Lens

Question 1

What are the functions of the lens?

Answer 1

Transmit and refract light
Accommodation
UV Absorption

Question 2

Describe accommodation.

Answer 2

Refractive surfaces are able to change curvature so the optical system can change focus

Question 3

What properties of the lens allow for transparency and light refraction?

Answer 3

Avascularity
Transparency
n> medium in which it resides (aqueous)
Appropriately curved refractive surfaces

Question 4

Does lens development occur throughout life?

Answer 4

Yes

Question 5

Describe the orientation of the lens within the eye.

Answer 5

Sits behind the iris
In front of vitreous
Medial part of iris rests on lens

Question 6

What is the thickness of the lens? What is the diameter of the lens? How do each change with age?

Answer 6

Thickness= 3.5 to 5mm
Gets thicker as we get older pushes on iris, AC becomes less deep, angle closure

Diameter= 6.5-9mm (infant to teen)
Stops growing unlike thickness

Question 7

What is the index of refraction of the central and peripheral portions of the lens? How does it change with age?

Answer 7

n= 1.42 centrally
n= 1.36 peripherally
Changes with cataracts and age as proteins are lost in medium

Question 8

How far is the lens from the corneal back surface?

Answer 8

3.0mm
Varies with myopia and hyperopia

Question 9

What is the radius of curvature of the back surface when accommodated and unaccommodated? How does it compare to front surface?

Answer 9

R unaccomm)= ~ 5.5 mm
R (accomm) = ~ 5.0 mm)
Back surface is much steeper, when accommodated front lens changes more drastically

Question 10

What is the radius of curvature of the front surface when accommodated and unaccommodated?

Answer 10

R (unaccomm) = ~ 11-12 mm
R (accomm) = ~6.5 MM
Refractive power and steepness increases with accommodation

Question 11

What property of the lens allows for spherical aberrations (blurriness at the edge of an image)? How does this differ in young people?

Answer 11

Spherical aberrations due to prolate shape
Young people are more oblate

Question 12

What is the structure of the lens from outside to inside?

Answer 12

Lens capsule
Lens epithelium
Lens fibers

Question 13

Describe the structure of the lens capsule. What is it produced by?

Answer 13

Modified basement membrane (very thick basal Lamina)
Produced by lens epithelium (anterior) and lens fibers (posterior)
Continuously produced throughout life
Variable thickness

Question 14

Describe the collagen structures within the lens capsule.

Answer 14

Type 5 collagen and Gags
Lamellar structure for transparency and elasticity (no elastin)
Flexibility helps mold cortex and for accommodation
Allows for diffusion of smaller molecules

Question 15

Describe the thickness of the lens capsule.

Answer 15

Paraequitorial (peripheral) region is thickest next to equator where zonules attach
Thin in posterior because no lens epithelium

Question 16

Where do zonules originate? What do they run between? Where do they insert into? How are they organized?

Answer 16

Originate from pars plana
Run between ciliary processes in valley
Inserts into para equatorial
Consult on either side of ciliary process, make triangle with apex on the lens and base in the pars plana

Question 17

What are the functions of the lens epithelium?

Answer 17

Secrete the lens capsule
Transport nutrients into the lens from the aqueous
Differentiate into the lens fibers

Question 18

Describe the lens epithelium.

Answer 18

Single layer of cells beneath anterior but not posterior lens capsule
Simple cuboidal epithelium that becomes more columnar nearer to equator
Basal surface against capsule
Lateral membranes with tight junctions
Cytoskeletal elements provide support and distribute mechanical stress
Proliferative capacity greatest at equator – germinative zone

Question 19

Describe the lens fibers.

Answer 19

Bulk of the lens
Long and thin, stretching from pole to pole
Arranged in concentric layers
Linked by gap junctions and interdigitations (ball and socket)
Synthesize crystallins

Question 20

What do lens fibers evolve from?

Answer 20

Lens epithelial cells (transition at germinative zone)

Question 21

What is the orders of the nuclei in the lens? How does the inner core compare to the outer core? Do those older layers have a nucleus?

Answer 21

Embryonic
Fetal
Infantile
Adult
Inner core is made up of oldest fibers that have no nucleus

Question 22

How does the cortex compare to the nucleus? Does the cortex have nuclei?

Answer 22

Newer lens fibers make up the cortex
Have nuclei

Question 23

What are sutures? How do they differ anteriorly and posteriorly?

Answer 23

Areas where fibers connect coming from opposite equatorial poles
Anterior are right side up (Y)
Posterior are upside down (Y)

Question 24

Describe the orientation of lens fibers.

Answer 24

Regular arrangement
Ball and socket attachment to each other

Question 25

Describe lens fibers. Where do lens fibers run within the lens?

Answer 25

Make up bulk of lens and evolve from lens epithelium to help lens grow
Run from anterior pole to posterior pole

Question 26

Where is the lens epithelium located (anteriorly or posteriorly)?

Answer 26

Anteriorly

Question 27

What area of the lens controls lens fibers evolution and dividing?

Answer 27

As you move toward germinative zone in peripheral, proliferation capacity increases.
Fibers closest to center are the oldest and get younger as you move outward (nucleus moves posteriorly)

Question 28

How does the lens change with age? Nucleus and cortex? How does this affect vision?

Answer 28

Gets thicker overall
The nucleus gets smaller and cortex gets larger as you age
In young person the nucleus takes up most of the lens
Increased light scatter

Question 29

In the unaccommodated state, how much of the eye’s power comes form the lens?

Answer 29

about 1/4-1/3 of the eye’s powe

Question 30

How does the unaccommodated eye compare to the accommodated eye?

Answer 30

Unaccommodated – lens is relatively flat
Accommodated – ciliary muscle contracts pulling ciliary body inward and forward, zonular tension decreases, lens steepens

Question 31

How does presbyopia affect the lens?

Answer 31

Loss of accommodation with age because lens capsule is less pliable