Structure Of Lens And Changes During Cataractogenesis

Structure of lens and changes during cataractogenesis

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Structure of Human Eye Wall of the eye ball contain 3 principle layers: 1. Outer fibrous layer. 

Sclera: Tough, fibrous, opaque coat.



Cornea: Clear, transparent, avascualar.

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Structure of Human Eye continued…

2. Middle Vascular layer. 

Choroid: bluish vascular structure ( uveal layer).



Ciliary body: continuation of choroid anteriorly, contains ciliary muscle.



Iris: pigmented, opaque muscular structure containing sphincter pupillae and dilator pupillae. 3

Structure of Human Eye continued…

3. Inner nervous layer:  Retina: Outer epithelial cells, inner nerve cells.  Other

structures:  Aqueous humour:  Vitreous

humour: 4

Structure of human lens Definition The lens is a transparent, encapsulated, biconvex body lies between iris and the vitreous body with no blood supply.

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Structure of lens under compound microscope

AP

PP

capsule epithelium

Cortex Lens fiber cells

Nuclear lens fiber cells 6

Development of Human lens

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Development of Human lens continued…

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Structure of mature lens Anterior pole

axis

equator Posterior pole

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Gross structure of lens 

Dimensions: ◦ Equatorial diameters  birth: 6.5mm  15 years of age: 9.0mm  90 years of age: 9.5mm

◦ Axial dimensions:  birth: 3.5 – 4.0mm  95 years of age: 4.75 – 5.0mm 10

Gross structure of lens continued...

◦ Radii of curvature:  reduce through out life.  Anterior surface: 10mm  Posterior surface: 6mm

◦ Refractive power:  Unaccomodated state: 20 diopters  Maximum accommodation state: 14 diopters ( 8 – 12 years of age)  Accommodation

decreases with age approaching ZERO after 50 years.



Weight: Adult lens is 1g. 11

capsule

The lens capsule 

Epithelial cells

Capsule is a transparent basement membrane completely surrounding the lens. ◦ thickness: in 35 years old lens.  at posterior pole: 4µm  at anterior pole: 21µm



Synthesis:  anterior capsule: epithelial cells.  posterior capsule: elongating fiber cells.



Major components: collagen type 4, laminin, entactin, heparan sulphate, proteoglycan and fibronectin. 12

The lens capsule continued…



Zonular fibers insert into capsule near the equator region called ZONULAR LAMELLA.



Functions of capsule: 1. During accommodation. 2. Barrier function.

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The lens epithelium 

Shape and size:  Polygonal cuboidal.  Height: 5-8µm  Width: 13µm.



Epithelial cell density:  Men: 3900 cells/mm sq  Woman: 5780cells/mm sq

 All

organelle are present and lateral membranes are connected by desmsomes and gap junctions. 14

The lens epithelium continued…



Cytoskeletal

elements:

actin, myosin, vimentin, microtubules, spectrin and alfa actinin.  Only a band of cell in the equatorial region remains mitotically active throughout life called as germinal zone.

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The lens fibers 

Shape: ◦ Young state: flattened, hexagonal cross sectional profile. ◦ Middle age: irregular profile.



Dimensions: ◦ Length: 7-10mm ◦ Width: 10-12µm ◦ Thickness: 1.5 - 2µm

 Corticle

fibers lack nucleus and all cell organelles. 16

The lens fibers continued…

Lateral membranes have interdigitations like ball and socket, tongue and groove (hook and eye) junctions

Note ball and socket joint interlocking at superficial cortical fiber edges. Planar surface of fibers interlock with next layer of offset fibers (removed).

Note alternating rows of hooks and complementary eyes. Spines hook into next row of eyes to interlock layers like Velcro. 17

The lens fibers continued…

 The

lens fiber cells are joined by desmosomes and gap junctions.



Crystallin proteins:  90% of the total mass of the fiber.  40% of the wet weight of the lens fiber.



Crystallin concentration:  In cortex: 15%  In nucleus: 70%  responsible for gradient refractive index.

