Physiology of lens and cataract formation mechanism

1. PHYSIOLOGY OF LENS AND
CATARACTOGENESIS
DR. SRISTI THAKUR
LEI, NAMS
1ST YEAR RESIDENT

2. LAYOUT
Physiological processes
concerned with the functioning
of lens
• Biochemical composition of lens
• Metabolism of lens
• Lens transparency
• Antioxidation mechanism
Cataractogenesis
• Introduction
• Congenital cataract
And Developmental cataract
• Acquired cataract

4. INTRODUCTION
• Transparent, avascular, biconvex, crystalline structure
• located between the iris and the vitreous in the patellar fossa
• aid in focussing light rays on retina.
• Suspended from the ciliary body by zonular fibres.
• formation of the human lens begins very early in embryogenesis i.e at 3rd
week of gestation.

5. BIOCHEMICAL COMPOSITION OF LENS
64%
water 35% wet wtproteins
Others : lipids, amino acids, carbohydrates,
inorganic ions, ascorbic acid, glutathione
1%

6. 1. LENS PROTEINS (35% WET WT)
Alfa
A
Alfa B
1/3r
d
55%
Gamma
crystallin
s

7. SOLUBLE PROTEINS
Alpha crystallins
• 31.7%
• Highest molecular wt
600 kilodaltons (kDa)
• 2 subunits: A(thiol
group) and B
• Polymer (50 monomer)
• prevent the complete
denaturation and
insolubilization of the
other crystallins.
Beta crystallins
• 53.4%
• 3 polypeptide chains
• Mol wt 23kDa
• Relatively high thiol
content
• Coded by 7 gene
• beta H (beta high
molecular weight) and
• beta L (beta low
molecular weight)
fractions.
Gamma crystallins
• 15%
• Monomers
• Lowest molecular
weight 20 kDa or less
• High in nucleus
• Coded by 4 gene

8. INSOLUBLE PROTEINS
• can be further separated into 2 fractions - soluble and insoluble in 8 molar
urea
• Urea-soluble fraction:
• Protein vimentin
• Beaded filaments - made from the proteins phakinin and filensin
• Urea-insoluble fraction:
• Major intrinsic protein (MIP)

9. Albuminoid :
• Chief insoluble protein
• Molecular weight – 3,70,000
• 12.5% of total protein
• partly digested by urea

10. • As the lens ages - soluble alpha crystallins are gradually converted into
insoluble albuminoids.
• Lens proteins are organ specific
• Individual can become sensitised to one’s own lens proteins
(phacoanaphylactic uveitis).

11. • Increase of water insoluble proteins with age
• lens protein aggregate form very large particles become water
insoluble scatter light increase opacity of lens
• Brunescent cataract – increase in amount of water-insoluble protein

12. Functions of lens proteins
• Refractive function
• Contributes to lens transparency
• Helps in change of shape during cell differentiation
• Involved in the assembly & disassembly of the lens cytoskeleton
• Provide the lens with stress-resistant & oxidative properties

13. 2. Amino acids
• 2 groups of amino acids :
1. Proteogenic - alanine, leucine, glutamic acid, aspartic acid glycine, valine,
phenylalanine, tyrosine, serine, isoleucine, lysine, histidine, methionine,
proline, threonine, and arginine
2. Non proteogenic - taurine, ornithine butyric acid

14. • Consist all the amino acids present in any tissue except tryptophan, cysteine
and hydroxy proline
• Concentration higher in lens than aqueous or vitreous humour
• Actively transported into the lens

15. 3. Carbohydrates
Free carbohydrates
Derivatives of
sugar
• Glucose
• Fructose
• Glycogen
• Sorbitol
• Inositol
• Ascorbic acid
• Gluconic acid
• Glucosamine

16. • FRUCTOSE FORM GLUCOSE
• GLYCOGEN – IN TRACES..CONC VARIES WITH AGE.
• .LOCALISED IN NUCLEUS AND APPEARS TO
REPLACE GAMMA CRYSTALLIN NORMALY PRESENT
THERE TO INCREASE REFRACTIVE INDEX
• IONOSITOL- INVOLE IN METABOLISM OF
PHOSPHOLIPID

17. 4. Lens lipid
• Cholesterol (50-60%)
• Phospholipids – sphingomyelin, cephalin, isolecithin, lipoproteins
• Glycosphingolipids
• Functions-
• Principal constituents of lens cell membrane
• Also associated with lens epithelial cell division

18. Chemical composition of lens
Aqueous humour Lens
Water 99% 66% of wet wt.
Sodium 144 20 mM
Potassium 4.5 125mM
Chloride 110 18mM
Glucose
Lactic acid
6
7.4
1 mM
14 mM
Glutathione
Inositol
0
0.1
12mM
5.9mM
Amino acids
Proteins
5
0.04%
25 mM
33% of wet wt.

