The document summarizes the anatomy, physiology, and biochemistry of the lens. It discusses the lens's highly organized structure of specialized cells that form its biconvex, transparent shape. It describes the lens's role in the eye's optical system and its dimensions. Key aspects of lens physiology covered include its water content, protein composition, metabolism, and transport mechanisms that maintain transparency. The document also examines the lens's accommodative ability and various theories about the mechanism of accommodation.

1. ANATOMY , PHYSIOLOGY
& BIOCHEMSTRY OF LENS
Presenter : Dr. Om Patel
Moderator : Dr. Suryakant
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2. • Highly organised system of specialised cells
• Biconvex, transparent, crystalline structure
• Divides anterior and posterior segments
• Important component of optical system of the eye
Introduction

3. Position
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4. 7/8/2015 4

5. DIMENSIONS OF LENS
10 mm 6 mm
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6. • 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

7. 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

8. • 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 fibers
ii]Central zone-cuboidal cells
i) Transports solutes between lens and aqueous
ii) Secretes capsular material
Anterior Lens Epithelium

9. 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

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

11. NUCLEUS

12. • 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

13. • 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

14. Nucleus
• Later the fibres grow
asymmetrically giving a
complicated dendritic
pattern in the infantile
and adult nucleus

15. 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 cortex
o Deep cortex

16. Layers of lens on slit lamp
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

17. Surgical anatomy

18. 18Footer Text
GYABB

19. 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 7/8/2015 19

20. Parts Of Ciliary Zonules
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21. Types Of Zonules
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22. PHYSIOLOGY OF LENS
COMPOSITION
22

23. 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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24. LENS PROTEINS
Soluble lens crystallins
• 88% of total lens protein
1) 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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25. Lipids
• Mainly cholesterol, sphingomyelin, lipoproteins
• Lubricating cement substance between lens fibres
• Cholesterol and lecithin content increases with age
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26. LENTICULAR METABOLISM
• Continous supply of ATP required for-
1. Transport of ions and aminoacids
2. Maintanence of lens dehydration
3. Continous protein synthesis
• Major site – epithelium
• Source of nutrient supply - aqueous
humour
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27. 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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28. 7/8/2015 28

29. 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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30. 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 7/8/2015 30

31. Transport mechanism
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32. Water and Electrolyte Transport
7/8/2015Footer Text 32Pump Leak Mechanism

33. 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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34. Maintenance of transparency
• Thin epithelium
• Regular arrangement of lens fibers
• Little cellular organelles
• Little extracellular space
• Lamellar conformation of lens proteins
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35. Maintenance of transparency
• Relative dehydration
• Semipermeable character of lens capsule
• Avascularity
• Autooxidation
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36. 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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37. 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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38. 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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39. 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
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40. Changes During Accommodation
• Slackening of zonules
• Changes in curvature
• Anterior pole
• Axial thickness of lens

41. Changes During Accommodation
• Tension of lens capsule
• Pupillary constriction and
convergence
• Choroid & ORA serrata

42. 1
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