 The

border between the apical membrane of the anterior epithelium and the apical membrane of the elongating fiber is known as Epithelium Fiber Interphase. 18

The lens sutures during 8 – 9 months of fetal life.  Only secondary lens fibers are responsible.  Symmetrical Y pattern in the anterior section and a symmetrical inverted Y pattern at the posterior section appears.  Appears

One lens fiber attached to the limb of the Y near the anterior pole is attached to the fork of opposite Y near the posterior pole and vice versa. 19

The lens sutures continued…

 Branches

of the suture:

In early adulthood : 6-9 o In middle to old age: 9-15  Finally a star shaped suture is formed at both the poles. o

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Lens crystallin proteins 40% of wet weight of lens fiber.  High concentration of crystallin (400mg/ml) than that of a typical cell.  Crystallin proteins classification: 

1. Classical. 2. Taxon spesific.

• Adult human lens do not produce taxon specific crystallins. 21

Lens crystallin proteins continued…

α- crystallins are the members of small heat shock proteins having chaperone activity.  α- crystallins are also enzymes- serine threonine autokinase activity.  α- crystallins are flexible and the complexes are plastic.  β- crystallins have a tendency to form multimers but γ- crystallins exist as monomers. 

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Lens crystallin proteins continued…

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Six β- crystallin polypeptides: βA1, βA3, βA4, βB1, βB2, βB3

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Three γ- crystallins: γS, γC, γD



For the electrophoretic study lens protein components are separated as: 1. Water soluble fraction. 2. Urea soluble fraction. 3. Detergent soluble fraction. 23

Structural properties maintaining transparency of the lens. 1. 2.

3.

4.

Organization of the lens fiber in the lens. The refractive power of the lens is altered as the lens grows, maintaining focal point constant. Variable concentration of crystallin proteins causes gradient of refractive index partially corrects for spherical aberration. All the cell organelles are absent in cortical and nuclear fibers. 24

Metabolic reactions maintaining transparency of the lens 1. ATP dependent bicarbonate pump activity:

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Metabolic reactions maintaining transparency of the lens continued…

2. Glucose metabolism in lens:  glycolysis: 85%  hexose monophosphate pathway: 10%  Citric acid cycle: 3% presumably by the cells located at the periphery.

3. Low oxygen tension (15mmHg or 2%of O2) protect the lens from oxidative damage. 4. Ascorbic acid concentration is 20 times in the aqueous humour than the blood (Blood level: 23-85µM). 26

Metabolic reactions maintaining transparency of the lens continued…

5.

Reduced glutathione is present in high concentration in aqueous humour: 4-6mM.

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Metabolic reactions maintaining transparency of the lens continued…

6.

High concentration of transferrin in epithelial cells, catalase and glutathione peroxidase prevent oxidative stress. transferrin catalase Fe+2

Glutathione peroxidase

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Cataract 

Definition: Cataract is defined as any opacity the lens causing scattering of the light transmitted.



Major classification of cataract: 1.Etiological classification 2.Morphological classification 3.Maturity of the cataract 4.Age onset

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Etiological classification

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Etiological classification continued…

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Classification of cataract according to maturity

1. 2. 3.

4. 5.

Immature Mature Intumescent Hypermature Morganian

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Classification of cataract according to age onset

1. 2. 3.

4. 5.

Congenital Juvenile Senile Infantile Presenile

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Cataractogenesis

General mechanism of cataract formation

1. 2.

3. 4. 5. 6. 7.

Opacification of previously clear lens fiber. Formation of new opaque fibers. Deposition of the granular material instead of fibers. Accumulation of pigments. Opacification of the lens epithelium. Deposition of extraneous material. General mechanism involve oxidation, osmotic effect, phase separation, chemical modification of proteins. 34

Age related changes during cataractogenesis



Morphologic changes: Capsule thickens, appearance of organelle changes, epithelial cell density decreases, loss of polygonal cross sectional profile, vacuoles and multilamellar bodies observed, plasma membrane disrupted.



Biophysical changes: Optical quality decreased, increase in the absorbance near blue region - tritanopia like defects observed in older people, accommodation loss and reaches to zero by the age 50 years. 35

Age related changes during cataractogenesis continued…

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Physiologic changes: ◦ Membrane potential decreases,  at 20 years: -50mV  at 60 years: -30mV



Na+ and Ca2+ concentration increases after the age 40.



