crystalline Lens basic sciences over view

LOGOLOGO

Crystalline lens

By Dr.Sreekanth Ramachandran

LOGOLens Facts… The lens is a transparent, biconvex, crystalline

structure placed between iris and the vitreous in

a saucer shaped depression ,the patellar fossa.

Its diameter is 9-10 mm and thickness varies with

age from 3.5 mm (at birth) to 5 mm (at extreme of

age).

Its weight varies from 135 mg (0-9 years) to

255 mg (40-80 years of age).

Celcus drew lens in

AD 30 as a center of

eye globe and “locus

vacuus” anterior to it

LOGO It has got two surfaces:

the anterior surface is less convex (radius of curvature 10 mm) than the posterior (radius of curvature 6 mm).

These two surfaces meet at the equator.

Its refractive index is 1.39 and total power is 15-16 D.

The accommodative power of lens varies

with age, being 14-16 D (at birth); 7-8 D (at 25

years of age) and 1-2 D (at 50 years of age).

Lens is a unique structure, which

contains cells of a single type, in various

stages of differentiation

LOGOLens is suspended in eye by Zonules which are inserted

on anterior surface and equatorial lens capsule and

attached to ciliary body. Zonular fibres are series of

fibrillin rich fibre.

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LENS EMBROLOGY

The development of the eyeball starts around

day 22 of gestation and it is around 2mm in

length.

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EYE DEVELOPMENT

Neural

plate

thickening

sulcus

vesicle

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LOGOSimultaneously, the optic vesicle gets converted into optic cup due to differential growth of its margins

The optic cup surrounds the upper and the lateral sides of the lens

It is deficient in the inferior part known as the choroidal or fetal fissure. This closes by 6th

week. Failure to fuse leads to typical colobomas.

LOGOThe hyaloid vessels suppy the lens in the fetus

due to which it grows rapidly.

Later the hyaloid vessels disintegrate and the

remains forms the central retinal artery and vein.

The lens derives its nutrtion then by diffusion

from the aqeous humor and vitreous humor.

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LOGOLens vesicle is lined by single layer of cells

covered by basal lamina.

Cells from the posterior wall of vesicle

elongate rapidly to form the primary lens

fibers.

Their base remain anchored to the basal lamina

posteriorly and their apices grow towards the

anterior lens epithelium.

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Primary lens fibres are formed up to the 3rd

month of gestation.

These fibres are preserved as the compact core

of lens, called embryonic nucleus.

Hence, posterior aspect of the lens become

devoid of epithelium.

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Equatorial cells of the anterior epithelium

forms the secondary lens fibres.

Tips of the these fibres extend around the

primary fibres and they meet at the Y-shaped

anterior and posterior lens sutures.

LOGOTunica vasculosa lentis

During embryonic and fetal development lens

receives nourishment from this structure.

It completely encloses lens by aprox. 9 weeks.

It is formed by mesenchyme sorrounding lens.

In the early stage, it receives abundant blood

supply from hyaloid artery.

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Lens - Anatomy

Histologically lens consists of three major components:

1. Capsule

2. Anterior epithelium

3. Lens fibers.

LOGOLens capsule It is a thin , transperent, hyaline collagenous

membrane which surrounds lens completely.

It is secreted by lens epithelium anteriorly and by

elongating lens fibers posteriorly.

It is more thick anteriorly than posteriorly and at

equator than poles.

It is thinnest at the posterior pole.

It is mainly composed of type-IV collagen.

Lens capsule is the thickest

basement membrane of body.

LOGOCapsule thickness increase anterior with age

Posterior also but to a less extent

lamellar arrangement is a peculiarity

Loss with age that may be the reason of loss of

accommodation or a manifestation of the same

This extreme thinness of

the posterior capsule

makes it more vulnerable

for posterior capsular

tear or rent during

cataract surgery

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Zonular lamella?/Pericapsular membrane?

LOGOThe superficial part of capsule layer of zonule

insertion

Rich in GAG And FIBRONECTIN

May aid for zonular adhesive mechanism

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Anterior lens epithelium It is a single layer of cuboidal nucleated epithelial cells.

Almost all metabolic, synthetic and transport process of lens occurs here.

zones of lens epithelium

a) central zone

b) Intermediate zone

c) Pre equatortial or germinative zone

Simple cuboidal

epithelium consists of

epithelial cells in a single

layer which have the same

height, width and depth

LOGOCentral zone

It consists of cuboidal cells.

These cells are stable and their number reduces with the age.

Normally, these cells do not mitoses.

But can do so in response to various insults

LOGOb)Intermidiate zone :- It contains small but more cylindrical cells.

