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ANATOMY , PHYSIOLOGY

& BIOCHEMSTRY OF LENS

Presenter : Dr. Om PatelModerator : Dr. Suryakant

• Highly organised system of specialised cells

• Biconvex, transparent, crystalline structure

• Divides anterior and posterior segments

• Important component of optical system of the eye

Introduction

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Position

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DIMENSIONS OF LENS

10 mm 6 mm

• Refractive index- 1.38 ( cortex) 1.42 ( nucleus)

• Refractive power- 16-17 D

• Accomodative power- 14-16 D at birth 7-8 D at 25 yrs 1-2 D at 50 yrs

Lens capsule• It is a thin , transparent

elastic membrane which surrounds lens completely

• Thicker anteriorly than posteriorly

• Thicker at the equator than the poles, thinnest at the posterior pole

• Composed of type 4 collagen and GAGs

• Single layer of nucleated epithelial cells inter-connected 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

Anterior Lens Epithelium

Lens fibres• Form the main bulk of the lens

• Long thin transparent cells firmly packed with diameter of 4-7 micron and length upto 12mm

• Newly formed fibres contain all the organelles and as the fibres mature the nucleus disappears

• The cells are linked by gap junctions and interdigitations that look like ball and socket

NUCLEUS

• EMBRYONIC NUCLEUS- the apices of these cells grow

towards anterior lens epithelium These are PRIMARY LENS FIBRES

• The nuclei of these cells are present anteriorly forming Nuclear bow

• Rest of the nuclei are formed from the SECONDARY LENS FIBRES

• These are the fibres formed from the anterior epithelium throughout life

• Initially the fibres of the fetal nucleus reach both the anterior and posterior pole.

• Later they are not able to extend all the way and instead meet at radiating lines which appear as an erect Y anteriorly and inverted Y posteriorly

Nucleus

Nucleus• Later the fibres grow

asymmetrically giving a complicated dendritic pattern in the infantile and adult nucleus

Cortex• The most newly formed (youngest) lens

fibres are present most peripherally outside the adult nucleus

• It is further divided into: o Superficial cortex o Intermediate cortexoDeep cortex

Layers of lens on slit lamp

1. Capsule2. Superficial cortex :

a) C1α- subcapsular clear zone

b) C1β- first zone of disjunction c) C2 – second cortical clear zone3. Deep cortex : a) C3 – bright light scattering zone b) C4- clear zone of cortex

4. Nucleus

Surgical anatomy

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GYABB

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Ciliary Zonules• Suspensory ligaments

• Series of fibres arising 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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PHYSIOLOGY OF LENS

COMPOSITION

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WATER• 65% of total volume

• Dehydrated state

• Tightly packed fibres with minimum extracellular spaces

• Dehydration is maintained by active sodium pump

• Important factor maintaining lens transparency and refractive index

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LENS PROTEINSSoluble lens crystallins• 88% of total lens protein1) Alpha crystallin• Largest crystallin• Accounts for 31% total lens protein

2) Beta crystallin • Most abundant - 55% total lens protein

3) Gamma crystallin• Smallest crystallin• Age related loss

Insoluble albuminoids – 12%

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Lipids• Mainly cholesterol, sphingomyelin, lipoproteins

• Lubricating cement substance between lens fibres

• Cholesterol and lecithin content increases with age

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LENTICULAR METABOLISM

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

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

humour

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Glucose metabolism• Main source of energy

• Glucose enters lens by simple diffusion and facilitated diffusion

• Epithelial cells- GLUT-1• Lens fibre cells-GLUT-3

• Glucose is rapidly metabolized. So that level of free glucose in lens is <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 only 2 moles of ATP• Lactate generated undergoes 2 pathways of

metabolism• Diffusion from lens into aqueous• Further metabolism via Kreb’s cycle

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

• Important source of NADPH and Pentose

• Required for other metabolic pathways e.g. sorbitol pathway and glutathione reductase

4)Sorbitol pathway• Accounts for 5% of glucose metabolism

• 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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Transport mechanism

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Water and Electrolyte Transport

Pump Leak Mechanism

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Amino Acid Transport• Also included in pump leak concept

• Three types of pumps – for acidic, basic and neutral aminoacids

• Inside the lens aminoacids are utilised for protein formation and energy production or diffuse back in to aqueous by “leak”

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• Thin epithelium

• Regular arrangement of lens fibers

• Little cellular organelles

• Little extracellular space

• Lamellar conformation of lens proteins

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• Relative dehydration

• Semipermeable character of lens capsule

• Avascularity

• Autooxidation

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ACCOMODATION• Mechanism by which diverging rays coming

from near object can be focused on retina

• Ability to focus from far to near and near to far

• Range of accomodation

• Amplitude of accomodation

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Theories of accomodation

1)Helmholtz theory-ciliary muscle contraction

relaxation of zonules

lens-spherical

• Aging lens rigid—difficulty to change shape

• Classical theory

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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

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3) Coleman’s (hydrualic suspension) theory—

• Lens zonules and anterior vitreous - diaphragm

• Ciliary muscle contraction - alters pressure gradient b/w aqueous and vitreous

• Anterior movement of diaphragm

• Alteration in shape

• Presbyopia – increased lens volume – reduced response to pressure gradient

Changes During Accommodation

• Slackening of zonules

• Changes in curvature

• Anterior pole

• Axial thickness of lens

Changes During Accommodation

• Tension of lens capsule

• Pupillary constriction and convergence

• Choroid & ORA serrata

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