Molecular physiology of transparency- The Lens. Lecture 4

Question 1

Whats unique about the lens optical power?

Answer 1

The lens’ optical power is dynamic (accomodation)

Question 2

What is unique about the lens’ refractive index?

Answer 2

The lens has a refractive index gradient to account for spherical aberration.

Question 3

What are three properties of the biological lens?

Answer 3

• Be transparent
• Have a curved surface that permits both near and far focusing (accomodation)
• Generate a refractive index gradient to account for spherical aberration.

Question 4

What is focal length?

Answer 4

Distance between the the lens and the retina

Question 5

Why must the lens correct for spherical aberration?

Answer 5

The cornea introduces positive spherical aberration and the lens must use negative spherical aberration to correct for it.

Question 6

What is the term used for loss of lens transparency?

Answer 6

Cataract

Question 7

How is the lens transparent?

Answer 7

Its unique structure eliminates light scattering.

Question 8

What is unique about the lens?

Answer 8

It is avascular

The fibre cells lose nuclie and organelles.

Question 9

What are the cells of the lens?

Answer 9

Epithelium
Differentiated Fibre Cells (Outer cortex)
Mature fibre cells (Nucleus)

Question 10

What do fibre cells promote?

Answer 10

Fibre cell structures promote regular packing of fibre cells that restrict the size of the extracellular space.

Question 11

Is the lens polarised?

Answer 11

Yes the lens is polarised with Ant + Post poles and an equator.

Question 12

Describe the transitioning of cells over time?

Answer 12

The transition of epithelial cells into Differentiating Fibre cells in the outer cortex continues throughout life. Older mature fibre cells become internalised and are retained in the nucleus.

Question 13

When is the age gradient set up?

Answer 13

The age gradient is set up in the embryo

Question 14

Describe the lens embryonic development:

Answer 14

1) Optic cup
2) Lens placed
3 & 4) Formation of the lens vesicle
5) Elongation of the Primary fibre cells
6) Embryonic Lens
7) Internalisation of the embryonic nucleus by Secondary fibre cells.

Question 15

What portion of the lens does the epithelium originally surround?

Answer 15

The entire lens. There is posterior and anterior epithelium (remembering that the lens is polarised.)

Question 16

During embryonic development how are primary fibre cells formed?

Answer 16

In response to growth factors secreted by the retina, the posterior epithelial cells elongate to fill the lens vesicle. The anterior epithelium remains a single layer of cells.

Question 17

Describe the differentiation of secondary fibre cells:

Answer 17

Secondary fibre cells are differentiated from anterior epithelial cells at the equator . The secondary fibre cells internalise the primary fibre cells.

Question 18

What does fibre cell differentiation involve?

Answer 18

• Elongation
• Expression of fibre specific proteins
• Loss of cell uncle and cell organelles.

Question 19

What are fibre specific proteins called?

Answer 19

Crystallins

Question 20

What must the lens have in order to achieve a high refractive index?

Answer 20

The lens must express a high concentration of cytoplasmic proteins called crystallins.

Question 21

What do crystallins account for?

Answer 21

Crystallins account or 40% of the wet weight of the lens.

Question 22

How are crystallins classified?

Answer 22

AS either classical crystallins or taxon specific

Question 23

What are taxon specific crystallins?

Answer 23

Proteins such as enzymes that have being recruited to increase protein concentration

Question 24

What is the function of classical proteins?

Answer 24

Classical crystallins i.e Alpha crystalline have Chaperone activity and prevent protein aggregation

Question 25

What does aggregation of proteins lead to?

Answer 25

Aggregation of crystallins lead to light scattering and cataract formation

Question 26

What do crystallins also do?

Answer 26

The gradient of crystalline concentration corrects for spherical abberation

Question 27

What is the profile of differentiated fibre cells?

Answer 27

Differentiated fibre cells adapt a classical hexagonal cross section profile that facilitates TIGHT PACKING AND MINIMISES EXTRACELLULAR SPACE

Question 28

How is the hexagonal profile of DF cells maintained?

Answer 28

• Cytoskeleton of lens specific beaded filaments
• Specialised cell:cell junctions
  i. e
• Ball and socket joints
• Ridges (tongue and groove)
• Gap functions

Question 29

What is lens transparency due to?

Answer 29

Fibre cell structure

Question 30

Is lens transparency passive?

Answer 30

No it is an active process and must be maintained

Question 31

Does the lens have a circulation system?

Answer 31

Yes an internal circulation system

It is however avascular

Question 32

What drives the internal circulation system?

Answer 32

The internal circulation system an electrically mediated system and is driven by a spatially segregated ion transport process.

Ion flow in at the poles and out at the equator.

Question 33

Describe fibre cell connections

Answer 33

Fibre cell’s have cellular fusions and high expressions of gap junctions.

Question 34

What sets the membrane potential for the lens?

Answer 34

The membrane potential for the lens is set by the expression of Na+ and K+ (by coupling), in epithelial cells.

Question 35

Describe the circulation of ions in the lens.

Answer 35

Ions a draw into the epithelium by Na and K pumps.
The ions diffuse through the extracellular space in the lens created by the fibre cell hexagonal structure. They diffuse into the fibre cells through Na, Cl leak channels. They then are efflux through a series of gap junctions between cells out of the lens. Thus completing circulation

Question 36

What does the circulation of ions result in?

Answer 36

Movement of ions precipitates the osmosis of water and within the water solutes, nutrients. The efflux of ions and water removes waste.

Question 37

Whats the physiological constraint of the lens structure?

Answer 37

Loss of organelles means that fibre cells gather the bulk of its energy from anaerobic glycolysis.

Lens is too large to rely on passive diffusion

Question 38

Why does the lens need a specialised internal circulation system?

Answer 38

To control its steady state volume as fibre cells do not posses Na and K pumps to do so.
- Therefore balance ion influx and efflux.

Supply metabolites to mature fibre cells.

Maintain the gradient of refractive index.

Question 39

What do fibre cells form within the lens when their membranes contain fusions?

Answer 39

Fibre cell “growth” rings. This means that the level of protein is uniform within each ring, creating the refractive index gradient towards the centre of the eye.