The document provides information on the anatomy, physiology, and diseases of the lens. It discusses the lens's biconvex shape and ability to change shape to accommodate. The lens is divided into an anterior and posterior epithelium, cortex, and nucleus. It maintains transparency through organized fiber structure, hydration, and antioxidants. The lens focuses light and accommodates through changes in shape mediated by the ciliary body and zonules. Aging and various diseases can impact the lens's structure and function.
8. 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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9. 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 fibers
ii]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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10. 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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11. • 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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15. Cortex
• Youngest lens fibres
• It is located peripherally, and is
composed of secondary fibres formed
continuously after puberty
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16. 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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21. 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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25. 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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27. 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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29. LENS PROTEINS
Water soluble lens crystallins
• 90% of total lens protein
1) Alpha crystallin
• Largest crystallin
• Accounts for 31% total lens protein
2) Beta crystallin
• Most abundant crystallin, accounts for 55% total lens protein
• Most heterogenous group, 6 distinct subgroups
3) Gamma crystallin
• Smallest crystallin
• Least abundant-2%
Water insoluble proteins
• Insoluble albuminoids-12%
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30. 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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31. 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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32. • 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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34. 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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36. 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
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Convergence of visual axis 36
38. Theories of accomadation
1)Helmholtz theory-ciliary muscle contraction
relaxation of zonules
lens-spherical
• aging—lens rigid—difficulty to change shape
• Classical theory
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39. 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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40. 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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41. LENTICULAR METABOLISM
• Continous supply of ATP required for-
1. Transport of ions and aminoacids
2. Maintanence of lens dehydration
3. Continous protein synthesis
4. GSH synthesis
• Major site – epithelium
• Source of nutrient supply-aqueous humour
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42. 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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44. 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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45. 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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46. 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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Fibre cell cycle stops-CDK INHIBITORS AND Rb geneElongation—microtubuleesFibredifferensiation—fibroblast gf and insulin likgfOrganelle degradation-15 lipoxygenase
Adhesive junctions by microtubules—early stagesBall and socket junctions– on reaching the sutures—membrane intact during accomodation
•Pars orbicularis –The part of the zonules which lie over pars plana.•Zonular plexus- part of the zonules that lie between the cilliary processes.•Zonular fork¬- the point of angulation of the zonule, which lies at the mid zone of cilliary valleys.•Zonular limbs- consists of◦Anterior zonular limb: passes from pars plana to preequatorial part of the lens.◦Posterior zonular limb: passes from pars plicata to postequatorial part of the lens.◦Equatorial Zonular limb: passes from pars pliacata to lens equator.Pars orbicularis –The part of the zonules which lie over pars plana.Zonular plexus- part of the zonules that lie between the cilliary processes.Zonular fork¬- the point of angulation of the zonule, which lies at the mid zone of cilliary valleys.Zonular limbs- consists ofAnterior zonular limb: passes from pars plana to preequatorial part of the lens.Posterior zonular limb: passes from pars plicata to postequatorial part of the lens.Equatorial Zonular limb: passes from pars pliacata to lens equator.
Hyaloidzonules are the single layers of fibers which connect the anterior hyaloid of vitreous at the border of the patellar fossa to pars plana and pars plicata.
Alpha-chaperone activity-heat shock protein-prevent protein aggregation and precipitationBeta gamma—buffering of calcium in lens fiber cell cytoplasm
The composition enables the lens to have a refractive index considerably greater than its fluid environment and yet remain sufficiently hydrated to be deformable during the process of accommodation
Ascorbic acid also is pro-oxidant
Lens if not formed– absence of corneal endothelium abnormaldifferensiation of corneal stroma absence of iris,ciliarybody,anterior chamber
CatarcatNuclera sclerosis
Insoluble esp alpha crystallins—bind to hydrophobic domains of misfolded proteins