ANATOMY OF LENS AND FACTORS AFFECTING LENS TRANSPARENCY (1) - Copy.pptx

ANATOMY OF LENS AND FACTORS
AFFECTING LENS TRANSPARENCY
PRESENTED BY:-
DR. SHRISTI SRIVASTAVA

INTRODUCTION:-
 Lens is a transparent, biconvex, crystalline
structure placed between iris and vitreous in a
saucer shaped depression known as PATELLAR
FOSSA.
 Lens is an asymmetric oblate spheroid.
 The posterior surface of the lens capsule is in
close contact with the vitreous in this fossa and
is attached to it in a circular area with
LIGAMENTUM HYALOIDEOCAPSULARE (WIEGERT’S
LIGAMENT).
 Inside this circle, between hyaloid face and the
lens capsule is a small cavity or potential space
called as RETROLENTAL OR BERGER’S SPACE.

GROSS ANATOMY:-
 Location:-In posterior chamber of anterior segment of eye.
 Shape:-Biconvex
 Equatorial diameter:-at birth- 6.5mm, increases to 9-10mm in second decade and
then remains constant.
 Thickness:- at birth- 3.5 mm and at extreme of age- 5mm
 Weight:-varies with age
 at birth- 65mg
 increases rapidly to 125mg by end of first year
 150mg at end of first decade
 260mg at 70-80 years of age.

 Surfaces:- two surfaces
o Anterior-radius of curvature 10mm(less convex than posterior
surface)
o Posterior- radius of curvature is 6mm
 The two surfaces meet at equator.
 Equator is almost circular and has an undulated or rippled
appearance.
 POLES:- two poles anterior and posterior
 Anterior pole is about 3mm from back of cornea.
 REFRACTIVE INDEX:-
o of lens is 1.39
o Of cortex is 1.38
o Of nucleus is 1.42

 Refractive power of lens is 16-17 dioptres.
 Accommodative power:- varies with age
o At birth- 14-16 dioptres
o At 25 years of age- 7-8 dioptres
o At 50 years- 1-2 dioptre
 Colour:- varies with age
o In infants- transparent
o In young adults:- colourless
o At 30 years :- yellow tinge
o Old age:- amber coloured
 Consistency of lens cortex differs from the nucleus, the former being softer than the latter.

LENS EMBRYOLOGY:-
 Lens develops from surface ectoderm.
 Lens placode formation occurs at 27 days of gestation, which
forms lens pit and enters into optic vesicle and thus, converts
optic vesicle into optic cup and it forms a circular structure in
the eye known as LENS VESICLE.

 In embryonic life, structure of lens is nearly circular with cuboidal
epithelial cells all throughout it.
 Later on , the posterior lens epithelium starts invaginating in the
empty space to form the lens fibres.

STRUCTURE OF LENS
Lens is a crystalline structure that is avascular and
devoid of any nerves or connective tissue
MICROSCOPIC LAYERS OF ADULT LENS ARE:-
FROM ANTERIOR TO
POSTERIOR ARE:-
1. ANTERIOR CAPSULE
2. ANTERIOR EPITHELIUM
3. LENS FIBERS
4. POSTERIOR CAPSULE

LENS CAPSULE
 It is a thin, transparent, hyaline collagenous membrane which surrounds the lens completely.
 It is highly elastic but doesn’t contain any elastic tissue.
 Produced continuously throughout life
 Capsule is secreted by the basal cell area of the lens epithelium anteriorly and by the basal
area of elongated fibres posteriorly.
 The lens capsule is thickest basement membrane in the body
 PAS +ve
 On microscopic exam it shows lamellar appearance which contain fine filaments.
 The lens capsule is composed of TYPE 4 COLLAGEN.
 CAPSULE is-
thicker anteriorly than posteriorly
thicker at equator than poles
thinnest at posterior pole

ANTERIOR LENS EPITHELIUM
 Single layer of cuboidal nucleated epithelial cells which lies deep to
anteriorcapsule.
 Mostmetabolicallyactivepartofthelens,containsallcellorganelles
 Na-KATPasesandacidphosphatasesarelocalizedontheapico-lateral
membraneofanteriorepithelium.
 In the equatorial region, these cells become columnar, are actively
dividing&elongatingtoformnewlensfibersthroughoutlife.
 Thereisnoposteriorepitheliuminadultlens.

