Human lens anatomy,physiology

1. LOGOLOGO
Crystalline lens
By Dr.Sreekanth Ramachandran

2. LOGO
Lens Facts…
 The lens is a transparent, biconvex, crystalline
structure placed between iris and the vitreous in
a saucer shaped depression ,the patellar fossa.
 Its diameter is 9-10 mm and thickness varies with
age from 3.5 mm (at birth) to 5 mm (at extreme of
age).
Its weight varies from 135 mg (0-9 years) to
255 mg (40-80 years of age).
Celcus drew lens in
AD 30 as a center of
eye globe and “locus
vacuus” anterior to it

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 It has got two surfaces:
 the anterior surface is less convex (radius of curvature 10 mm)
than the posterior (radius of curvature 6 mm).
 These two surfaces meet at the equator.
 Its refractive index is 1.39 and total power is 15-16 D.
 The accommodative power of lens varies
with age, being 14-16 D (at birth); 7-8 D (at 25
years of age) and 1-2 D (at 50 years of age).
Lens is a unique structure, which
contains cells of a single type, in various
stages of differentiation

4. LOGOLens is suspended in eye by Zonules which are inserted
on anterior surface and equatorial lens capsule and
attached to ciliary body. Zonular fibres are series of
fibrillin rich fibre.

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6. LOGO
LENS EMBROLOGY
The development of the eyeball starts around
day 22 of gestation and it is around 2mm in
length.

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EYE DEVELOPMENT
Neural
plate
thickening
sulcus
vesicle

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10. LOGO Simultaneously, the optic vesicle gets
converted into optic cup due to differential
growth of its margins
 The optic cup surrounds the upper and the
lateral sides of the lens
 It is deficient in the inferior part known as the
choroidal or fetal fissure. This closes by 6th
week. Failure to fuse leads to typical
colobomas.

11. LOGOThe hyaloid vessels suppy the lens in the fetus
due to which it grows rapidly.
Later the hyaloid vessels disintegrate and the
remains forms the central retinal artery and vein.
The lens derives its nutrtion then by diffusion
from the aqeous humor and vitreous humor.

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Lens vesicle is lined by single layer of cells
covered by basal lamina.
Cells from the posterior wall of vesicle
elongate rapidly to form the primary lens
fibers.
Their base remain anchored to the basal lamina
posteriorly and their apices grow towards the
anterior lens epithelium.

14. LOGO
Primary lens fibres are formed up to the 3rd
month of gestation.
These fibres are preserved as the compact core
of lens, called embryonic nucleus.
Hence, posterior aspect of the lens become
devoid of epithelium.

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Equatorial cells of the anterior epithelium
forms the secondary lens fibres.
Tips of the these fibres extend around the
primary fibres and they meet at the Y-shaped
anterior and posterior lens sutures.

17. LOGO
Tunica vasculosa lentis
During embryonic and fetal development lens
receives nourishment from this structure.
It completely encloses lens by aprox. 9 weeks.
It is formed by mesenchyme sorrounding lens.
In the early stage, it receives abundant blood
supply from hyaloid artery.

18. LOGO
Lens - Anatomy
Histologically lens consists of three major
components:
1. Capsule
2. Anterior epithelium
3. Lens fibers.

19. LOGOLens capsule
 It is a thin , transperent, hyaline collagenous
membrane which surrounds lens completely.
 It is secreted by lens epithelium anteriorly and by
elongating lens fibers posteriorly.
 It is more thick anteriorly than posteriorly and at
equator than poles.
 It is thinnest at the posterior pole.
 It is mainly composed of type-IV collagen.
Lens capsule is the thickest
basement membrane of body.

20. LOGO
Capsule thickness increase anterior with age
Posterior also but to a less extent
lamellar arrangement is a peculiarity
Loss with age that may be the reason of loss of
accommodation or a manifestation of the same
This extreme thinness of
the posterior capsule
makes it more vulnerable
for posterior capsular
tear or rent during
cataract surgery

21. LOGO
Zonular lamella?/Pericapsular membrane?

22. LOGO
The superficial part of capsule layer of zonule
insertion
Rich in GAG And FIBRONECTIN
May aid for zonular adhesive mechanism

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Anterior lens epithelium
 It is a single layer of cuboidal nucleated epithelial cells.
 Almost all metabolic, synthetic and transport process of lens
occurs here.
zones of lens epithelium
a) central zone
b) Intermediate zone
c) Pre equatortial or germinative zone
Simple cuboidal
epithelium consists of
epithelial cells in a single
layer which have the same
height, width and depth

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Central zone
 It consists of cuboidal cells.
 These cells are stable and their number reduces with the age.
 Normally, these cells do not mitoses.
 But can do so in response to various insults

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b)Intermidiate zone :-
 It contains small but more cylindrical cells.
 They can mitoses occasionally.
c)Germinative zone :-
 It consists of columnar cells.
 These are actively dividing and elongating to form new lens
fibers..
 Alpha crystalline +
 Protected from radiation

26. LOGO
Functions;
– Centrally  transport of substances between the
aqueous humor & the lens interior.
– Equatorially  mitotic division & differentiation
into lens fibers.