They can mitoses occasionally.

c)Germinative zone :- It consists of columnar cells.

These are actively dividing and elongating to form new lens

fibers..

Alpha crystalline +

Protected from radiation

LOGOFunctions;

– Centrally transport of substances between the

aqueous humor & the lens interior.

– Equatorially mitotic division & differentiation

into lens fibers.

LOGOMembrane specialization

Cytoskeltal proteins are also abundant here

(Geodomes).They thought to perform

structural function

LOGOFiber elongation

• Fibers constitute the main mass of the lens.

• The fibers are formed by the multiplication and differentiation

of the lens epithelial cells at the equator.

• As the basal part of the cell elongates, the process moves along

the internal surface of the capsule in a posterior direction.

• As the apical part of the cell elongates, it slips beneath the

internal surface of adjacent lens cells.

Little information about epithelial

mitosis in human is available ,but in

rat it falls with age

LOGO• At first, the nucleus of fibers remains intact; but later it

fragments and disappears (This process continues throughout life).

• Previous generations of cells are repeatedly pushed into the lens substance. As the cell progressively elongates anteriorly, the nucleus moves anteriorly, so that it takes up a position anterior to the nuclei of the more superficial cells. This anterior movement of the nuclei as the fibers pass deeper produces the nuclear pattern known as the lens bow.

Please look

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More clear?

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Fibres The lens fiber is a hexagonal prism in cross-section.

The fibers run meridionally from the posterior to the

anterior lens surface in U-shape.

The earliest formed fibres are those in the center or nucleus

of the lens; the later fibres form the outer or cortex of the

lens.

Fibres are thinner posteriorly

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Is it fiber/fibre?

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Nucleus and cortex

Outer part cortex inner part nucleus

Exact junction is arbitrary....

LOGOThe lens nucleus is divided into:

– Embryonic nucleus: up to 3 months of gestation,

– Fetal nucleus: 3 months of gestation till birth,

– Infantile nucleus: birth to puberty,

– Adult nucleus: after puberty

LOGO• In the fetus, the ends of opposing lens fibers in the same layer

abut (border) in a manner producing patterns known as

sutures.

• The anterior suture is an erect Y-shape, while the posterior is

an inverted Y. As the lens increases in size, the lens fibers are

unable to stretch the antero-posterior distance, so that

progressively more complicated suture patterns are formed.

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Why the sutures?

LOGOAccommodating the growth and packing of

lens fibers while retaining the cross-sectional

configuration

9 point star in mature lens

LOGO• The fibers are tightly packed, there being very little intercellular

space. The lens fibers are held together by the interlocking of

their adjacent plasma membranes as ball-and-socket type of

interdigitations.

• It is interesting to note that the interdigitations are less

complicated in the superficial zones of the lens, and this may

permit molding of the lens shape during accommodation.

•Lens fibers exhibit numerous gap

junctions which may explain how deep

lens fibers can survive some distance from

the surface, and away from a source of

nourishment

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• During development, the lens fiber cells lose their nuclei and

the cytoplasmic organelles become specialized for the

production of lens proteins (crystallins);

• Crystallins constitute up to 60% of the lens fiber mass, which

is the highest protein concentration in the body.

• The differing concentrations of the crystallins in different parts

of the lens produce regional differences in the refractive index.

•This probably compensates for the

spherical and chromatic aberrations that

might exist if the concentrations of the

crystallins were uniform throughout the

lens!!

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LOGOCortex :

Peripheral part

It has the youngest fibres.

Histologically ,section through the equator

shows the hexagonal structure of lens fibres.

and bound together by ground substance.

LOGOApplied aspect

1. Capsule

2. Superficial cortex :

a) C1α- subcapsular clear

zone

b) C1β- first zone of disjunction

c) C2 – second cortical clear zone.

3. Deep cortex :

a) C3 – bright light scattering zone

b) C4- clear zone of cortex.

4. Nucleus.

Which grading is this?

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The Oxford system of lens zoning

LOGOSuspension The lens is held in position by a series of delicate, radially

arranged fibers collectively known as the suspensory

ligament of the lens, or zonule.

The zonule fibers arise from the epithelium of the ciliary

processes and run toward the equator of the lens.

The fibers fuse to form about 140 bundles .

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• The larger bundles are straight and reach the lens capsule in

front of the equator.. Together they form the anterior zonular

sheet.

• The smaller fibers curve backward and are attached to the

posterior surface of the lens to form the posterior zonular

sheet.

• The third group passes from the tip of the ciliary processes

almost directly inward to be inserted at the equator

LOGO• As the zonular fibers reach the lens, they break up into fine fibers

that become embedded in the outer part of the lens capsule.

