Crystalline lens

• does refractive index vary across the lens, and how?

  yes, higher refractive index in the nucleus then the cortex of the lens.
• describe the capsule of the lens

  how does it act as a physical barrier?

  how is it adapted to help accommodate?

  thicker black line surrounding the lens.

  
  

  it acts as a physical barrier and doesn't have a blood supply so we need nutrients to get in from aqueous humour but keep bacteria out.

  Glucose provides energy, mostly anaerobic respiration, waste product is lactate so we want it to get out the lens by the barrier.

  
  

  It's a modified basement membrane, made of collagen so robust but also flexible so it allows the lens to change shape during accommodation. the difference in thickness of the capsule helps when accommodating.
• The epithelium of the crystalline lens:

  - where is it located?

  - what junctions are involved?

  - central cells? (shape, structure)

  - equator cells? (structure)

  under the capsule, only on the anterior surface of the lens extending up to the equator

  
  

  desmosomes and gap junctions connect to cortex

  
  

  central cells are cuboidal but are hexagonal in shape. They are interdigitated to help in the transport of getting nutrients in and waste out through the barrier of the capsule.

  
  

  equator cells are columnar, not tightly bound. produce lens fibres.
• Stroma of the crystalline lens

  - structure

  - junctions


  hexagonal, very long thin fibres. the fibres we find in the middle have been there for the longest, gradually overtime producing a ring of fibres. newest fires in cortex. oldest fibres in nucleus

  fibres meet at y shaped structures: erect anterior, inverted posterior (elongate). many layers/lamellae

  
  

  need gap junctions so nutrients can get in. interlocking junctions helps with the flexibility in lens shape during accommodation.
• Zonules of Zinn

  - what doe they do?

  - relaxed state?

  - accommodated state?

  allow lens to get thicker and thinner - accommodate. keeps the lens in suspension, attach the ciliary body to the equator of the lens. Vitreous attachment formed.

  
  

  relaxed state: ciliary body muscle is pulling lens tail and relaxes. zonules under tension, lens is pulled to it's flattest position. (we don't need close focus or accommodate).

  
  

  accommodate: ciliary body muscles contract and pulled towards each other. zonular tension released so lens can take up fatter shape to give us the power we need for close focus. lens surface steepen, increasing overall power.
• Transparency, how is light scattering minimised by? (4)

  - soluble proteins called crystallin's increases the refractive index whilst maintain transparency. They are dissolved in the lens fibres

  - Narrow cell membranes, so there is less 'things' for light to bounce off of and be scattered around. This also minimises the space in the cells.

  - No nuclei or organelles, less scattering of light to be bounced off of

  - highly regular arrangement
• what effect does refractive index have on transparency?

  it changes over distance and needs to be less then the wavelength of light.
• how is low water content maintained?

  what is sodium potassium pump?

  tightly regulated electrolyte balance - sodium potassium pump in epithelium.

  draws potassium in and kicks sodium out of the cell so water is drawn out of the cell via osmosis.
• Metabolism.

  -what is the main energy source, where does it come from?

  -where has the highest metabolic activity?

  glucose, comes from aqueous mainly from anaerobic respiration (glycolysis).

  
  

  some aerobic respiration happens in epithelium where it has the highest metabolic activity.
• What is the paradox of aerobic life?

  how does it cause damage, and what does it cause?

  what does the lens have to prevent this?

  oxygen is vital but dangerous to biological tissue.

  Most oxygen is reduced to water, but 2% becomes reactive oxygen species or hydrogen peroxide.

  These cause damage to protein and membranes, leading to cataract and muscular degeneration.

  The lens has natural antioxidants which help to neutralise the hydrogen peroxide into water, and reduce the oxygen species to prevent protein an tissue damage.
• Effect of aging on the crystalline lens?

  - metabolic (what is hexokinase)?

  - molecular?

  - structural?

  - optical?

  - reduction in metabolic activity because the antioxidant aren't working as well as they used to, and hexokinase doesn't work properly (enzyme that breaks the glucose down so we can get energy in the eye).

  
  

  - lens becomes insoluble and begins to become more solid. Build up of protein that have colour to them. The sodium potassium pump isn't effective so water is unable to leave the lens.

  
  

  - lens can't change shape as well as it could due to the amount of layers being added to it. (Unable to accommodate as well). Lens becomes less flexible, we get gaps between the cells (vacuoles forcing the fibres apart).

  
  

  - increase scatter of light. Brunescence (effect of colour vision - becomes more brown).
• cataract (nuclear), what happens?

  myopic shift

  signs?

  in the middle of the lens which is the nucleus.

  the proteins become less soluble and they aggregate together so we can get a colour change, no longer clear now ang the light is scattered more in the centre.

  This can cause myopic shift (short sighted, increase refractive index mean increase power of eye so light is being focused in front of the retina).

  
  

  patient may report problems with distance vision but will find it easier to read.
• cataract (cortical)

  cause? (4)

  appear as?

  due to sunlight, swelling and breakdown of lens fibres and crystalline aggregation.

  
  

  appear as vacuoles and water clefts. affected fibres often appear as spokes (more in outer layers).
• Cataract (posterior subcapsular)

  causes? (3)

  drugs?

  when may vision be affected?

  due to swelling, breakdown of lens fibres and abnormal epithelial cells more to posterior pole and clump together on the back surface of the lens under the capsule.

  
  

  drug induced - steroids

  
  

  vision affected in bright light (miosis), and when reading (miosis).
• cataract

  - diabetes

  -downs syndrome


  - high glucose levels, too much glucose forcing itself into the lens so the lens uses sorbitol pathways but it's too big to get out, so stays in the lens. Leads up to water uptake by osmosis and you can get quick refractive index change and myopic shift can occur. Nuclear cataract may appear earlier.

  
  

  - born with it (congenital). cataract appears in late childhood
• 4 other causes of cataract?

  - trauma

  - radiation (X rays, IR)

  - chronic uveitis (inflammation)

  - high myopia
• Cataract surgery?

  - small limbal incision.

  - portion of anterior surface of capsule removed.

  - phacoemulsification probe used to break up and remove cloudy lens.

  - intraocular lens inserted and placed in remaining capsular bag

  - typically no sutures (stitches) required