19. Glucose metabolism
• Main source of energy
• Lens requires continuous supply of energy for
• Active transport of ions and amino acids
• Maintenance of lens dehydration and transparency
• Synthesis of protein and glutathione
• Most of the energy produced is utilized in the epithelium which is
the major site of all active transport processes

20. Glucose metabolism in lens

21. • Most of glucose transported into lens phosphorylated to glucose 6-phosphate by
enzyme hexokinase
• Rxn 70-100 times sloer than that of other enzymes involved in lens glycolysis-so rate
limited
• G6p enters 1 of 2 metabolic pathways : anerobic glycolysis or HMP shunt
• Most active-anerobic – provides most of high energy phosphate bond required for
lens metabolism
• ADP to ATP occurs at 2 steps along the pathway from glucose metabolism to lactate
• less efficient as only 2 atps produced for each glucose molecule utilized
• Aerobic produce 36 molecules atp from each molecule in glucose in citric acid
cycle(0xidative metabolism)
• Low o2 tension In lens , only 3%of lens glucose passes through the krebs citric acid
cycle to produce atp—approx. 25% lens ATP

22. Pathways of glucose metabolism
S.N
o
Pathway Main
Intermediates
End products %
Of
O2
ATP
gain
1. Glycolytic G6P, fructose 1,6 -
diphosphate, PA
Lactic acid 80 2
2. Kreb’s cycle TCA CO2, H20 3 36
3. HMP shunt Pentoses CO2, NADPH 10 -
4. Sorbitol Sorbitol, fructose Lactic acid 5 2

23. Clinical correlation
 Sugar cataract-
1. True diabetic cataract - accumulation of sorbitol within the lens and
accompanying changes in hydration
-increased non-enzymatic glycosylation (glycation) of lens proteins,
or greater oxidative stress from alternations in lens metabolism.
1. Galactosemic cataract - accumulation of galactose and galactitol within
lens.

24. Sugar cataract
Diabetic cataract;
Snowflake or snowstorm
cataract
Galactosemic cataract;
oil drop cataract

25. GLUTATHIONE
A tripeptide : glutamate, cysteine and glycine
• 3.5-5.5 mmol/g wet weight
• More in the epithelial layer
• Half life-1 to 2 days
Functions:
• Maintenance of protein thiols in the reduced state
• Protection of protein involved in cation transport & permeability
• Protection against oxidative damage
• Removal of xenobiotics

27. Antioxidant mechanism
Reactive oxygen species from cell metabolism and photochemical
reactions
Superoxide anions, hydroxyl free radicals, hydroperoxyl radicals, singlet
oxygen, H2O2
a) Cause damage in the bases of DNA
b) Polymerize & cross link protein.
c) Peroxidize membrane lipids which forms cross links between membrane
proteins & lipids causing opacity of lens.

29. Clinical correlation
• Reduced glutathione levels in lens epithelial cells or whole lenses causes cell
damage and cataract formation
• Higher oxygen levels near the crystalline lens induce nuclear cataract
• With vitreous syneresis or after vitrectomy, the lens has much greater
exposure to oxygen levels from the choroid - induces nuclear sclerosis.

30.  Protein metabolism
 Protein synthesis
◦ Synthesized from free amino acids which are actively transported from
aqueous
◦ Requires ATP and the appropriate RNA template
◦ Occurs throughout life (crystallins & MIP)
 Protein breakdown
Endopeptidases Exopeptidases
Protein Peptides A.A
Ca++ & Mg

31. Transport of water and ions

32. Maintenance of lens water and cation balance
• Transparency highly dependent on the structural and macro molecules
components of the lens; perturbation of cellular hydration can lead to
opacification
• Lens capsule freely permeable to water, ions and proteins with molecular
weight upto 70 kda
• Epithelial cells and fibers possess a number of different channels, pumps, and
transporters that enable transepithelial movement to and from the extracellular
milieu

33. • Cation balance between inside and outside of the lens is due to
- Permeability properties of the lens cell membrane
- Activity of the sodium pump ( reside within the cell membrane of the
lens epithelium and each lens fiber)
• Sodium pump functions by pumping sodium ion out while taking
potassium ions in
• Inhibition of NA+ K+ ATPase leads to loss of cation balance and elevated
water content in the lens.

