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anatomy,development and physiology of eye lens

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  2. 2. ANATOMY1. Gross anatomy• Biconvex, transparent, crystalline structure• Ability to change shape• divides eye into anterior and posterior segments• 2 surfaces-anterior and posterior 8/25/2012 2
  3. 3. ciliary body Zonules Lens Zonules ciliary body8/25/2012 3
  4. 4. Position8/25/2012 4
  5. 5. LOCATION OF LENS8/25/2012 5
  6. 6. DIMENSIONS OF LENS 10 mm 6 mm 8/25/2012 6
  7. 7. Histology of lens8/25/2012 7
  8. 8. A)Capsule• Acellular elastic structure• basement membrane -type 4 collagen+sulphated GAG• Variable thickness• Zonules run from ciliary processes and fuse onto outer layer of capsule• Main function is to mold the shape of the lens in response to tension from zonules8/25/2012 8
  9. 9. B)Anterior epithelium• Single layer of nucleated epithelial cells interconnected with gap junctions and desmosomes• The only metabolically active part of lens• Functionally divided into 2 zones-i] Pre-equatorial zone- columnar cells Actively dividing and differentiating into lens fibersii]Central zone-cuboidal cells i) Transports solutes between lens and aqueous ii) Secretes capsular material• Maintains the osmotic balance of lens 8/25/2012 9
  10. 10. C)Lens substance• Composed of lens fibres• Consist of primary and secondary fibers• Produced by mitosis of epithelial cells in the pre- equatorial zone• They elongate and undergo differentiation with pyknocytosis and eventual loss of cell oraganelles and nucleus• This is an important factor in the transparency of the lens8/25/2012 10
  11. 11. • As lens fibres elongate and new ones form, the older ones are pushed towards the depth of the lens• Lens bow-shifting of lens nuclei forms a visible line in the equator• Fibres are arranged in zones in which fibres growing from opposite directions meet in sutures• Consist of nucleus and cortex8/25/2012 11
  13. 13. 8/25/2012 13
  14. 14. 8/25/2012 14
  15. 15. Cortex• Youngest lens fibres• It is located peripherally, and is composed of secondary fibres formed continuously after puberty8/25/2012 15
  16. 16. Nucleus• Central part with oldest fibres• Dense and compactly arranged lens fibres• Higher refractive index than capsule• Different zones depending on period of development8/25/2012 16
  17. 17. epinucleus8/25/2012 17
  18. 18. 8/25/2012 18
  19. 19. Anterior Suture posterior suture( erect Y ) inverted Y 8/25/2012 19
  20. 20. Sutures8/25/2012 20
  21. 21. Zonules of zinn• Suspensory ligaments/ciliary zonules• Series of fibres from ciliary process• Holds the lens in position• Assist action of ciliary muscle• Attached to lens capsule at zonular lamella 8/25/2012 21
  22. 22. Parts of ciliary zonules8/25/2012 22
  23. 23. Types of zonules8/25/2012 23
  24. 24. Zonular spaces8/25/2012 24
  25. 25. EMBRYOLOGY1)Formation of lens vesicle• 4 weeks• Optic vesicle induces lens placode from ectoderm• Lens placode invaginates and becomes lens pit• Optic vesicle also invaginates and becomes optic cup• Lens pit separates from ectoderm to become the lens vesicle8/25/2012 25
  26. 26. 8/25/2012 26
  27. 27. 2)Formation of lens fibers and zonules• Primary lens fibres—fibres formed upto 3rd month Cells in posterior portion of lens vesicle elongate to fill vesicle forms emryonic nucleus• Secondary lens fibres—3rd month to entire life Cells in anterior portion of vesicle divide actively and elongate includes all other nucleus• Lens capsule-produced by anterior epithelial cells• Lens zonules—from neuroectoderm in ciliary area(3rd – 5th month)• Tunica vasculosa lentis--nourishment to embyonic lens branch of hyaloid artery disappears before birth 8/25/2012 27
  29. 29. LENS PROTEINSWater soluble lens crystallins• 90% of total lens protein1) Alpha crystallin• Largest crystallin• Accounts for 31% total lens protein2) Beta crystallin• Most abundant crystallin, accounts for 55% total lens protein• Most heterogenous group, 6 distinct subgroups3) Gamma crystallin• Smallest crystallin• Least abundant-2%Water insoluble proteins• Insoluble albuminoids-12%8/25/2012 29
  30. 30. WATER• Lens –dehydrated state• Unique arrangement of proteins within lens fibres• Low protein osmotic activity within lens• Tightly packed fibres with minimum extracellular spaces• Lens epithelium transports water into the fibre mass• Half of the water -protein hydration• Water excreted via aquaporin in the equator into aqueous• Important factor maintaining lens transparency8/25/2012 30
