Ocular: Crystalline Lens Flashcards
The lens is a ________ structure
Biconvex
Radius of curvature for the anterior lens:
8-14mm
Radius of curvature of the posterior lens:
5-8mm
What is the size of the lens at birth?
6.5mm
What is the diameter of the lens in adults?
10mm diameter
What is the thickness of the unaccommodated lens in an adults?
4mm
Does the diameter of the lens increase with age?
No
Does the thickness of the lens increase with age?
Yes, at 0.02mm/year
Lens accounts for _____-_____D of the total dioptric power of the eye
+15 - +20D
Range of dioptric power of the lens is reduced to ____D by age 40
+8D
Range of dioptric power at age 60:
+1-2D
Does the lens have the same index of refraction throughout the surface? why?
NO, due to non-uniform distribution of crystalline proteins in the lens fibers
What tissue makes up the crystalline lens?
Specialized epithelial tissue responsible for fine-tuning the image projected onto the retina
Protein concentration of the lens:
Highest protein concentration of any tissue in the body!!
The lens has a _______ refractive index than the medium in which it is suspended
Higher
Refractive power of the lens depends on the __________ and the __________ of the proteins within the lens
Concentration and shape
What alters the protein concentration in the lens?
-H2O movement in diabetes
-polymerization
Why is the various refractive power of the lens important?
It permits the dioptric apparatus to focus on objects that are near or far
Lens has about ___D at rest, and about ____D more with accommodation
15D, 15D
What suspends the lens?
Zonules
Where do the zonules insert into the lens?
Into the lens capsule near the equator
Where do the zonules originate? \
Non-pigmented ciliary epithelium
What are zonules composed of?
The protein fibrillin
What is responsible for the alterations in the lens curvature that occurs during accommodation?
Changes in the tension that are applied to the zonules
The lens equator is ____ away from the ciliary processes of the pars plicata
0.5mm
How thick is the lens capsule anteriorly?
20 microns
Is the lens capsule in contact with the anterior lens epithelium?
Yes, directly
What is the anterior lens capsule secreted by?
Anterior lens epithelium
In the posterior lens, is there an epithelium?
No
What secretes the posterior lens capsule?
The lens fibers
How thick is the posterior lens capsule?
3 microns thick
What is the lens capsule composed of? (Hint which collagen)
Type IV collagen fibers embedded in a matrix of glycoproteins and sulfated proteoglycans (allows for stretch up to 60% of circumference without tearing)
Are there elastic fibers in the lens capsule?
NO
Does the anterior capsule change size with age?
Yes, 1/3 larger by age 65
2 main functions of lens capsule:
-diffusion barrier that is freely permeable to low-molecular weight compounds
-makes the lens pliable to pull of zonular fibers during accommodation
What is the zonular lamella?
The outermost layer of the lens capsule where the zonules will attach
What cells make up the lens epithelium? Is there epithelium on both anterior and posterior sides?
-Monolayer of cuboidal cells
-ONLY anterior
Where do lens epithelial cells proliferate from?
Equator (germinative zone)
In the lens, the cells can stretch and become ________ shaped
Columnar
What percent of pumps are located in the anterior epithelium?
90%
3 functions of the lens epithelium:
-form lens fibers at germinative zone
-central portion involved in transport of substances from aqueous to lens interior
-secretes lens capsule
How does nutrient and ion exchange at the lens epithelium-fiber interface occur?
Numerous gap junction
Secondary lens fibers make up the ____ of the lens
Bulk
Where do secondary lens fibers originate and what type of arrangement do they take on?
From the equator and take up meridional arrangement
Basal portion of the lens fibers move posterior along the _________ _________
Posterior capsule
As the lens fiber is pushed towards the center, which portion elongates? And where does it situate itself?
The apical portion elongates, situates itself under the anterior lens epithelium
What happens during lens fiber differentiation?
-Fiber cells withdraw from the cycle and elongate significantly
-they express large amounts of crystallin protein
-acquire several specializations to their plasma membrane
-degrade all membrane bound organelles
Each lens fiber is ___ long and __-shaped
10mm long and U-shaped
Ends of the lens fibers meet with lens fiber ends from the other side and form _________
Sutures
Lens fibers that lose nuclei and detach from the capsule are…
Compacted on older fibers toward the center of the lens
What causes Y sutures to be formed?
