anatomy & physiology of lens

THE ANATOMY & PHYSIOLOGY
OF LENS
Presented by
DR RAKESH JAISWAL

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.

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

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.

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

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

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.

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.

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

(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)

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.

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.

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.

 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.

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.

(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.

(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.

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.

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%

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.

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.

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.

 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.

 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.

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.

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.

 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

 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.

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.

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.

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

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.

anatomy & physiology of lens

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