34. Pump-leak theory

35. • NA FLOWS IN THROUGH BACK OF LENS WITH
CONCENTRATION GRADIENT AND ACTIVELY
EXCHANGED FOR K BY EPITHELIUM
• K CONCENTRATED IN ANTERIOR LENS WHEREAS
NA IN POSTERIOR

36. • Active transport mechanism are lost if the capsule and attached epithelium
are removed from the lens but not if the capsule alone is removed by
enzymatic degradation(collagenase)
• Membrane transport process establishes the ion gradient across cell
membranes and generates extra cellular currents around the outside of lens.

37. • Unequal distribution of electrolytes across the lens cell membranes results
in an electrical potential difference between inside & outside of lens.
• Inside of lens is electronegative about -70mv, there is -23mv potential
difference between anterior & posterior surfaces of lens.
• Normal potential difference of about 70mv is readily altered by changes in
pump activity or membrane permeability.

38. Calcium homeostasis
•

39. • Critical to lens
• large transmembrane calcium gradient is maintained by calcium pump(ca+
atpase)
• Lens cell membrane impermeable to calcium
• Increased levels of calcium –depressed glucose metabolism , formation of
high molecular weight protein aggregates, and activation of destructive
proteases

40. Transport functions in the lens
•Simple diffusion
•Facilitated diffusion
Glucose
•Dependent upon the sodium gradientAmino acid
•specialized carrier-mediated transport
system
Choline,
myo inositol, ascorbic
acid
•Simple diffusionWaste products

41. LENS TRANSPARENCY
Lens transparency depends on:
A . Anatomical factors
1. Single layered lens epithelial cells.
2. Tight arrangement of fibers with little extracellular space.
3. Loss of cellular organelles and nucleus preventing scattering of light.
4. Permeability of lens capsule.
5. Pattern of distribution of the protein within the cells.
6. Avascularity of lens
41

42. B . Physiological factor:
1. Water and electrolyte balance of lens fiber maintaining relative
state of dehydration.
2. Presence of alpha crystallin protein preventing aggregation of
lens protein.
3. Low oxygen tension around the lens.
4. Auto-oxidative mechanism 42

43. Accomodation

44. • Mechanism by which the eye changes focus from distant to near images,
produced by a changes in lens shape resulting from the action of the ciliary
muscle on the zonular fibers.
• lens substance progressively loses its ability to change shape with age.
• After 40, rigidity of lens nucleus clinically reduces accommodation because
sclerotic nucleus can’t bulge anteriorly & change it’s anterior curvature.

46. • Accommodation is mediated by the parasympathetic fibers of cranial nerve
III (oculomotor).
• Parasympathomimetic drugs (pilocarpine) induce accommodation,
• Parasympatholytic medications (atropine) block accommodation.
• Drugs that relax the ciliary muscle are called cycloplegics.

47. Theory of accomodation

50. PRESBYOPIA
• Loss of accommodation due to aging
• Usually after 40 yrs
• According to theory of von helmholtz, as the crystalline Lens ages, it
becomes firmer and more sclerotic and resists deformation when the ciliary
muscles contracts
• Hence, it can not bulge enough anteriorly to increase the lens curvature
and dioptric power to focus at near.

51. Changes in aging lens
Accommodation changes:
• Amplitude of accommodation : amount of change in the eye’s refractive
power that is produced by accommodation; diminishes with age and may
be affected by some medications and diseases
10yrs=12-16d, 40yrs=4-8 d , 60yrs= <2 d
1. Decreased capsular elasticity
2. Increase in stiffness of lens substance
3. Radius of curvature of anterior capsule decreases lens rounder
4. Distance between anterior surface of lens & cornea decreases
5. Internal apical region of the ciliary body moves forward & inward

52. Changes in aging lens
Morphological changes:
•  In weight and thickness of the lens
• Epithelial cells become thinner with flat nuclei , develop electron dense
bodies and vacuoles which increases density of surface & cytoskeleton .
proliferative capacity decreases.
• Lens fibers- loss of plasma membrane proteins & cytoskeletal proteins
• Capsule becomes thick and loses its elasticity.
•  Disulfide bond with  sulfhydryl group of lens proteins → conversion
of soluble lens proteins into insoluble proteins → lens opacification

53. Changes in aging lens
• Biophysical changes:
• Colorless/pale yellow → darker yellow to brown or black.
•  In lens transparency(aggregation of lens protein)
• Increase in light scattering
• Fluorescence property increases with the age
• Absorption of UV & visible light increases with age.

