The document provides information on the anatomy and physiology of the lens. It discusses the position, dimensions, surfaces, parts and zones of the lens. It describes the biochemistry of the lens including its water, protein, amino acid, carbohydrate and lipid content. It explains the metabolic activities of the lens such as glucose metabolism and protein synthesis and breakdown. It discusses permeability, transport mechanisms and the role of various components in maintaining lens transparency.

1. THE ANATOMY & PHYSIOLOGY
OF LENS
Presented by
DR RAKESH JAISWAL

2. ANATOMY OF LENS
DEFINITION : Lens is a transparent, biconvex, crystalline
structure
Position of Lens in Eye Ball :
 Lens lies between post surface of iris & the vitreous in a
saucer shaped depression c/a patellar fossa.
 Post surface of lens is in contact with vitreous &
attached to it in a circular area with Wiegert’s ligament.
 There is a potential space between post lens capsule &
Wiegert’s ligament c/a Berger’s space.

4. Dimension of Lens :
 Equatorial diameter of lens in adult is 9-10mm.
 During birth approximately 6.5 mm & attains max
diameter in IInd
decade of life .
 Axial diameter (Thickness)
- A birth about 3.5mm
- At extreme of Age 5mm
Weight :
 135 mg = 0 – 9 years.
 255 mg = 40 – 50 years

5. Surfaces of Lens :
Two Surfaces – (i) Anterior (ii) Posterior
 The ant. Surface is less convex & is a part of sphere
having radius of 8 to 14mm.
 The post surface is more convex & is a part of sphere
having radius of 4.5 to 7.5 mm.
 The two surfaces meet at the equator.
 Equator is almost circular and has an undulated
appearance.

6. Poles of Lens :
 Ant. Pole lies in centre of ant surface and is about 3mm
from the back of cornea.
 Post pole lies in centre of post surface.
Refractive Index :
 R/I of Lens is 1.39.
 R/I of Cortex – 1.38
 R/I of Nucleus – 1.42

7. Ref Power :
 About 16 – 17 D.
Accomodative Power :- varies with age
 At Birth - 14 to 16 D
 At 25 years - 7 to 8 D
 At 50 years - 1 to 2 D
Colour :- varies with age
 Colourless in infant & young adult
 Yellow tinge after 30 years
 Amber coloured in old age

8. PARTS OF LENS :
The Lens Capsule
 A bag like structure which surround the lens completely.
 It is a thin & transparent membrane like structure.
 Capsule does not contain any elastic tissue and is a
basement membrane like structure.
 Thickest basement membrane of body.
 On microscopic exam it shows lamellar appearance
which contain fine filaments.
 The lens capsule is composed of type IV collagen.

9. Thickness of Capsule
 At equator – 7 to 17
 Ant Pole – 8 to 14
 Post Pole – 2 to 4
Ant Lens Epithelium
 Single layer of cuboidal nucleated epithelial cells which
lies deep to ant capsule.
 All metabolic, synthetic & transport process of lens
occur in this layer.
 In the equatorial region, these cells become columnar,
are actively dividing & elongating to form new lens
fibres throughout life.
 There is no post epithelium.

11. ZONES OF LENS EPITHELIUM
(A) Central Zone :
Cuboidal cells
Nuclei rounded & located apically
Normally do not mitose
May mistose in certain injuries & produce spindle
shaped cells with lead to ant sub cap cat.
(eg in atopic dermatitis & glucoma)
(B) Intermediate Zone:
Smaller & more cylindrial cells located peripheral to
central zone.
Nuclei round & central
Mitose occassionaly

12. (C) Germinative Zone :
Most peripheral columnar cells, located just
preequatorial.
Nuclei flattend & lie in cell axis.
Actively dividing to from new cells c migrate post to
form lens fibres.
Dysplasia of this zone may case post subcap-cat.
(e.g.–Radiation (a) & Neurofibromatosis II)

13. LENS FIBRERS :
 Epithelial cells elongate to form lens fibres.
 At first lens fibres are formed from post epithelium &
later on from equatorial region of ant epithelium.
 The cytoplasm of cells of newly formed fibres contains
rich ribosomes indicated elevated protein synthesis.
 The nucleus disappear later on.
 These is ball & socket and tongue & groove
interdigitation between cells.
 Initial fibres are arranged as two γ-shaped sutures, the
ant erect- γ & post inverted- γ .
 In later stage lens suture arranged in complicated
dendritic patterns.

