crystalline lens basic sciences over view
TRANSCRIPT
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LOGOLOGO
Crystalline lens
By Dr.Sreekanth Ramachandran
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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
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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
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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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LOGO
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LOGO
LENS EMBROLOGY
The development of the eyeball starts around
day 22 of gestation and it is around 2mm in
length.
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LOGO
EYE DEVELOPMENT
Neural
plate
thickening
sulcus
vesicle
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LOGO
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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.
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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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LOGO
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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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LOGO
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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LOGO
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LOGO
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.
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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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LOGO
Lens - Anatomy
Histologically lens consists of three major components:
1. Capsule
2. Anterior epithelium
3. Lens fibers.
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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.
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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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LOGO
Zonular lamella?/Pericapsular membrane?
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LOGOThe superficial part of capsule layer of zonule
insertion
Rich in GAG And FIBRONECTIN
May aid for zonular adhesive mechanism
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LOGO
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
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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
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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
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LOGOFunctions;
– Centrally transport of substances between the
aqueous humor & the lens interior.
– Equatorially mitotic division & differentiation
into lens fibers.
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LOGOMembrane specialization
Cytoskeltal proteins are also abundant here
(Geodomes).They thought to perform
structural function
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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
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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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LOGO
More clear?
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LOGO
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....
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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
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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?
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LOGOAccommodating the growth and packing of
lens fibers while retaining the cross-sectional
configuration
9 point star in mature lens
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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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LOGO
• 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.
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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
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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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LOGO
• 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
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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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LOGO
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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LOGO
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
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LOGOOXIDATIVE DAMAGE AND
PROTECTIVE MECHANISMS FREE RADICALS-SCAVENGED BY GLUTATHIONE
• VITAMIN E AND ASCORBIC ACID IN LENS ALSO ACT AS
FREE RADICAL SCAVENGERS
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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
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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
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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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LOGO
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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LOGO
RISK FACTORS Age >50
Low SES
Smoking and alcoholism
Exposure to steroids and radiations
Myopia,DM,HTN,renal failure etc.,
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LOGOLOGO