Chapter 10: Lens

Question 1

What characteristics must the lens have in order to be able to concentrate light onto the retina?

Answer 1

Must be transparent, and have a higher index of refraction than that of the surrounding fluids

Question 2

What is the lens capsule?

Answer 2

it is an elastic basement membrane that encloses the lens
Is acellular
composed mainly of Type IV collagen

Question 3

What are some characteristics of the anterior and posterior lens?

Answer 3

The anterior lens is thicker (15.5 micrometers) comapred to the posterior lens (2.8 micrometers) due to secretion of capsular material by anterior lens epithelium throughout life.

Question 4

Where do the zonular fibers insert into the lens?

Answer 4

The zonular fibers insert into the anterior and posterior lens capsule around the equator.

Question 5

Is the lens capsule a permeable membrane?

Answer 5

The lens capsule is permeable to the flow of ions, h20, ions, small molecules, and proteins (equal to or smaller than albumin)

Question 6

What is the lens epithelium? What is its function?

Answer 6

It is a single layer of cells that covers the anterior surface and have full metabolic capabilities.

Its primary role is regulating water and ion balance of the lens

Question 7

What is the germinative zone of the lens? Where is it located?

Answer 7

It is the active replicative or miotic zone of the lens, and is located as a ring anterior to the equator.

Question 8

What is the nucleus/cortex of the cell? What is it composed of?

Answer 8

The nucleus/cortex of the lens compose to substance of the lens, and consist of lens fibers produced from the epithelium of the germinative zones.

Question 9

How long do fibers of the lens survive?

Answer 9

Lens fibers last forever, from birth until death. They are never sloughed off, and the proteins are thought to be the longest lasting of any tissue in the entire body.

Question 10

Describe the formation of new lens fibers at the germinative zones of the lens?

Answer 10

New fibers form, elongate, and differentiate. Eventually, their nuclei and organelles disintegrate (process not well understood)

Question 11

What is the chemical compostion of the lens membrane?

Answer 11

The membrane contains a high content of saturated fatty acids, a high cholesterol: phospholipid ratio, and a high concentration of sphingomyelin.
All contribue to the packing and low fluidity of membrane

Question 12

How does the lens protein content compare to other tissues in the body? What proteins constitute the majority of lens proteins?

Answer 12

The lens has the highest protein content of any tissue in the body.
The crystallin proteins constitute b/w 90-95% of total lens proteins

Question 13

What are the crystallin proteins of the lens? What are some of their characteristics?

Answer 13

Crystallins are h20-soluble proteins expressed in the lens and other bodily tissues, and members of the heat shock protein family.

They must be very stable in structure and must remain soluble under conditions of increased protein concentration without forming aggregates.

Question 14

What are characteristics of Alpha Crystallins? What is their function?

Answer 14

They are the largest of the crystallins (600-800 kD)

They bind to proteins that are beginning to denature and prevent further denaturation/aggregation

Question 15

What are some characteristics of the Beta/Gamma Crystallins?

Answer 15

Beta Crystallins- composed of polypeptides ranging from 23-32 kD
Gamm Crystallins- monomeric proteins with a molecular mass of around 20 kD. Majority of their expression occurs early in development, and are most concentrated in the nuclear region of the lens

Question 16

What are the taxon-specific crystallins? What are their function?

Answer 16

Most are oxidoreductases, whose function is to find pyridine nucleotides that have bound to UV light, thereby protecting the eye.

Question 17

What are the cytoskeletal elements of the lens? What unqiue proteins are found and what is their role?

Answer 17

Actin filaments, Vimentin (intermediate filaments), and tubulin (microtubules)
Proteins: Phakinin and Filensin–found only in lens fibers, provide structural support and compose the cytoskeletal structure of the “beaded filament”
MIP= major intrinic protein, another lens fiber-specific protein, is an aquaporin.

Question 18

What is the “beaded filament” of the lens?

Answer 18

Composed of unique lens proteins phakinin and finensin, provides structural support and play a role in the differentiation, motility, shape change, and organization of cytoplasm

Question 19

Where does the lens obtain nutrients from?

Answer 19

Nutrients are obtained from surrounding fluid. Metabolic wastes are also released into this fluid.

Question 20

How do ion concentrations in the lens compare to that of the aqueous humor? What is responsible for the ionic balance of the lens and where is it found?

Answer 20

In the lens, Na+=10mmol/L and K= 120 mmol/L compared to the aqueous Na=150mmol/L and K=5 mmol/L
The Na+K+ATPase pump found on the anterior lens surface is responsible for maintaining lens ionic balance (Na pumped out, K pumped in)

Question 21

How do the lens fibers communicate with one another?

Answer 21

Via gap junctions, composed of members of the connexin family.

Question 22

How is energy produced in the lens?

Answer 22

ATP is produced primarily through anaerobic glycolysis in the metabolically active cells of the anterior epithelium
Glycolysis used due to decreased 02 tension

HMP shunt is also used, but under conditions of oxidative stress b/c it regenerates NADPH

Question 23

What is a “True Diabetic Cataract?”

Answer 23

a rapidly developing snowflake cataract that occurs in patients with poorly controlled Type 1 DM

Question 24

What kind of cataract do people with type 2 DM develop?

Answer 24

These patients dont develop a specific type of cataract, but do have a higher prevalence of age-associated cataracts that develop at a younger age.

Question 25

How does sorbitol contribute to cataract formation?

Answer 25

Under conditions of hyperglycemia, sorbitol builds up in the lens–> increased osmotic pressure–> H20 enters cells-> lens swells–> membrane damage–> cataract formation