17. Nucleus :
The nucleus contains oldest fibres.
The embryonic nucleus formed between 1 to 3 mth
of gestation & is inner most part.
Outside the embryonic layer foetal nucleus formed
from 3 mth. of gestation till birth.
Infantile nucleus formed from birth to puberty.
Adult nucleus correspond to lensin adult life.
The embryonic nucleus & foetal nucleus size remain
constant throughout life.
Cortex :
Cortex is peripheral part which lies just outside the
adult nucleus.
It is formed by youngest lens fibres.

19. CILIARY ZONULES :
 A series of fibres which hold the lens in position &
enable the ciliary muscle to act on lens.
 Run from ciliary body and fuse into outer layer of
capsule around equatorial region.
 Transparent, stiff and non elastic.
 Diameter about 0.35 to 1.0 µ.
 Composed of microfibrills with a diameter of 8 to 40
nm.
 Made up of fibrillin with is a large glycoprotein.
 Fibrillin is secreted into extracellular matrix by
fibroblast & become incorporated into insoluble
microfibils.

20.  Mutation on chromosome-15 causes defective fibrillin
formation in Marfan’s syndrome and causing ectopia
lentis.
 Zonular fibres are three different type.
 Ist Type — thick, wavy and 1 µ in diameter.
 IInd Type — thin and flat.
 IIIrd Type — very fine and run in circular course.

21. RECENT CONCEPT ABOUT ZONULAR FIBRES :
(A) MAIN ZONULAR FIBRES:–
Most of zonules arise from the post end of pars
plana upto 1.5 mm of ora serrata.
From here they run into a continuous course upto
edge of lens.
Main fibres divided into four zones.
(1) Pars Orbicularis:–After arising zonular fibres run
forward over parst plana upto post margin of pars
plicata.
(2) Zonular Plexuses:–
 At post margin of pars plicata the zonular fibres
formes the zonular plexus.
 The plexuses pass into valleys of cilliary processes.
 Here they firmly attached to the base of valley &
called as tension fibres.

22. (3) Zonular Fork:–
At the anterior margin of pars plicata zonular
plexuses form a zonular bundle.
This bundle turn to right angle toward lens.
(4) Zonular Limb:–
The zonular fork divides into three zonular limbs.
(a) Ant Zonular Limb–Zonular fibres c insert at
1.5 mm. Anteriorly from equator.
Decrease in number with increasing age.
(b) Equatorial Zonular Limb–
Fibres inserted into capsule of equatorial
region.
(c) Post Zonular Limb–
Inserted into posterior capsule in 2-3 layers from
post edge of equator is about 1.25 mm.

23. (B) Hyaloid Zonule
Connecting Ant hyaloid with pars plana & pars
plicata.
(C) Hyalocapsular Zonule
Probably correspond to ligament of wiegert.
(D) Circumferential Zonular Girdle
Ant Cilliary Girdle–Binds cilliary processes with Ant
hyaloid membrane.
Post Cilliary Girdle–Binds pars plana 1-2 mm of
ora serrata with ant hyaloid membrane.

26. PHYSIOLOGY OF LENS
 Biochemical composition :– Lens contains
 Water – 65%
 Protein – 34%
 Lipid, Carbohydrate, Ascorbic Acid, Glutathion,
Amino acid & Inorganic ions-1%
WATER CONTENT OF LENS
Lens is relatively dehydrated organ.
Dehydration is maintained by active Na+ pump
within cell membrane of epithelium & each lens fibre.
80% water is free & rest 20% bound.
In normal lens there is no significant alteration in
hydration with age.

27. PROTEIN CONTENT
Higher than that of any organ of body.
Soluble fraction c/a crystalline.
Insoluble fraction c/a albuminoid.
Young lens fibres contain more soluble fraction
than older fibres.
 Soluble Fraction (crystallins)
α – crystalline — 31.7%
β – crystalline — 53.4%
γ – crystalline — 1.5%
 Insoluble Fraction (Albuminoids) – 12.5%
 Other Proteins :— Mucoprotein – 0.8%
Nucleoprotein – 0.07%

28. SOLUBLE PROTEINS
Structural protein c make bult of refractive fibres.
Synthesis takes place in equatorial part of lens &
on the surface of lens.
α-Crystallins:–Having highest molecular weight (10)6
M.W. of A chain – 19,500
M.W. of B chain – 22,500
α-Crystallins is a polymer made by fifty monomers.
β-Crystallins:–M.W. – 5x104
to 2x105
β-Crystallin have high
thiol content & disulphide linkage
γ -Crystallin
 Composed of monomers only.
 γ-crystallin level is high in nucleus than cortex.
 Having four fraction which are immunologically identical
except fraction II.