ZONES OF LENS EPITHELIUM
 (A) Central Zone :
 Cuboidal cells
 Nuclei rounded & located apically
 Normally do not show mitosis
 May undergo mitosis in certain injuries & produce
spindle shaped cells with lead to anterior
subcapsular cataract(example:-Shield cataract in
atopic dermatitis & Glaukomflecken seen after
attack of Acute congestive glaucoma).
 (B) Intermediate Zone:
 Smaller & more cylindrical cells located peripheral to
central zone.
 Nuclei round & central
 Mitose occasionaly.

(C) Germinative /EquatorialZone :
 Most peripheral columnar cells, located just pre-equatorial.
 Nuclei flattened & lie in plane of cell axis.
 Actively dividing to from new cells which migrate posteriorly to form
secondary lens fibers
 Continue to divide throughout life
 Responsible for development of PCO after cataract surgery
 These cells are extremely susceptible to irradiation.
 Dysplasia of these transitional zone cells may cause-
 Posterior subcapsular cataract
 Seen in–Radiation Cataract ,myotonic dystrophy and
Neurofibromatosis II.

LENS FIBERS
 The epithelial cells elongate to form the lens
fibres.
 Initially formed by posterior epithelium k/a
PRIMARY LENS FIBRE which runs from
posterior to anterior to fill lens vesicle but,
later on, they are derived from cells of
equatorial region of anterior epithelium k/a
SECONDARY LENS FIBRE
 Successively, the new lens fibres are laid on
the deeper fibres.
 The superficial new fibre are nucleated with
elongation of the cell; the nuclei assume a
relatively more anterior position.
 As the new fibres are laid down, the
anterior shifted nucleus forms a line convex
forward at the equator, known as LENS OR

STRUCTURAL ARRANGEMENT OF LENS FIBRES
 The initial fibres forming the FETAL NUCLEUS surrounding
embryonic nucleus are arranged in such away that they
terminate with two Y-shaped sutures:- Anterior Upright Y &
Posterior inverted Y.
 Later in gestation, irregular and asymmetrical growth of
lens sutures forms complicated dendritic patterns.
 The formation of sutures enables the shape of the lens to
change from spherical to a flattened biconvex shape.

ZONAL ARRANGEMENT OF LENS FIBRES
 NUCLEUS:-
 Nucleus contains oldest fibres.
 Fibers of nucleus are compactly arranged- nucleus is
harder in consistency.
 The size of embryonic and fetal nuclei remains constant
while that of adult nucleus is always increasing.
 CORTEX:-
 It comprises the youngest most recently formed lens fibres.
 Fibres of cortex are loosely arranged.

CILIARY ZONULES
 The ciliary zonules( zonules of zinn or suspensory
ligaments of lens) consists of a series of fibres which
runs from ciliary body and fuse into the outer layer of
the lens capsule around the equatorial zone.
 Thus, they hold the lens in position and enable the
ciliary muscle to act on it.
 Majority of zonules arise from posterior end of pars
plana and 1.5 mm of ora serrata
 Zonule fibres are:-
1. Pars orbicularis- from pars plana
2. Zonular plexus- b/w ciliary process in region of pars
plicata
3. Zonular fork
4. Anterior zonular limb
5. Equatorial zonular limb
6. Posterior zonular limb

 CANAL OF HANNOVER:-
The space between the anterior and posterior limb of the
zonules.
 CANAL OF PETIT:-
The space between the hyaloid zonule and the posterior
zonule.
The hyaloid zonule comprises a single layer of zonules
connecting the anterior hyaloid at the border of the patellar
fossa with the pars plana and pars plicata.
Ant.
Zonular
limb
Canal of
hannover
Hyaloid
zonule

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

PROTEIN CONTENT
 Protein content of lens is Higher than that of any organ of body.
 Water Soluble fraction k/a crystalline.
 Water Insoluble fraction k/a albuminoid.
 Young lens fibers contain more soluble fraction than older fibers.
 Three types of Soluble Fraction (Crystallin)
 α – crystalline — 31.7%(Highest molecular weight)
 β – crystalline — 53.4%(Most abundant proteins)
 γ – crystalline — 1.5% (a/w cry-g gene i.e, a/w congenital
cataract)
 Insoluble Fraction (Albuminoids) – 12.5%
Glutathione- maintains proteins in reduced state and retain lens pump
integrity

ELECTROLYTES
 Sodium is present
comparatively more
in aqueous than in
lens.
 Potassium is the
predominant cation
in lens.