54. Changes in aging lens
Biochemical changes:
• Changes in the cellular junctions and alteration on cation permeability →
constant K+ level,↑Na (40meq/L)→ ↑optical density
• Decrease in MIP reduces cell to cell communication.
• ↑ Cholesterol:phospholipid ratio→↓ in membrane fluidity
• Membrane potential  es(due to change in free Ca levels)from –50mv (at
age of 20 yrs) to –20mv (at the age of 80 yrs)
• Glutathione and ascorbate levels ↓
• Superoxide dismutase and catalase activity ↓

55. Changes in aging lens
Changes in crystallins:
• Loss of α – crystallins from soluble proteins of lens nucleus and b
crystalline become more polydisperse
• Loss of γ – crystallins and increase in disulphide bond
• ↑ Insolubility  accumulation of HMW aggregates

56. CATARACTOGENESIS

57. Cataract means opacification of crystalline lens and its capsule
• d/t Precipitation, denaturation, coagulation or agglutination of
soluble protein
Risk factors:
• Heredity
• Exposure to radiation
• Dietary factors

58. • Severe diarrhoea
• Diabetes
• Renal failure
• Hypertension & diuretics
• Myopia
• Misc – smoking, alcohol, glaucoma, steroids

59. 59

60. ETIOPATHOGENESIS

61. • Cataract-caused by degeneration and opacification of lens fibres
• Lens fibres already formed
• Formation of aberrant lens fibre and deposition of other material
in there place
• Disorganisation of lens fibre and abnormalities in lens protein
• Loss of transpararency

62. • Any factors physical or chemical which disturb
the critical intra and extra cellular equilibrium
of water and electrolytes and deranges the
colloid system within fibres—bring
opacification

63. • Aberrant lens fibre germinal epithelium loses its
ability to form normal fibres –there is posterior
subcapsular cortical cataract
• Fibrous metaplasia of fibres - complicated cataract
• Epithelial cell necrosis - focal opacification of lens
epithelium as glaucomfleken in acute ACG

64. ETIOPATHOGENESIS
• Biochemically 3 factors evident in cataract formation
1. Hydration – soft cataract
2. Denaturation of lens protein
3. Usual degenerative change (hard caratact)

65. 1. Hydration
• Frequently actual droplet of fluids gather under capsule forming lacunae
between fibres
• Entire tissue swells
• Intumesence
• Become opaque
• To some extent process reversible and opacities thus formed may clear up as
in juvenile insulin dependent diabetic patient whose lens become clearer
after control of hyperglycemia

66. • Hydration due to osmotic changes within lens or to change in
semipermeability of capsule impaired
• Inactive insoluble protein and anti oxidative mechanism less effective
• Process dramatic in traumatic cataract when capsule ruptures and lens
fibres swells and bulge out into anterior chamber

67. 2. Denaturation of lens protein
• Protein denatured with increase in insoluble protein
• Dense opacity produced
• Irreversible opacity don’t clear up
• Mostly in young lens and cortex of adult lens where metabolism active
(Rare in old and inactive fibres)
• Hard-slow sclerosis

68. 3. Aging
• Semipermeability of capsule impaired
• Inactive insoluble protein increase and antioxidative mechanism less
effective
• Normal lens-sulphydryl containing reduced glutathione and vit c—both
decrease with age and in cataract
• Deficiency, either of aa(tryptophan or vit b2(riboflavin) or by
administration of toxic substances naphthalene, lactose, galactose, selenite,
thallium etc

69. • Dinitrophenol (for slimming) and paradichlorobenzene(in insecticide) -
lens opacity in posterior cortex
• Cyanate from cigarrete smoke and from urea in renal failure and
dehydration—carbamylation and protein denaturation (as do sugars by
glycation in diabetes)
• Hypocalcemia - same result - alter ionic balance - cataract of parathyroid
tetany
• Cataractous changes - use of stronger anticholinesterase group of miotics
and after prolongrd systemic use of corticosteroid

70. AGE RELATED CATARACTS
• • Most commonest
• • B/L and asymmetrical
• • Three main types
1.Nuclear cataracts
2.Cortical cataracts
3.Posterior subcapsular
cataracts 70

71. NUCLEAR CATARACT
• Most common type, >60%
• In asian population, cortical cataract
predominates
• associated with the oxidative damage to the
proteins and lipids, leading to hardening of the
lens nucleus and increased light
Scattering
• Hardening increases refractive index
myopic shift second sight 71