• When the eye is at rest (meiosis), the elastic lens capsule is under

tension, causing the lens constantly to attempt a globular rather than

a discoid shape.

• The pull of the radiating fibers of the zonule tends to keep the elastic

lens flattened, permitting the eye to focus on distant objects.

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CILIARY ZONULES Zonules of zinn or suspensory ligaments of lens

Series of fibres which run from ciliary body and fuse in to

outerlayer of lens capsule around equatorial zone

Holds lens in position and enable ciliary muscle to act

Structure

• Transparent stiff and non elastic

• Composed of glycoproteins and muco polysaccharides

• Susceptible to hydrolysis by alpha chymotrypsin,beneficial in

ECCE

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LOGOTypes of zonules

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Zonular spaces

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BIOCHEMISTRY OF THE LENS

MAIN CONTENTS ARE

WATER (65%)

PROTEINS(34%)

LIPIDS,CHO AND TRACE

ELEMENTS(1%)

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WATER

relatively dehydrated organ

cortex more hydrated than nucleus

80% is free and 20% is bound

low water is natural consequence of need for

maintining refractive index

no significant alteration in hydration with age

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PROTEINS

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Lens - CrystallineLens fibres contain high concentrations of

crystallins.

Crystallins represent the major proteins of the lens

(constitute 90% of total protein content of lens).

Crystallin has the following constituents:

Alpha

Beta and,

Gamma

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WATER INSOLUBLE PROTIENS UREA SOLUBLE

.CYTOSKELETAL PROTEINS

.VIMENTIN AND BEADED FILAMENTS

.GENETIC DISRUPTION OF BEADED FILAMENTS L/T

CATARACT FORMATION

• UREA INSOLUBLE

.MEMBRANE INHIBITORY PROTEINS

.MEMBRANE PROTEINS

.SERVES TO REDUCE LIGHT SCATTERING BETWEEN

CELLS

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LENS METABOLISM

Continous supply of ATP required for-

1. Transport of ions and aminoacids

2. Maintanence of lens dehydration

3. Continous protein synthesis

4. GSH synthesis

Major site – epithelium

Source of nutrient supply-aqueous humour

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WITH AGE lens proteins proteolyse dissembly of fibres aggregation

of water insoluble proteins scatter light opacification of

lens

glutathione is essential for maintaining a reduced environment

any depletion cause cataract

LOGOOXIDATIVE DAMAGE AND

PROTECTIVE MECHANISMS FREE RADICALS-SCAVENGED BY GLUTATHIONE

• VITAMIN E AND ASCORBIC ACID IN LENS ALSO ACT AS

FREE RADICAL SCAVENGERS

LOGOEXPOSURE TO LONG TERM

HYPERBARIC

OXYGENOPACIFICATION OF LENS

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Lens - Functions The lens serves two major functions:

– Focusing of visible light rays on the fovea

– Preventing damaging ultra-violet radiation from reaching

the retina

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PHYSIOLOGY OF LENS

LOGOMAINTANENCE OF LENS WATER AND

CATION BALANCE MOST IMPORTANT MECH FOR MAINTAINING LENS

TRANSPARENCY

MAINLY BY ACTIVE AND PASSIVE TRANSPORT

MECHANISMS

ACTIVE- Amino acids,K,Na,INOSITOL ETC. 90% of energy in

the form of ATP utilised here

PASSIVE-Water,ions and waste products of metabolism

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WATER AND ELECTROLYTE TRANSPORT

pump leak mechanism

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LOGOTransport of amino acids Also included in pump leak concept

Three types of pumps

Inside the lens aa are utilised for protein formtion and energy

production or diffuse back in to aqueous by leak

LOGOFACTORS MAINTAINING

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

Antioxidants

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Changes during aging1)Changes in structure

Leads to opacities –nuclear sclerosis—senile cataract

2)Less elasticity of lens— loss of power of accomodation—

presbyopia

3)Overall reduction in light transmission

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LOGOCATARCTOGENESIS

INCREASED AGE LEADS TO INCREASE WEIGHT AND THICKNESS OF LENS

LENS UNDERGO COMPRESSION AND HARDENING(NUCLEAR SCLEROSIS)

AGGREGATION OF PROTIENS ALSO CAUSES FLUCTUATIONS IN RI OF

LENS,LIGHT SCATTERING AND DECREASED TRANSPARENCY

CHEMICAL MODIFICATIONS ALSO INCREASES PIGMENTATION GIVING

RED YELLOW COLOURS TO LENS

K,GLUTATHIONE AND Ca AND Na

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RISK FACTORS Age >50

Low SES

Smoking and alcoholism

Exposure to steroids and radiations

Myopia,DM,HTN,renal failure etc.,

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