2. ANATOMY
1. Gross anatomy
• Biconvex, transparent, encapsulated,crystalline and avascular structure placed
between iris and vitreous
• divides eye into anterior and posterior segments
2.Functions
• To Maintain its own clarity
• To Refracts light
• To Provides accomodation
• No blood supply or innervation after foetal development
• Depends entirely on aqueous humour
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3. EMBRYOLOGY
OPTIC VESICLES (25 days of gestation)
LENS PLACODE (27 days of gestation)
LENS VESICLE(30 days of gestation)
LENS
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4. 1)Formation of lens vesicle
• 4 weeks
• Optic vesicle induces lens placode from ectoderm
• Lens placode invaginates and becomes lens pit
• Optic vesicle also invaginates and becomes optic cup
• Lens pit separates from ectoderm to become the lens vesicle
2)Formation of lens fibers and zonules
• Primary lens fibres—fibres formed upto 3rd month
Cells in posterior portion of lens vesicle elongate to fill vesicle forms
emryonic nucleus
• Secondary lens fibres—3rd month to entire life
Cells in anterior portion of vesicle divide actively and elongate
includes all other nucleus
• Lens capsule-produced by anterior epithelial cells
• Lens zonules—from neuroectoderm in ciliary area(3rd – 5th month)
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10. IMPORTANCE OF REFRACTIVE INDEX :
• If RI of lens is too low- Index hypermetropia
• If RI of lens is too high- Index myopia
OLD AGE-RI of cortex relatively increases and
approximates that of nucleus-less converging power – eye becomes
hypermetropic
CATARACT- increase in RI OF NUCLEUS- eye becomes myopic
DIABETES- high blood sugar level- myopia
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12. CAPSULE
• Elastic transparent membrane
• Type 4 collagen
• Thicker anterior than posterior and at equator than at pole.thinnest at
post pole
• Thickest basement membrane of the body
• Light microscopy appears as a homogenous structure but electron
microscopy shows lamellar pattern
• Zonular fibres get attached to it at the equator
• Age related loss of accomodation result from change in capsular
elasticity which may be due to loss of lamellae
• Permeable to water
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13. ANTERIOR EPITHELIUM
• Single layer of cuboidal nucleated epithelial cells
• 5 lakh cells in mature lens.density more in women than men
• The only metabolically active part of lens
• Density increases toward the periphery.
• No posterior epithelium as this is used up in formation of
primary lens fibres which occupy centre of the nucleus
• 3zonescentral
intermediate
germinative
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14. 1.Central zone:
Polygonal,round nuclei. Do not normally mitose , but they can do in response to
damage.
Metaplasia of these cells into spindle shaped myofibroblast like cells can
lead to anterior sub capsular cataract as in shield cataract and
glaucomflecken.
2. Intermediate zone:
Cylindrical cells,mitose occasionally
3.Germative zone:
-Coloumnar cells,located pre equatorially
-Actively dividing cells
-Extremely susceptible to irradiation.
-Dysplasia of these transitional cells can lead on to PSC as seen in: radiation
cataract, myotonic dystrophy, and NF2
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15. LENS FIBRES
•
•
•
•
•
The epithelial cells elongate to form the lens fibres.
At first lens are formed from posterior epithelium which runs
from posterior to anterior to fill the lens vesicle.
But later on, the lens fibres are derived from the cells of the
equatorial region of the anterior epithelium.
The deposition of succesive generation of lens fibres is
associated with formation of nuclear bow , in which flattened
nuclei form an arch forwards when traced into the deeper
portions of lens.
This forms an S or C shaped curve in meridional section.
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17. •
•
•
The deep older fibres about 150 µm into cortex, lose their
nuclei and thus represent the termination of nuclear bow.
The fibres are laid down in concentric layers the outermost of
which lie in cortex of lens and innermost in core or nucleus.
The fibres are thinner posteriorly . Tips of fibres meet those
of other fibres to form sutures
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19. STRUCTURAL ARRANGEMENT OF LENS FIBRES
• Lens sutures are formed by the arrangement of inter digitations
of the apical cell processes and basal cell processes
• Fetal nucleus surround embryonic nucleus in such a way that
they terminate with two Y shaped sutures . Anteriorly-upright
Y and posteriorly –inverted Y.
• Later in gestation and following birth, growth of lens is
irregular and due to asymmetrical growth complicated
dendritic patterns are observed(9 point suture)
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21. BIOMICROSCOPY
1.Capsule
2.Superficial Cortex
C1 alpha- first cortical clear zone or subcapsular
clear zone.
C1 beta- first zone of disjunction seen as bright
narrow zone of discontinuity.
C2- second cortical clear zone or subclear zone
of cortex.
3. Deep cortex
C3-bright light scattering zone of cortex
C4-clear zone of cortex
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23. Tunica vasculosa lentis
• During embroynic and fetal
development , the lens
receives nourishment via an
intricate vascular capsule, it
is formed from the hyaloid
artery
• Disappears shortly after birth
• Sometimes a remanent persist
called mittendorf dot
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24. Grading of nuclear hardness
• Important for setting the parameters of phaco
machine
• GRADE1
– Whitish/Green Yellow
• GRADE2
– Yellow
• GRADE3
– Amber
• GRADE4
– Brown
• GRADE5
– Black
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25. 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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29. Functions of lens
• Refraction
Accounts for 35% of total refractive power of eye (15D out of total
of 58D)
• Light transmission
Focusing of visible light rays on the fovea
Preventing, damaging- ultra-violet radiation, from reaching
the retina
• Accomodation
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30. Changes during aging
1)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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Fibre cell cycle stops-CDK INHIBITORS AND Rb geneElongation—microtubuleesFibredifferensiation—fibroblast gf and insulin likgfOrganelle degradation-15 lipoxygenase
•Pars orbicularis –The part of the zonules which lie over pars plana.•Zonular plexus- part of the zonules that lie between the cilliary processes.•Zonular fork¬- the point of angulation of the zonule, which lies at the mid zone of cilliary valleys.•Zonular limbs- consists of◦Anterior zonular limb: passes from pars plana to preequatorial part of the lens.◦Posterior zonular limb: passes from pars plicata to postequatorial part of the lens.◦Equatorial Zonular limb: passes from pars pliacata to lens equator.Pars orbicularis –The part of the zonules which lie over pars plana.Zonular plexus- part of the zonules that lie between the cilliary processes.Zonular fork¬- the point of angulation of the zonule, which lies at the mid zone of cilliary valleys.Zonular limbs- consists ofAnterior zonular limb: passes from pars plana to preequatorial part of the lens.Posterior zonular limb: passes from pars plicata to postequatorial part of the lens.Equatorial Zonular limb: passes from pars pliacata to lens equator.
Hyaloidzonules are the single layers of fibers which connect the anterior hyaloid of vitreous at the border of the patellar fossa to pars plana and pars plicata.
Lens if not formed– absence of corneal endothelium abnormaldifferensiation of corneal stroma absence of iris,ciliarybody,anterior chamber
Insoluble esp alpha crystallins—bind to hydrophobic domains of misfolded proteins
