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 cellsNuclei rounded & located apicallyNormally do not mitoseMay 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 -shaped sutures, the

ant erect- & post inverted- . 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 . 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 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) – crystalline — 31.7% – crystalline — 53.4%

– crystalline — 1.5% Insoluble Fraction (Albuminoids) – 12.5% Other Proteins :— Mucoprotein – 0.8%

Nucleoprotein – 0.07%

SOLUBLE PROTEINSStructural protein c make bult of refractive fibres.Synthesis takes place in equatorial part of lens & on the surface of lens.

-Crystallins:–Having highest molecular weight (10)6M.W. of A chain – 19,500M.W. of B chain – 22,500

-Crystallins is a polymer made by fifty monomers.-Crystallins:–M.W. – 5x104 to 2x105 -Crystallin have high

thiol content & disulphide linkage-Crystallin Composed of monomers only. -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 –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 ACIDSLens 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.

• THANK YOU