What is the function of the lens?
Fine tunes projection of images onto the retina
What does the lens need to achieve its function?
- Be transparent
- Higher refractive index than media in which it is suspended
- Curved surfaces that permit both near and far focussing
What is loss of lens transparency called?
Cataract
Describe the tissue architecture of the adult lens
- Epithelial cells on anterior exterior
- transition into differentiating fibre cells in outer cortex throughout life
- Older mature fibre cells internalised and retained in nucleus
- age gradient set up in embryo
7 stages of Lens embryonic development
- Optic cup
- Lens placode
3 & 4. Formation of lens vesicle - Elongation of primary fibre cells
- Embryonic lens
- Internalisation of embryonic nucleus by secondary fibre cells
What do primary fibre cells form?
Lens embryonic nucleus
What causes primary fibre cells to form the lens embryonic nucleus?
Growth factors secreted by the retina, which cause posterior epithelial cells to elongate to fill the lens vesicle. The anterior epithelium remains a single layer of cells
Where are secondary fibre cells derived from?
Anterior epithelial cells located at the equator
What do secondary fibre cells do to primary fibres?
Internalise them
What does fibre cell differentiation involve?
- Elongation
- Loss of cell nuclei and cell organelles
- Expression of fibre specific proteins (e.g. crystallins)
What does the refractive index gradient in the lens do?
Correct for spherical aberration
How does the lens achieve a high refractive index?
Express a high concentration of cytoplasmic proteins called crystallins
What percent of wet weight of lens are crystallins?
Over 40%
What are crystallins?
- Proteins such as enzymes that have been recruited to increase protein concentration
- Other crystallins (alpha-crystallin) have chaperone activity and prevent protein aggregation
What does an aggregation of crystallins lead to?
Light scattering and cataract formation
What profile/structure do DF cells adopt?
Hexagonal cross-sectional profile that facilitates tight packing and minimises extracellular space
How is the hexagonal cross-sectional profile of DF cells maintained?
- Cytoskeleton of lens-specific beaded filaments
- Specialised cell-to-cell junctions
Example of some of the specialised cell-to-cell junctions in DF cell formation?
- Ball and socket joints
- Ridges (tongue and groove)
- Square arrays
- Gap junctions
