crystallin is present in a variety of cells throughout the body, not just the lens
True. Such as in the heart and skeletal muscle.
T/F: 6 β- and 3 γ-crystallins are found in the lens
True
What percent of the lens is water?
65%
What is the function of microtubules in lens fibers?
Stabilizing the fiber cell membrane and
Transporting vesicles to the apical and basal ends of elongating fiber cells
Also gives lens its birefringent property
T/F: Lens fibers cell membranes are composed of high proportion of cholesterol and sphingomyelin
True
Lens fiber cell membranes are rigid because they contain what?
highest proportion of cholesterol
Why do nuclear fibers have 3X more cholestrol than cortical fibers
Because cholestrol content in fiber cells increases as they mature.
Describe lens growth through out the human lifespan?
Growth is rapid in the embryo and 1st postnatal year
Growth slows between ages 1-10
and then continues to grow at a slower, almost linear rate throughout life
What is the tunica vasculosa lentis?
Capillaries that supply the posterior part of the lens. Theses capillaries arise from the hyloid artery and regress during the 2nd trimester
Capillaries at the anterior surface of the lens arise from which blood vessels?
from blood vessels of the iris stroma
T/F: The lens plays as anterior segment organizer in early stage of embryogenesis.
True
What happens if there is no lens in early stage of embryogenesis?
No corneal endothelium, abnormal differentiation of corneal stroma, absence of iris, absence of cilliary body and absence of anterior chamber.
Through which process does the lens get most of its energy?
Glycolysis
Why does the lens have low PH towards the center (deeper ends of lens fibers) than it does on the peripherally?
It is because the lens gets most of its energy from glycolysis and lactic acid accumulates as an end product of glycolysis.
What are some potential sources that could lead to the production of free radicals?
- oxidative phosphorylation by the lens and
* the presence of DNA, proteins, nucleoside-containing metabolites, flavonoids, and pigments (which absorb UV light)
T/F: there is low oxygen tension around the lens.
True (approximately 15 mm Hg)
What is GLUTATHIONE?
Glutathione is a tripeptide of: glutamine, cysteine, and glycine. It provides most of the protection against oxidative damage for the human lens.
What happens when gluthathione levels are low in the lens?
Cell damage and cataract formation
Which has more ascorbic acid (vitamin C); the Blood or the aqueous humor?
Aqueous humor. There is 20x more ascorbic acid in the aqueous humor compared to the blood.
T/F: there is 20x more ascorbic acid in the aqueous humor compared to the blood
True
What is the role of ascorbic acid in the lens?
protection against oxidative damage
T/F: Lens epithelial and superficial fiber cells have numerous mitochondria
True. Needed for glycolysis and oxidative pathways.
T/F: The lens can get glucose from Aqueous humor?
True. There is glucose in the aqueous humor that the lens can use. This glucose comes from facilitated diffusion from ciliary epithelium.
Does the lens have high K+ content or high Na+?
The lens has high K+ and low Na+. (There is low Na+, and Cl- inside the lens to maintain low/water balance)
T/F: Aqueous has more Na+, low K+
True. This is the opposite of what is found in the lens and ensures water balance.
What maintains the water electrolyte balance is maintained
Sodium-potassium pump
Where in the lens is the sodium-potassium pump located?
In the lens epithelium (which has mitochondria because this pump requires energy!)
What do chromophores do?
Absorb short visible wavelength of light.
Chromophores accumulate in the human eye as one ages. This is why patients with cataracts may have decreased ability to see shades of blue.
How are brunescent or nigrescent cataracts formed?
By accumulation of chromophores.
What effect does high concentration of chromophores have?
Increased light absorbance
Reduced visual acuity
and lead to formation of brunescent or nigrescent cataracts.
T/F: Brunescent cataract is more common in developing countries
True
T/F: Protein content inside a lens increase with age.
True
Cataract lenses have more water-insouble or soluble proteins?
Cataract lenses have more water-insoluble proteins. soluble proteins can turn into insoluble proteins via cross-linking.
What kind of cataract occurs most in the oldest fiber cells, those formed during embryonic and fetal life?
Nuclear Cataracts
What kind of cataract occur in cells formed later in life
Cortical Cataracts
Cataract that result from light scattering by a plaque of swollen cells at the posterior pole of the lens is called ?
PSCs: Posterior subcapsular cataracts
T/F: Early on the the opacity in cortical cataract is limited to the center.
False: early on in cortical cataract the opacity is limited to the periphery
T/F: Cortical cataract most often occur in the inferior nasal quadrant
True
What are the risk factors for nuclear catracts?
- Increased oxidative damage (i.e. in patients with vitrectomy)
- Age
In posterior subcapsular cataracts, what causes the swollen cells at the posterior pole?
abnormal migration of epithelial cells or aberrant differentiation of cells
What is the most common complication Extracapsular Cataract Extraction?
formation of secondary cataracts that is posterior capsular opacification (PCO)
What is Intracapuslar Cataract Extraction?
When the entire lens is removed to remove the cataract (older technique and found mostly in developing countries)
what is “morgagnian” cataract?
cataract progressed until it effects the entire lens
T/F: PSC cataract is more commonly found in conjunction with nuclear or cortical opacities
True
What is Extracapsular Cataract Extraction?
Remove a portion of the anterior lens epithelium and capsule
Extract the nuclear and cortical fiber cells
Implant an intraocular lens (IOL) in a capsular bag
What is posterior capsular opacification (PCO)?
Common complication: formation of secondary cataracts after extracapsular cataract extraction.
What are Elschnig’s pearls?
aka “lentoid bodies” are when the lens epithelial cells near the equator persist after surgery and migrate beneath the IOL onto the denuded posterior capsule
What are Anterior Polar Cataracts?
An early-onset cataract where an opaque plaque is formed near the center of the lens epithelium.
T/F: Diabetes increases the risk for early-onset cataracts
True
T/F Long-term exposure to high-dose steroids increases the risk of PSC cataract
True
What is glass-blowers cataract?
Long-term exposure to infrared light and focused microwaves can increase risk of cataracts.