As the lens increases in size, fibers are unable to stretch anteroposterior and form this Y pattern
How is transparency and high refractive index of crystalline lens maintained?
-precise alignment of lens fibers
-minimal intercellular space
-accumulation of crystallin
-hexagonal shape maximizes packing
Disruption of the organization of the lens fiber cells can destroy the transparency. This is called _______________
Cataract formation
Fiber membranes can become more interdigitated by forming _____________ junctions
Ball-and-socket junctions
What do ball and socket junctions do?
Stabilize the lateral membranes of the fiber cells and make sure they remain tightly connected during accommodation
Lens fiber cells are linked to their neighbors along the _________ membranes by ______
Lateral, cadherins
What holds the lens fibers together during accommodation?
Interlocking ball-and-socket junctions and N-cadherin-containing cell-cell adhesion complexes
Why is minimizing the intracellular space important?
For reducing light scattering and maximize transparency
What happens when the lens fiber lose their nuclei?
The organelles of the fiber become specialized to form lens crystallins (alpha and beta) that make up 40% of the fiber
Crystalline concentration varies across the lens which provides a…
Refractive index gradient that is higher in the nucleus than the outer cortical surface
Embryonic nucleus division of the lens:
-Earliest mass at the center of the lens composed of primary lens fibers
-2 months gestation
-does not increase in size
Fetal nucleus:
-formed before birth
-contains Y sutures
-does not increase in size
Adult nucleus:
-fibers made between birth and sexual maturation
-increase in size throughout life
-they sclerose and become yellow with age
Lens cortex
Newly formed fibers that contain nuclei and surround the adult nucleus
What is the leading cause of elderly blindness worldwide?
Cataracts
What is the most common surgical procedure in the aged population?
Cataract surgery
A nuclear cataract is do to a decline in ___________ that makes the lens fibers susceptible to __________ damage
Glutathione, oxidative
How do cortical cataracts form?
-decrease in glutathione activity
-increase in calcium, sodium and water
-water forms lakes which separate the cells
-separation in cells create water vacuoles
-water vacuoles cause light scatter
-cells burst causing the proteins to expose
-proteins become oxidized which forms the cataract
How do posterior subscapular cataracts form?
Epithelial-like cells migrate from the equatorial region and accumulate at the posterior pole forming an opacity
What is posterior capsule opacification?
Most common complication after cataract surgery
What percent of patients experience posterior capsule opacification after cataract surgery?
10%-50% of patients
How is posterior capsule opacification treated?
Neodymium:yttrium-aluminum-garnet (YAG) laser
How does the lens protect the retina?
It protects the retina from harmful UV effects
Scattering of ____ and ____ light protects the retina, but what is the trade off?
-UV, blue
-trade off is that lens places itself at greater cataract development
How is the lens protected from UV light?
-low O2 concentration
-cornea absorbs most short wavelength
-lens lipids are saturated
-DNA of lens epithelium is protected behind iris
-DNA is autophaged as lens fiber develops
-strong reductive capacity
What causes the lens to be weakend by its anatomy?
It can stuff cells inwards which can further compact the lens
What are two protectors against oxidative damage in the lens?
-glutathione
-ascorbic acid
What helps lens maintain transparency at the cellular level?
-small lens fibers
-uniformity of lens fibers
-regularity of packing
-avascular
What maintains lens transparency at the molecular level?
-proteins are uniform
-proteins are small
-proteins [ ] increases towards nucleus that creates a RI gradient
How does water enter the back of the lens?
Osmotic pressure
Flow of water through the lens
-Water flows through the cortex
-avoids the center of the lens
-pumped out of anterior surface vis Na+/K+ ATPase pumps
Why are gap junctions important in the lens?
Maintenance of metabolic homeostasis and transparency of the lens
Relative pupillary block:
Too narrow lens-iris diaphragm
3 ways to create a relative pupillary block:
-narrow iris and pressure from posterior chamber displaces iris into the anterior chamber angle (CAG)
-lens grows too large
-lens imbibe water and swells
Diabetic cataract formation
Insulin insensitive-build up of blood glucose causes an increase in sorbitol production which depletes NADPH that causes cataract formation because glutathione needs NADPH to reduce free radicals
Change in lens osmolarity causes changes in:
-thickness
-ROC
-Rx
