Lens - Anatomy, Physiology and Biochemistry

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
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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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