crystalline lens

• function of lens

  -refracts light to focus it on foveal region on retina

  -is 1/3 of optical power of the eye 20D (2/3 from cornea)

  -refractive power can change

  -lens controls accommodation by changing lens shape done by zonules which connect the lens to ciliary body
• accommodation w near objects

  -retinal blur occurs

  -ciliary muscles contract diameter gets smaller

  -anterior zonule fibres loosen

  -lens changes shape becomes thicker curvature increases , more refraction occurs
• accommodation w far objects

  no retinal blur

  -ciliary muscle diameter increases as they relax, further away from lens

  -anterior zonule fibres become tight

  -lens shape becomes thinner so we can focus on objects far away
• shape of lens

  -biconvex and is transparent for minimal light scatter

  -posterior side of lens has higher degree of curvature (5-8um, smaller more steeper)

  -anterior :8-14um larger less curvature
• Lens dimensions

  -poles: centre of anterior and posterior surface

  -thickness: distance from anterior to posterior (unaccommodated lens increases by 0.2mm each year w age

  -equator : largest circumference of lens

  
• Lens structural divisions

  -made of long thin fibres arranged concentric/circular shells

  -capsule : outer layer

  -anterior epithelium

  -cortex (newest layer)

  -nucleus (oldest part) : adult, foetal and embryonal
• lens capsule- external surface

  -thickened basement membrane made of collagen fibrils,glycosaminoglycans , is acellular

  -is synthesised by anterior epithelium

  -acts as anchoring site for zonule fibres

  -permeable to water small solute and proteins: acts as reservoir for growth factor that regular lens metabolic processes
• zonules insertion into lens

  -thousands of zonule join lens to ciliary body

  -zonules originate from non pigmented ciliary epith (pars plana) and valleys between ciliary processes /pars plocates

  -zonules end at lens capsule
• what are zonules

  -microfibrils

  -these offer mechanical strength and elasticity/distensibility

  -made of extracellular matrix w elastin and fibrillin proteins
• lens epithelium

  -anterior: made of simple epithelia, basal surface is adjacent to capsule, apical surface towards lens centre

  -has cell junctions : desmosomes/gap junctions for nutrients to lens

  -posterior epith:not present as it was used up to make primary lens fibres during foetal development
• function of anterior epith

  -secretes capsule

  -forms new lens fibres

  -active T of ions and water for lens transparency, metabolically active transport metabolits to lens from aqueous humour
• lens fibres

  -cortex and nucleus made up of shells made of lens fibres, new fibres laid down outer layer

  -each fibre has hexagonal cross section

  -lens fibres were once epith cells but now don’t have nucleus or organelles

  -now have the protein crystallins: contributes to refractive index and transparency
• what do lens fibres contain

  -water

  -10-20% insoluble proteins: cell membrane proteins and cytoskeleton proteins (actin) provides structure

  -80-90% soluble proteins: crystallin which is water soluble and durable
• crystallin arrangment

  -prodiced in columnar epith (differentiate into fibres)

  -packed regulary for lens transparency

  -crystallins are highly stable
• how is lens fibre formed

  Molecules/hormones called growth factors cause proliferation/migration of epithelial cell which are differentiated into lens fibres

  -growth factors come from aqueous thru capsule

  -high conc of g factors at equatorial region
• stages where lens fibres formed.

  -epithelial cells migrate towards equator change from squamous to cuboidal to columnar

  -epithelial cells then elongate/rotate becoming fibres

  -elongating end reaches elongating fibre from opposite end join forming suture

  -basal end detaches losing organelles (nucleus,mitochondria)

  -elongated protein(lens fibre) produces crystalline,aquaporins,gap junctions

  -these continue to extend making layers, thicker lens
• How fibres are locked together

  -sutures: end of one fibre joins to end of another

  -ball and socket joint: fibres locked together like and key and lock
• lens fibres suture pattern

  young lens: simple pattern smaller lens w less shells and fibres

  Adult lens: complex suture w more fibres and shells w increasing age