29. INSOLUBLE PROTEINS (Albuminoid)
M.W. – 3,70,000
Amino acid composition is similar to alpha-
crystallin.
Most of albuminoid is urea soluble & appears to be
derived from α–crystalline.
OTHER PROTEINS
Glycoprotein – Protein bound with sugar with
covalent bond.
Nucleoprotein, Phosphoprotein, Lipoproteins etc.
Lens proteins are organ specific and an individual
can become sensitized to one’s own lens protein.

30. AMINO ACIDS
Lens contains all amino acids except tryptophan,
cysteine & hydroxy proline.
Amino acids actively transported from aquous
humour to lens.
Amino acid concentration of lens is not affected by
aging.
CARBOHYDRATES
 Glucose:–Level of glucose in lens is 1/10th
of aqueous,
where glucose concentration has been found to be 100
mg%.
 Fructose:–Produced from glucose.

31.  Glycogen:–Lenticular glycogen is localised principally in
nucleus.
 Sorbitol:–Presence of sorbitol has been demonstrated
in many species lens.
 Inositol:–Presence is demonstrated in lens but function
is unknown.
LIPIDS
 Total lipid of human lens amount to about 2.5% of wet
weight.
 Main substances are cholesterol & various
phospholipids.
 65% of lenticular lipid are bound to protein.

32.  Feldman and Feldman have demonstrated that in
cataracts the concentration of free lipid increases &
lipoprotein decreases.
GLUTATHIONE
 Glutathione present in lens varies from 3.5 to 5.5 mg%
of wet weight.
 It’s amount altered with age.
 It is a tripeptide & consist of 3 amino acids I.e. glycin,
cysteine and glutamic acid.
 Glutathion contributes the redox system of lens micro-
environmental.
 More than 95% of glutathion is reduced state.

33. ASCORBIC ACID
 The mean value of ascorbic acid in human lens is 30 mg
% of wet weight of lens.
 It is neither synthesized nor actively transported into
lens.
 The precise role of ascorbic acid in lens metabolism is
not established.

34. METABOLIC ACTIVITIES OF LENS
 GLUCOSE METABOLISM
 Lens requires energy in form of ATP for it’s various
metabolic activities.
 This energy (ATP) is achieved by glucose metabolism.
 10-20% of ATP used in Protein synthesis.
 Rest ATP used for transport of ions, amino acid,
maintenance of lens dehydration & transparency.
 Most of ATP used at epithelial level.
 3-4 mg. glucose/day is utilized by lens.

35.  Glucose deprivation in lens can cause utilization of other
sources i.e. ATP, Sorbitol, furctos become hydrated, thus
loss of transparency.
 GLUCOSE METABOLISM
(a) Anaerobic glycolysis
(b) Kreb’s cycle
(c) HMP shunt
(d) Sorbitol Pathway

38.  PROTEIN METABOLISM
SYNTHESIS
 From free amino-acids which are actively transported
into lens from aqueous.
 Peptides formed from amino acids with requires ATP &
RNA template.
 Rate of protein synthesis is slow in nucleus than other
part of lens
BREAK DOWN
 Protein catalyzed by enzyme peptidases & proteases.
 Normally the process of autolysis is inhibited.

39. PERMEABILITY & TRANSPORT MECHANISM
ACTIVE TRANSPORT (90% of ATP used)
 Transport of amino acid, K+
, taurine, inositol & extrusion
of Na+
.
PASSIVE TRANSPORT :
 Occurs across the lens capsule for water, ions & waste
product of metabolism (lactic acid & CO2).
 Lens capsule is permeable to low molecular weight
compound & restrict the larger colloidals.

40. WATER AND ELECTROLYTE TRANSPORT :
Cation Pump :
 Functioning at level of ant lens epithelium.
 With the help of ATP, Na+
is actively extruted & uptake of
K+
takes place.
 This process of active transport stimulates passive
diffusion & c/a pump & leak theory.
Lens as Osmometer :
 Lens considered as a single giant cell, which swells up in
hypertonic media.
 Increase in Na+
& K+
increase osmolarily & causes lens
swelling & loss of transparency.

43. Transport of Amino Acid :
 Transport of AA takes place by pump & leak mechanism.
Glucose Transport :
 By simple diffusion & facilitated diffusion.

45. LENS TRANSPARENCY :
 Avascularity of Lens.
 Single layer of epithelial cells.
 Semipermeable nature of lens capsule.
 Sparisty of highly packed lens cells.
 Characteristic arrangement of lens protein.
 Pump mechanism of lens fibres.
 Auto – Oxidation –
Reduced Glutathion keeps the lens protein in reduced
state & provides integrity of cell membrane pump.

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