Molecular physiology of transparency- The Lens. Lecture 4 Flashcards
Whats unique about the lens optical power?
The lens’ optical power is dynamic (accomodation)
What is unique about the lens’ refractive index?
The lens has a refractive index gradient to account for spherical aberration.
What are three properties of the biological lens?
- Be transparent
- Have a curved surface that permits both near and far focusing (accomodation)
- Generate a refractive index gradient to account for spherical aberration.
What is focal length?
Distance between the the lens and the retina
Why must the lens correct for spherical aberration?
The cornea introduces positive spherical aberration and the lens must use negative spherical aberration to correct for it.
What is the term used for loss of lens transparency?
Cataract
How is the lens transparent?
Its unique structure eliminates light scattering.
What is unique about the lens?
It is avascular
The fibre cells lose nuclie and organelles.
What are the cells of the lens?
Epithelium
Differentiated Fibre Cells (Outer cortex)
Mature fibre cells (Nucleus)
What do fibre cells promote?
Fibre cell structures promote regular packing of fibre cells that restrict the size of the extracellular space.
Is the lens polarised?
Yes the lens is polarised with Ant + Post poles and an equator.
Describe the transitioning of cells over time?
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.
When is the age gradient set up?
The age gradient is set up in the embryo
Describe the lens embryonic development:
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.
What portion of the lens does the epithelium originally surround?
The entire lens. There is posterior and anterior epithelium (remembering that the lens is polarised.)
During embryonic development how are primary fibre cells formed?
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.
Describe the differentiation of secondary fibre cells:
Secondary fibre cells are differentiated from anterior epithelial cells at the equator . The secondary fibre cells internalise the primary fibre cells.
What does fibre cell differentiation involve?
- Elongation
- Expression of fibre specific proteins
- Loss of cell uncle and cell organelles.
What are fibre specific proteins called?
Crystallins
What must the lens have in order to achieve a high refractive index?
The lens must express a high concentration of cytoplasmic proteins called crystallins.
What do crystallins account for?
Crystallins account or 40% of the wet weight of the lens.
How are crystallins classified?
AS either classical crystallins or taxon specific
What are taxon specific crystallins?
Proteins such as enzymes that have being recruited to increase protein concentration
What is the function of classical proteins?
Classical crystallins i.e Alpha crystalline have Chaperone activity and prevent protein aggregation
What does aggregation of proteins lead to?
Aggregation of crystallins lead to light scattering and cataract formation
What do crystallins also do?
The gradient of crystalline concentration corrects for spherical abberation
What is the profile of differentiated fibre cells?
Differentiated fibre cells adapt a classical hexagonal cross section profile that facilitates TIGHT PACKING AND MINIMISES EXTRACELLULAR SPACE
How is the hexagonal profile of DF cells maintained?
- Cytoskeleton of lens specific beaded filaments
- Specialised cell:cell junctions
i. e - Ball and socket joints
- Ridges (tongue and groove)
- Gap functions
What is lens transparency due to?
Fibre cell structure
Is lens transparency passive?
No it is an active process and must be maintained
Does the lens have a circulation system?
Yes an internal circulation system
It is however avascular
What drives the internal circulation system?
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.
Describe fibre cell connections
Fibre cell’s have cellular fusions and high expressions of gap junctions.
What sets the membrane potential for the lens?
The membrane potential for the lens is set by the expression of Na+ and K+ (by coupling), in epithelial cells.
Describe the circulation of ions in the lens.
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
What does the circulation of ions result in?
Movement of ions precipitates the osmosis of water and within the water solutes, nutrients. The efflux of ions and water removes waste.
Whats the physiological constraint of the lens structure?
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
Why does the lens need a specialised internal circulation system?
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.
What do fibre cells form within the lens when their membranes contain fusions?
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.
