LENS-ANATOMY AND PHYSIOLOGY

VISHNU NARAYANAN M.R

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ANATOMY

1. Gross anatomy• Biconvex, transparent, crystalline structure • Ability to change shape• divides eye into anterior and posterior segments• 2 surfaces-anterior and posterior

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Lens

Zonules

Zonules

ciliary body

ciliary body

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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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Histology of lensCAPSULE

ANTERIOR EPITHELIUM

LENS FIBRES

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A)Capsule

• Acellular elastic structure• basement membrane -type 4 collagen+sulphated GAG• Variable thickness• Zonules run from ciliary processes and fuse onto outer

layer of capsule• Main function is to mold the shape of the lens in

response to tension from zonules

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B)Anterior epithelium

• Single layer of nucleated epithelial cells interconnected with gap junctions and desmosomes

• The only metabolically active part of lens• Functionally divided into 2 zones-i] Pre-equatorial zone- columnar cells Actively dividing and differentiating into lens fibersii]Central zone-cuboidal cells i) Transports solutes between lens and aqueous ii) Secretes capsular material• Maintains the osmotic balance of lens

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C)Lens substance

• Composed of lens fibres• Consist of primary and secondary fibers• Produced by mitosis of epithelial cells in the pre-

equatorial zone• They elongate and undergo differentiation with

pyknocytosis and eventual loss of cell oraganelles and nucleus

• This is an important factor in the transparency of the lens

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• As lens fibres elongate and new ones form, the older ones are pushed towards the depth of the lens

• Lens bow-shifting of lens nuclei forms a visible line in the equator

• Fibres are arranged in zones in which fibres growing from opposite directions meet in sutures

• Consist of nucleus and cortex

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LENS FIBRES-PHOTOMICROGRAPH

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Cortex

• Youngest lens fibres• It is located peripherally, and is composed

of secondary fibres formed continuously after puberty

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Nucleus

• Central part with oldest fibres• Dense and compactly arranged lens fibres• Higher refractive index than capsule• Different zones depending on period of development

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epinucleus

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Anterior Suture

( erect Y )

posterior sutureinverted Y

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Sutures

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Zonules of zinn

• Suspensory ligaments/ciliary zonules

• Series of fibres from ciliary process

• Holds the lens in position

• Assist action of ciliary muscle

• Attached to lens capsule at zonular lamella

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Parts of ciliary zonules

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Types of zonules

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Zonular spaces

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EMBRYOLOGY

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

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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)• Tunica vasculosa lentis--nourishment to embyonic lens branch of hyaloid artery disappears before birth

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PHYSIOLOGY OF LENS

COMPOSITION

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LENS PROTEINSWater soluble lens crystallins• 90% of total lens protein1) Alpha crystallin• Largest crystallin• Accounts for 31% total lens protein2) Beta crystallin • Most abundant crystallin, accounts for 55% total lens protein• Most heterogenous group, 6 distinct subgroups3) Gamma crystallin• Smallest crystallin• Least abundant-2%

Water insoluble proteins• Insoluble albuminoids-12%

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WATER

• Lens –dehydrated state• Unique arrangement of proteins within lens fibres• Low protein osmotic activity within lens• Tightly packed fibres with minimum extracellular spaces• Lens epithelium transports water into the fibre mass• Half of the water -protein hydration• Water excreted via aquaporin in the equator into

aqueous• Important factor maintaining lens transparency

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Electrolytes

• Active transport of ions and low molecular weight metabolite takes place between lens and aqueous humour

• Epithelium contains Na-K-ATPase and a calmodulin- dependent Ca-activated ATPase for the active transport of electrolytes

• Fibre cells contain large concentrations of negatively charged crystallines

• Positively charged cations enter the lens cell to maintain electrical neutrality

• pH- 6.9-7.2• Aminoacids transported in lens via energy dependent carrier

mechanisms

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• Lipids-high concentration of cholesterol and sphingomyelin membrane rigidity• Glutathione –major antioxidant in lens synthesized by GSH in lens present in reduced state• Ascorbic acid-synthesized by cililary body into aqueous antioxidant property• Inositol- osmolyte membrane rigidity• Taurine- osmolyte antioxidant

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Pump-leak mechanism

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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• Organizer of anterior segment

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REFRACTIVE PROPERTIES

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ACCOMODATION

• Lens has the capacity to change the focussing power of the eye for distant and near vision

• Accomodation can be divided into physical process- measure of change in shape of lens physiological process- measure of ciliary muscle contraction• Near reflex-Contraction of ciliary muscles Contraction of pupils Convergence of visual axis

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Theories of accomadation

1)Helmholtz theory-ciliary muscle contraction

relaxation of zonules

lens-spherical

• aging—lens rigid—difficulty to change shape• Classical theory

2) Schachar’s theory– ciliary muscle contraction

equatorial zonules tensed

shape changes in lens

Aging—diameter of lens grow—less space for proper functioning of ciliary muscles

3) Coleman’s theory—• Lens and zonules—diaphragm• Shape altered by pressure difference b/w aqueous and vitreous• Ciliary muscle-alters pressure gradient

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Maintenance of 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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LENTICULAR METABOLISM

• Continous supply of ATP required for-1. Transport of ions and aminoacids2. Maintanence of lens dehydration3. Continous protein synthesis4. GSH synthesis

• Major site – epithelium• Source of nutrient supply-aqueous humour

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Glucose metabolism

• Energy production entirely dependent on glucose metabolism

• Glucose enters lens by simple diffusion and facilitated diffusion

• Epithelial cells- GLUT-1• Lens fibre cells-GLUT-3• Glucose is rapidly metabolized via glycolysis so that

level of free glucose in lens < 1/10 level in aqueous

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1)Anaerobic metabolism• Accounts for 85% of glucose metabolism by lens• Provides > 70% of energy for lens• 1 mole of glucose gives 2 moles of ATP• Lactate generated undergoes 2 pathways of metabolism• Further metabolism via Kreb’s cycle• Diffusion from lens into aqueous

2)Aerobic metabolism (Krebs cycle)• Limited to epithelium• 1 mole of glucose gives 38 moles of ATP• Only 3% of lens glucose metabolized by this pathway• But generates up to 20% of total ATP needs of lens

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3)Hexose monophosphate shunt• Accounts for 5% of glucose metabolism by lens• Important source of NADPH required for other

metabolic pathways e.g. sorbitol pathway and glutathione reductase

4)Sorbitol pathway• Accounts for 5% of glucose metabolism by lens• When sorbitol accumulates within cells of lens, it sets

up an osmotic gradient that induces influx of water and lens swelling, and ultimate loss of lens transparency

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Changes during aging

1)Changes in structure• Crystallins—proteolysis,decresed solubility,aggregation racemization and deamidation of aminoacids• Cytoskeleton—proteolysis and insolubilization—disassembly

of fibres• 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• Microspherophalus• 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 uveatits• Loss of accomodation-aging,cataract,surgery

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