Relative lens permeability increases. 36

Biochemical changes in the lens with aging



In general: ◦ metabolic activities decline. ◦ Proteins undergo post translational, covalent, conformational modifications ◦ Enzymes loose activity and become more heat labile.

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Biochemical changes in the lens with aging continued…

SDS-PAGE protein profile confirms chemical modification and partial degradation, cleavage product of native protein. 38

Biochemical changes in the lens with aging continued…

α- crystallin concentration decrease whereas γS, β- crystallins increase.  At the age of 42 years approximately 50% of α, β and γ-crystallins become water insoluble.  Major plasma membrane protein MIP-26 undergo radical modification.  Racemization of Asp, Met, Tyr and deamidation of Gln, Asn of the crystallin proteins occur. 

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Biochemical changes in the lens with aging continued…

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Cytoskeletal proteins like vimentin, intermediate filaments are disassembled because of insolubalisation and proteolysis.

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Increased non-enzymatic glycation of crystallin proteins increase high molecualr aggregates, scattering the light.



Intercellular transport decreases have control over oxidative stress.

can not

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Changes during cataractogenesis in diabetes

Human lens is affected only in severe diabetes.  Two important recations responsible: 

1. Non-enzymatic glycation of crystallin proteins. 2. Increased activity of aldose reductase.

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Non-enzymatic glycation of crystallin proteins

Difference between glycosilation and glycation.  Glycation reaction principle.  Non-enzymatic glycation of crystallin proteins occurs at amino group of Lys reciduces.  In diabetes the process occurs twice as often as in normal individual of comparable age.  Crystallin proteins become insoluble and make high molecular weight aggregates. 

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Step: 1. Ca+2 promotes the binding of protein of RMM 43KD to the plasma membrane. Step: 2. Crystallin becomes bound to the protein by disulphide bridges forming light scattering aggregates. 43

Increased activity of aldose reductase



Km of aldose reductase for glucose is about 200mM.(Normal concentration of blood glucose is 4-6.1mM/L)

D-glucose

Aldose reductase

D-sorbitol

Aldehyde dehydrogenase

D-fructose 44

Changes during cataractogenesis in galactosemia 

Galactokinase or galactose-1-phosphate uridyl transferase deficiency.

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Changes during cataractogenesis caused by ionising radiation 

X- rays affect germinative zone of the epithelial layer.



Organisation of the fiber cell is disrupted.



Membrane permiability increases. Synthesis of protein, potassium, glutathione concentration decline. Sodium concentration increases. 46

Changes during cataractogenesis caused by non ionising radiation High lifetime exposure to UV light causes cortical cataract.  UV-B reaching the eye is mainly responsible than UV-A reagion.  Mechanism is by free radical damage.  Long term exposure to infra red and high energy microwaves can cause cataract characteristically referred as “Glass Blowers” cataract. 

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Normal Vs Cataractous Lens

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Non surgical management of cataract

1.

Proper correction with the glasses, mydriatic agents, use of dark glasses.

2.

Medical treatment to delay progression of cataract: i.

Aldose reductase inhibitor: oral aspirin 50100mg/kg, quercetin 200-400mg/kg, topical sorbinil and sulidac drops. 51

Non surgical management of cataract continued…

Antioxidants: β-carotene, α-tocopherol, ascorbic acid. iii. Membrane stabilising agents: Benzadec and Benzyl alcohol(0.07%) iv. Miscellaneous: Iodides of calcium, potassium, homiopathic drugs like cinireria, maritima.  Unfortunately none of the drug have been conclusively proven to be anticataractogenic. ii.

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References: 1. 2.

3. 4. 5. 6. 7.

Grays Anatomy Wills biochemical basis of medicine. Text book of biochemistry with clinical correlation. – Thomas. Devlin. Text book of biochemistry. – Bhagwan. The text book of opthomalogy, volume 3, Lens & Cataract. - Norman. S. Jaffe, Joseph. Horwitz. Adler’s physiology of the eye: Clinical applications.-Paul. L. Kaufman, Albert ALM. Essentials of opthomalogy- Sanar. K. Basak. 53

Thank You Thank You

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