Metabolic activities of lens
 Glucose metabolism is the main source of energy.
 The lens requires a continuous supply of ATP for active transport of ions, amino acids,
maintenance of lens dehydration, and lens transparency, and for a continuous protein
and GSH(reduced glutathione) synthesis.
 Glucose from the aqueous and vitreous diffuses into the lens and is rapidly metabolized
through four main pathways:-
1. Anaerobic glycolysis
2. Krebs cycle
3. Hexose monophosphate shunt
4. Sorbitol pathway

 ANAEROBIC GLYCOLYSIS:-
• About 80% of the lens glucose is metabolized through anaerobic glycolysis.
• End product of glucose metabolism is lactic acid.
• Metabolism of one molecule of glucose by anaerobic glycolysis yields only 2 molecules of ATP.
 KREBS CYCLE:-
• Only 3% of lens glucose is metabolized via Kreb’s cycle.
• But since efficiency of the pathway is more i.e 1 mol of glucose produces 36 mol of ATP, so it generates about 20%
of the total ATP production from glucose in lens.
 HEXOSE MONOPHOSPHATE SHUNT:-
• HMP shunt uses glucose-6-phosphate as substrate and doesn’t produce ATP.
• It produces pentose and reduced NADPH which is utilized to maintain lens glutathione in reduced state and is also
a necessary factor in sorbitol pathway.

 SORBITOL PATHWAY:-
• Under normal conditions, only 5% of glucose used by lens is metabolized by sorbitol
pathway.
• No ATP is generated.
• In diabetes mellitus, glucose is converted into sorbitol by enzyme aldose reductase
Sorbitol remains in lens and imbibes water(hygroscopic nature)
Lens swells up due to hydration
Leads to frequent change of glasses in diabetic patients.

PATHWAYS OF GLUCOSE METABOLISM IN LENS

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
colloids.
 Transport of AA takes place by pump & leak mechanism.
Glucose Transport-
 By simple diffusion & facilitated diffusion
Transport of Amino Acid

WATER AND ELECTROLYTE TRANSPORT
Cation Pump Mechanism :
 Functioning at level of anterior lens
epithelium.
 With the help of ATP, active extrusion of
Na+ coupled with uptake of K+ takes place.
 This process of active transport stimulates
passive diffusion & is k/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 osmolarity &
causes lens swelling & loss of
transparency.

FACTORS AFFECTING LENS TRANSPARENCY
 Presence of crystal clear proteins.
 Avascularity of lens.
 Lamellar Arrangement of nuclear and cortical lens fibres.
 Presence of free radical scavengers such as REDUCED GLUTATHIONE prevents
oxidation.
 Single layer of epithelial cells.
 Semipermeable character of lens capsule.
 Pump mechanism of lens fibre, thus maintaining relative dehydration of lens.

 Biochemically 3 factors that lead to development of lens opacity-
1. Hydration -droplets of fluid gather under capsule forming lacunae
entire tissue swells [intumescence]
opacity[SOFT CATARACT]
To some extent it is reversible, e.g. inn juvenile insulin dependant diabetes , lens
become clear after control of hyperglycemia
Hydration may be due to osmotic changes within the lens or changes in the
semipermeability of the lens capsule
In traumatic cataract , capsule is ruptured and lens fibres swell and bulge in the
AC
2.Denaturation of lens protein- due to increase in insoluble protein
This is irreversible
3.Slow sclerosis- in nucleus
Leads to HARD CATARACT

Risk factors for cataract Pathomechanism of loss of
transparency
Advancing age Oxidative damage to membrane and
proteins
Sunlight [uvA and uvB] Hydration &denaturation of proteins
Severe diarrhoeal disease Opacification of lens fibreswith fibrous
metaplasia
Vitamin A,C,E deficiency Opacification of lems epithelium
Diabetes Accumulation of pigmented molecules
[3OH kineurenine & chromophores
leading to yellowing]
Smoking , corticosteroids Disturbance in osmotic balance
Genetic Failure in ion pumps[increase in
intralenticular Ca and Na and decrease
in K]

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