72. • lens normally exists in an extremely hypoxic Environment.
• Patients treated with long-term hyperbaric oxygen therapy develop a
myopic shift nuclear cataracts
• Post vitrectomy and age related degeneration of Vitreous also plays
significan role in nuclear cataracts
72

73. CORTICAL CATARACT
• First appear at age of onset of presbyopia
• Mature fibres on surface of cells are affected
• Most common site is inferonasal quadrant
• Starts at periphery and takes years to obscure
vision
Risk factors
• Exposure to sunlight
• Thinner lens
• Dm 73

74. MECHANISMS
• Disruption of pumps
• Physical or chemical damage to cell plasma proteins
• Damage to ca homeostasis
• Glutathione loss
74

75. POSTERIOR
SUBCAPSULAR
CATARACT
• Caused by cluster of swollen cells at posterior
pole of lens just below capsule
• Opacity in optical axis, disabling
Risk factors
• Steroid intake
• Exposure to radiation
• Trauma
75

76. SUGAR CATARACT
• Associated with galactosaemia and daibetes mellitus
• Galactosaemic cataract: seen in galactosaemia
(a.Classical galactosaemia due to galactose-1-p04
uridyltransferase deficiency, b. Due to deficiency of
galactokinase); oil drop cataract
• True diabetic cataract: snowflake or snowstorm cataract
76

77. SUGAR CATARACT CONTD..
PATHOGENESIS:
1. SORBITOL, ALDOSE REDUCTASE AND OSMOTIC HYPOTHESIS:
77

78. 2.Theory of non-enzymatic glycosylation: most important theory of
etiology of diabetic cataract till date
Increase glucose non-enzymatic glycosylation
Of lens proteins conformational change cataract
78

79. RADIATION CATARACT
• Due to damage to the germinative zones of lens epithelium
Infrared(heat) cataract:
• posterior subcapsular cataract
• seen in workers of glass industries so called glass blower’s and glass
worker’s cataract
Non ionizing radiations-uv rays:
• UV-B not UV-A responsible for cortical cataract
• Mechanism in humans not clear
• May be due to excess formation of free radicals
• Mainly cortical cataracts are formed 79

80. 80

82. STEROID INDUCED CATARACT
• • Children more susceptible than adults
• • Mechanism- increase glucose levels
-Inhibition of na-k-atpase pump
-Loss of ATP
-Increased cation permeability
• • Common is posterior subcapsular opacities
due to aberrant differentiation and
Migration of epithelial cells.
82

83. ELECTRICAL INJURY
• • Cause protein coagulation and cataract
Formation
• • More likely when the transmission of
current Involves the patient's head
• • Initially, lens vacuoles appear in the
anterior midperiphery of the lens, followed
by linear Opacities in the anterior
subcapsular cortex
83

84. CATARACT DUE TO UVEITIS /INTRAOCULAR SURGERY /
RETINITIS PIGMENTOSA
• In chronic uveitis - due to uncontrolled and sustained inflammation and
prolonged use of steroids
• Following vitreo-retinal surgery -breakdown of blood vitreous barrier -
posterior subcapsular cataracts
• In RP– breakdown of the blood vitreous barrier by Destruction of
pigment epithelium.
84

85. DRUG-INDUCED CATARACT
85
STATINS
AMIODARONE• PHENOTHIAZI
NES:

86. TRAUMATIC CATARACT
• ROSETTE CATARACT
86

87. CATARACT IN CHALCOSIS
87
SUNFLOWER CATARACT AS IN WILSON DISEASE

88. MYOTONIC DYSTROPHIC CATARACT
• POLYCHROMATIC CRYSTALS IN CHRISTMAS TREE PATTERN
88

89. Present at birth
Involvement of the fetal
nucleus
Diameter less than 5.75mm
Pediatric: Congenital Cataract
89

90. TOXIC AGENTS
Corticosteroids Discoid, PSCC
Anticholinesterases Ant subcapsular
Chlorpromazine Yellow/ brown ant. Capsular/ stellate/ ant. Polar
Busulfan Pscc
Chloroquine White/flaky PSCC
Amiodarone Ant. Subcapsular
Cigarette smoking Nuclear
Iron Brown discoloration
Gold Golden ant. Capsular deposits

91. Reference
• American academy of ophthalmology [lens and cataract ] ,2014-
2015
• Parson’s disease of the eye, 20th edition
• Anthony j bron, ramesh c tripathi, brenda j tripathi, wolff’s
anatomy of the eye and orbit, 8th edition.
• Jack j kanski, brad bowling, clinical ophthalmology, 8th edition
• Internet resources: www.Oculist.Com