human lens anatomy

LENS-ANATOMY

LENS ANATOMY

SIVATEJA CHALLASSSIHMS

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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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EMBRYOLOGYOPTIC VESICLES (25 days of gestation)

LENS PLACODE (27 days of gestation)

LENS VESICLE(30 days of gestation)

LENS

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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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Position

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LOCATION OF LENS

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DIMENSIONS OF LENS

10 mm 6 mm

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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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STRUCTRE OF LENSCAPSULE

ANTERIOR EPITHELIUM

LENS FIBRES

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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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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• 3zones- central intermediate germinative

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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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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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• 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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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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Anterior Suture( erect Y )

posterior sutureinverted Y

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BIOMICROSCOPY1.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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4.NUCLEUS

epinucleus

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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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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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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 actStructure• Transparent stiff and non elastic• Composed of glycoproteins and muco polysaccharides• Susceptible to hydrolysis by alpha chymotrypsin,beneficial in

ECCE

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Zonular complex

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Types of zonules

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Zonular spaces

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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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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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DISEASES OF LENS

1)CONGENITAL DISORDERS• Coloboma• Lenticonus• Lentiglobus• Mittendorf dot

2)CATARACTS• Congenital• Acquired

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3) Miscellaneous disorders• Posterior capsular opacification• Aphakia• Pseudophakia• Exfoliation of capsule• Psuedoexfoliation and psuedoexfoliation glaucoma• Lens induced glaucoma• Ectopia lentis• Lens induced uveitis• Loss of accomodation-aging,cataract,surgery

4)Several systemic diseases are also associated with disorders of the lens