Lens Flashcards
What happens to alpha crystallins with age?
Alpha crystallins decrease dramatically with age; by age 45, there are NO alpha crystallins in the lens nucleus.
Alpha crystallins function as molecular chaperones, preventing degradation of other crystallins.
What is the consequence of a reduction in alpha crystallins?
An increase in degradation of lens fiber cells, contributing to cataract formation.
The degradation of lens fiber cells is a key factor in cataract development.
How much does the lens thickness increase per year?
0.02 mm per year.
What happens to the lens diameter after the teenage years?
The lens diameter is relatively stable.
How does the anterior lens capsule thickness change with age?
Increases with age.
What is the posterior lens capsule thickness like with age?
Relatively stable.
Where is the lens capsule thickest?
At the anterior mid-peripheral portion of the lens (pre-equatorial region).
The lens capsule is the thickest basement membrane in the entire body and is comprised of type IV collagen.
What happens to the radius of curvature of the lens with age?
The radius of curvature decreases, becoming more convex.
What theoretical effect does a more convex lens have on myopia?
It should result in the patient becoming more myopic due to age.
What change occurs to the center of the lens with age?
The center of the lens moves anteriorly, causing a decrease in anterior chamber depth.
How does amino acid concentration in the lens change with age?
Decreases with age.
What happens to glutathione activity with age?
Decreases.
What ions increase in concentration in the lens with age?
Na+, Ca2+, and H2O concentrations increase.
What factors contribute significantly to cataract formation with aging?
Increase in intracellular Ca2+, decrease in glutathione, and decrease in crystallins.
What happens to nuclear fibers as they age?
They begin to lose their nucleus and organelles and accumulate a yellow-brown pigment.
Where does nuclear sclerosis begin?
In the embryonic nucleus and expands to include fetal and adult nuclei.
What is the most common cataract due to aging?
Nuclear sclerosis.
What is produced through anaerobic glycolysis in the lens?
Over 70% of the glucose required by the lens is produced through anaerobic glycolysis.
What role does the Na+/K+ ATPase pump play in the lens?
It helps to establish a balance between H2O and ions within the lens to maintain lens transparency.
What happens to glucose if hexokinase is not available?
Glucose is converted to sorbitol via the enzyme aldose reductase.
What can excessive sorbitol in the lens cause?
It can create an osmotic gradient that favors the movement of H2O into the lens, causing lens swelling, fiber damage, and cataract formation.
How does diabetes affect the lens?
Excessive levels of glucose in diabetes lead to sorbitol accumulation, resulting in early cataract development and a shift in refractive error due to lens swelling.
What is the primary protector against oxidative damage in the lens?
Glutathione acts as a reducing agent and detoxifies hydrogen peroxide.
How is glutathione supplied to the lens?
It is transported into the lens from the aqueous and can also be synthesized from lens epithelial cells and superficial fiber cells.
What is the role of ascorbic acid in the lens?
Ascorbic acid helps to protect the lens from oxidative damage.
How does the concentration of ascorbic acid in the lens compare to the aqueous humor?
Ascorbic acid is found in a much higher concentration in the lens compared to the aqueous humor.
What maintains electrolyte balance in the lens?
Active Na+/K+ ATPase pumps on lens epithelial cell membranes pump Na+ into the aqueous humor and K+ into the lens.
What is the structure of lens fiber cells?
Lens fiber cells lack membrane-bound organelles and are packed closely together to minimize light scattering.
What is contained in the cytoplasm of lens fibers?
The cytoplasm of lens fibers is crowded with crystallins that minimize light scattering by destructive interference.
How is Ca2+ concentration managed in the lens?
Multiple transport processes monitor and limit the concentration of Ca2+ inside the lens to prevent cataract formation.
How is the embryonic nucleus formed?
The embryonic nucleus is formed from primary lens fibers of the posterior lens epithelium during embryological development.
What contributes to lens growth after embryonic development?
All remaining growth of the lens is due to the production of secondary lens fibers by the anterior lens epithelium.
Where does mitosis of fiber cells occur?
Mitosis of fiber cells occurs in the germinative zone of the anterior lens epithelium.
What happens to lens fiber cells after mitosis?
After mitosis, lens fiber cells migrate through the transition zone to the equator, losing organelles and acquiring crystallins.
Which lens component has the greatest metabolic demand?
The anterior lens epithelium has the greatest metabolic demand and contains a significant amount of mitochondria.
What is the role of aqueous humor for the anterior lens epithelium?
Aqueous humor provides nutrients, including glucose and O2, to the anterior lens epithelium for mitosis.
What is the function of Na+/K+ ATPase pumps in the lens?
Na+/K+ ATPase pumps on the epithelial cell membranes maintain the appropriate ion balance to control lens dehydration.
