Structure of Lens and Changes During Cataractogenesis

Structure of lens

and

changes during cataractogenesis

1

Structure of Human Eye

1. Outer fibrous layer.

Sclera: Tough, fibrous, opaque coat.

Cornea: Clear, transparent, avascualar.

2

Wall of the eye ball contain 3 principle layers:

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…

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

The lens is a transparent, encapsulated,

biconvex body lies between iris and the

vitreous body with no blood supply.

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Definition

AP

Structure of lens under compound microscope

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capsule

epithelium

Cortex Lens fiber cells

Nuclear lens fiber cells

PP

Development of Human lens

7

Development of Human lens continued…

8

Structure of mature lens

9

Anterior pole

Posterior poleaxis

equator

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

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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

The lens capsule

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

capsule

Epithelial cells

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.

13

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.

15

Shape:

◦ Young state: flattened,

hexagonal cross sectional

profile.

◦ Middle age: irregular

profile.

Dimensions:

◦ Length: 7-10mm

◦ Width: 10-12µm

◦ Thickness: 1.5 - 2µm

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The lens fibers

Corticle fibers lack nucleus and all cell organelles.

The lens fibers continued…

17

Note alternating rows of hooks and

complementary eyes. Spines hook

into next row of eyes to interlock

layers like Velcro.

Note ball and socket joint

interlocking at superficial cortical

fiber edges. Planar surface of

fibers interlock with next layer of

offset fibers (removed).

Lateral membranes have interdigitations like ball and

socket, tongue and groove (hook and eye) junctions

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.

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The lens sutures

Appears 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.

19

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.

20

The lens sutures continued…

Branches of the suture:

o In early adulthood : 6-9

o In middle to old age: 9-15

Finally a star shaped suture is formed at both the poles.

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.

22

Lens crystallin proteins continued…

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

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.

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Structural properties maintaining

transparency of the lens.

1. Organization of the lens fiber in the lens.

2. The refractive power of the lens is altered as

the lens grows, maintaining focal point

constant.

3. Variable concentration of crystallin proteins

causes gradient of refractive index partially

corrects for spherical aberration.

4. All the cell organelles are absent in cortical

and nuclear fibers.

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1. ATP dependent bicarbonate pump activity:

25

Metabolic reactions maintaining

transparency of the lens

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).

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5. Reduced glutathione is present in high

concentration in aqueous humour: 4-6mM.

27


6. High concentration of transferrin in

epithelial cells, catalase and glutathione

peroxidase prevent oxidative stress.

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Fe+2

transferrin

catalase

Glutathione

peroxidase

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. Immature

2. Mature

3. Intumescent

4. Hypermature

5. Morganian

32

Classification of cataract according to age onset

1. Congenital

2. Juvenile

3. Senile

4. Infantile

5. Presenile

33

Cataractogenesis

General mechanism of cataract formation

1. Opacification of previously clear lens fiber.

2. Formation of new opaque fibers.

3. Deposition of the granular material instead

of fibers.

4. Accumulation of pigments.

5. Opacification of the lens epithelium.

6. Deposition of extraneous material.

7. 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…

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.

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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.

37

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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.


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.

39


Cytoskeletal proteins like vimentin,

intermediate filaments are disassembled

because of insolubalisation and proteolysis.

Increased non-enzymatic glycation of

crystallin proteins increase high molecualr

aggregates, scattering the light.

Intercellular transport decreases can not

have control over oxidative stress.

40

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.

41

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.42

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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.

Increased activity of aldose reductase

Km of aldose reductase for glucose is

about 200mM.(Normal concentration of

blood glucose is 4-6.1mM/L)

44

Aldose reductase

D-fructose

Aldehyde

dehydrogenase

D-glucose D-sorbitol

Changes during cataractogenesis in

galactosemia

Galactokinase or galactose-1-phosphate

uridyl transferase deficiency.

45

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.

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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 50-

100mg/kg, quercetin 200-400mg/kg, topical

sorbinil and sulidac drops.

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Non surgical management of cataract continued…

ii. 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.

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References:

1. Grays Anatomy

2. Wills biochemical basis of medicine.

3. Text book of biochemistry with clinical

correlation. – Thomas. Devlin.

4. Text book of biochemistry. – Bhagwan.

5. The text book of opthomalogy, volume 3, Lens &

Cataract. - Norman. S. Jaffe, Joseph. Horwitz.

6. Adler’s physiology of the eye: Clinical

applications.-Paul. L. Kaufman, Albert ALM.

7. Essentials of opthomalogy- Sanar. K. Basak.

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Structure of Lens and Changes During Cataractogenesis