Physiology Of Hte Lens And Agigng Changes Of The Lens Flashcards
Anterior sutures
Upright Y
Posterior lens sutures
Upside down Y
Y sutures with age
More branches
Look like stars kinda
How much power does the lens provide
15D (1/3 of the total power)
What protects the retina from UV light
Lens
Lens composition
Water
Proteins
Others
Total lens proteins
Water soluble proteins (90%)
-crystallins, a, b, y
Alpha crystalline
Contributes to lens transparency and gives the lens a significantly higher RI than surrounding fluids
- offers resistance to the degradation of the other crystalline, beta nad gamma, and is called the molecular chaperone
- crystalline concentration varies among the lens, providing a refractive index gradient that is higher in the nucleus, than the outer cortical surface
- SA in the lens is reduced due to the gradient index system and peripheral flattening
Transparency on a cellular level of the lens
Small lens fibers Uniformity of the lens fibers Regularity of packing Lack of organelles Avascular
Transparency at a molecular level of lens
Proteins are uniform
Proteins are small
Proteins concentration increases towards nucleus, creates RI gradient that is higher in nucleus, than the outer cortical space
Transparency: water in the lens
Water is pumped out of the lens from the anterior surface by Na/K pump
- water enters the lens from the back because of osmotic pressure
- we do not want water getting into the lens
Mitosis activity of the lens
- mitosis of secondary fiber cells occurs in the germinative zone of the anterior lens epithelium
- after mitosis, lens fiber cells gradually migrate through the transition zone and into the equator where elongation occurs
Protectors against oxidative damage in the lens
Glutathione
Ascorbic acid
Glutathione in the lens
-primary protector against oxidative damage in the lens
-transported in to the lens from the aqueous and can be synthesized from lens epithelial cells and superficial fiber cells
0detoxifies hydrogen peroxide
Ascorbic acid in the lens vs aqueous
Much higher in the lens
Aging changes in the lens
Cataracts and reduction of accommodation
Ability of accommodation decreases by how much in presbyopia
Quarter of the age
40 yr old
15-(40x0.25)=5D
Nuclear cataract
- decline of glutathione, making the fibers susceptible to oxidative damage
- older nuclea fiber lose organelles and their nucleus and gain yellow-brown pigment
Age related cortical cataract
- decreased glutathione activity, increases Ca2+, Na, and water
- water forms lake, lakes separate cells, water vacuoles created, cause light scatter, cells burst, proteins exposed, proteins oxidized, cataracts are formed
Posterior subcapsular cataract
-epithelial like cells migrate from the equatorial region and accumulate at the positerio pole forming an opacity
Other lens aging changes
Decrease in crystallins, esp alpha Thickens Radius of curvature decreases Decreases AA Center of lens moves anterior with age
Lens in diabetes
Diabetic cataract
- insulin insensitive build up of blood glucose
- increase in sorbitol production (hydrophilic)
- increases osmotic pressure
- increases water uptake
- causes lens swelling
- causes cataract formation
- depletion of NADPH during sorbitol production
- glutathione requires NADPH to reduce free radicals
- oxidative stress
- polymeriztion of proteins
Refractive change in lens in diabetes
Change in osmolarity causes changes in
- thickness
- radius of curvature
- Rx
Unusual changes in RX may indicate uncontrolled diabetes
Unusual changes in RX may indicate
Uncontrolled diabetes