  31. 31. Electrolytes• Active transport of ions and low molecular weight metabolite takes place between lens and aqueous humour• Epithelium contains Na-K-ATPase and a calmodulin- dependent Ca-activated ATPase for the active transport of electrolytes• Fibre cells contain large concentrations of negatively charged crystallines• Positively charged cations enter the lens cell to maintain electrical neutrality• pH- 6.9-7.2• Aminoacids transported in lens via energy dependent carrier mechanisms8/25/2012 31
  32. 32. • Lipids-high concentration of cholesterol and sphingomyelin membrane rigidity• Glutathione –major antioxidant in lens synthesized by GSH in lens present in reduced state• Ascorbic acid-synthesized by cililary body into aqueous antioxidant property• Inositol- osmolyte membrane rigidity• Taurine- osmolyte antioxidant8/25/2012 32
  33. 33. Pump-leak mechanism8/25/2012 33
  34. 34. Functions of lens• Refraction Accounts for 35% of total refractive power of eye (15D out of total of 58D)• Light transmission Focusing of visible light rays on the fovea Preventing, damaging- ultra-violet radiation, from reaching the retina• Accomodation• Organizer of anterior segment8/25/2012 34
  35. 35. REFRACTIVE PROPERTIES8/25/2012 35
  36. 36. ACCOMODATION• Lens has the capacity to change the focussing power of the eye for distant and near vision• Accomodation can be divided into physical process- measure of change in shape of lens physiological process- measure of ciliary muscle contraction• Near reflex-Contraction of ciliary muscles Contraction of pupils8/25/2012 Convergence of visual axis 36
  37. 37. 8/25/2012 37
  38. 38. Theories of accomadation1)Helmholtz theory-ciliary muscle contraction relaxation of zonules lens-spherical• aging—lens rigid—difficulty to change shape• Classical theory8/25/2012 38
  39. 39. 2) Schachar’s theory– ciliary muscle contraction equatorial zonules tensed shape changes in lensAging—diameter of lens grow—less space for proper functioning ofciliary muscles3) Coleman’s theory—• Lens and zonules—diaphragm• Shape altered by pressure difference b/w aqueous and vitreous• Ciliary muscle-alters pressure gradient8/25/2012 39
  40. 40. Maintenance of transparency• Thin epithelium• Regular arrangement of lens fibers• Little cellular organelles• Little extracellular space• Orderly arrangement of lens proteins• Relative dehydration• Semipermeable character of lens capsule• Avascularity• Antioxidants8/25/2012 40
  41. 41. LENTICULAR METABOLISM• Continous supply of ATP required for-1. Transport of ions and aminoacids2. Maintanence of lens dehydration3. Continous protein synthesis4. GSH synthesis• Major site – epithelium• Source of nutrient supply-aqueous humour 8/25/2012 41
  42. 42. Glucose metabolism• Energy production entirely dependent on glucose metabolism• Glucose enters lens by simple diffusion and facilitated diffusion• Epithelial cells- GLUT-1• Lens fibre cells-GLUT-3• Glucose is rapidly metabolized via glycolysis so that level of free glucose in lens < 1/10 level in aqueous8/25/2012 42
  43. 43. 8/25/2012 43
  44. 44. 1)Anaerobic metabolism• Accounts for 85% of glucose metabolism by lens• Provides > 70% of energy for lens• 1 mole of glucose gives 2 moles of ATP• Lactate generated undergoes 2 pathways of metabolism• Further metabolism via Kreb’s cycle• Diffusion from lens into aqueous2)Aerobic metabolism (Krebs cycle)• Limited to epithelium• 1 mole of glucose gives 38 moles of ATP• Only 3% of lens glucose metabolized by this pathway• But generates up to 20% of total ATP needs of lens8/25/2012 44
  45. 45. 3)Hexose monophosphate shunt• Accounts for 5% of glucose metabolism by lens• Important source of NADPH required for other metabolic pathways e.g. sorbitol pathway and glutathione reductase4)Sorbitol pathway• Accounts for 5% of glucose metabolism by lens• When sorbitol accumulates within cells of lens, it sets up an osmotic gradient that induces influx of water and lens swelling, and ultimate loss of lens transparency8/25/2012 45
  46. 46. Changes during aging1)Changes in structure• Crystallins—proteolysis,decresed solubility,aggregation racemization and deamidation of aminoacids• Cytoskeleton—proteolysis and insolubilization—disassembly of fibres• Leads to opacities –nuclear sclerosis—senile cataract2)Less elasticity of lens— loss of power of accomodation—presbyopia3)Overall reduction in light transmission8/25/2012 46
  47. 47. DISEASES OF LENS1)CONGENITAL DISORDERS• Coloboma• Lenticonus• Lentiglobus• Microspherophalus• Mittendorf dot2)CATARACTS• Congenital• Acquired8/25/2012 47
  48. 48. 3) Miscellaneous disorders• Posterior capsular opacification• Aphakia• Pseudophakia• Exfoliation of capsule• Psuedoexfoliation and psuedoexfoliation glaucoma• Lens induced glaucoma• Ectopia lentis• Lens induced uveatits• Loss of accomodation-aging,cataract,surgery4)Several systemic diseases are also associated with disorders ofthe lens8/25/2012 48