27. LOGOMembrane specialization
Cytoskeltal proteins are also abundant here
(Geodomes).They thought to perform
structural function

28. LOGO
Fiber elongation
• Fibers constitute the main mass of the lens.
• The fibers are formed by the multiplication and differentiation
of the lens epithelial cells at the equator.
• As the basal part of the cell elongates, the process moves along
the internal surface of the capsule in a posterior direction.
• As the apical part of the cell elongates, it slips beneath the
internal surface of adjacent lens cells.
Little information about epithelial
mitosis in human is available ,but in
rat it falls with age

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• At first, the nucleus of fibers remains intact; but later it
fragments and disappears (This process continues throughout
life).
• Previous generations of cells are repeatedly pushed into the
lens substance. As the cell progressively elongates anteriorly,
the nucleus moves anteriorly, so that it takes up a position
anterior to the nuclei of the more superficial cells. This
anterior movement of the nuclei as the fibers pass deeper
produces the nuclear pattern known as the lens bow.
Please look

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More clear?

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Fibres
 The lens fiber is a hexagonal prism in cross-section.
 The fibers run meridionally from the posterior to the
anterior lens surface in U-shape.
 The earliest formed fibres are those in the center or nucleus
of the lens; the later fibres form the outer or cortex of the
lens.
Fibres are thinner posteriorly

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Is it fiber/fibre?

33. LOGO
Nucleus and cortex
Outer part cortex inner part nucleus
Exact junction is arbitrary....

34. LOGOThe lens nucleus is divided into:
– Embryonic nucleus: up to 3 months of gestation,
– Fetal nucleus: 3 months of gestation till birth,
– Infantile nucleus: birth to puberty,
– Adult nucleus: after puberty

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• In the fetus, the ends of opposing lens fibers in the same layer
abut (border) in a manner producing patterns known as
sutures.
• The anterior suture is an erect Y-shape, while the posterior is
an inverted Y. As the lens increases in size, the lens fibers are
unable to stretch the antero-posterior distance, so that
progressively more complicated suture patterns are formed.

36. LOGO
Why the sutures?

37. LOGOAccommodating the growth and packing of
lens fibers while retaining the cross-sectional
configuration
9 point star in mature lens

38. LOGO
• The fibers are tightly packed, there being very little intercellular
space. The lens fibers are held together by the interlocking of
their adjacent plasma membranes as ball-and-socket type of
interdigitations.
• It is interesting to note that the interdigitations are less
complicated in the superficial zones of the lens, and this may
permit molding of the lens shape during accommodation.
•Lens fibers exhibit numerous gap
junctions which may explain how deep
lens fibers can survive some distance from
the surface, and away from a source of
nourishment

39. LOGO
• During development, the lens fiber cells lose their nuclei and
the cytoplasmic organelles become specialized for the
production of lens proteins (crystallins);
• Crystallins constitute up to 60% of the lens fiber mass, which
is the highest protein concentration in the body.
• The differing concentrations of the crystallins in different parts
of the lens produce regional differences in the refractive index.
•This probably compensates for the
spherical and chromatic aberrations that
might exist if the concentrations of the
crystallins were uniform throughout the
lens!!

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41. LOGOCortex :
Peripheral part
It has the youngest fibres.
Histologically ,section through the equator
shows the hexagonal structure of lens fibres.
and bound together by ground substance.

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Applied aspect
1. Capsule
2. Superficial cortex :
a) C1α- subcapsular clear
zone
b) C1β- first zone of disjunction
c) C2 – second cortical clear zone.
3. Deep cortex :
a) C3 – bright light scattering zone
b) C4- clear zone of cortex.
4. Nucleus.
Which grading is this?

43. LOGO
The Oxford system of lens zoning

44. LOGOSuspension
 The lens is held in position by a series of delicate, radially
arranged fibers collectively known as the suspensory
ligament of the lens, or zonule.
 The zonule fibers arise from the epithelium of the ciliary
processes and run toward the equator of the lens.
 The fibers fuse to form about 140 bundles .

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• The larger bundles are straight and reach the lens capsule in
front of the equator.. Together they form the anterior zonular
sheet.
• The smaller fibers curve backward and are attached to the
posterior surface of the lens to form the posterior zonular
sheet.
• The third group passes from the tip of the ciliary processes
almost directly inward to be inserted at the equator

47. LOGO• As the zonular fibers reach the lens, they break up into fine fibers
that become embedded in the outer part of the lens capsule.
• When the eye is at rest (meiosis), the elastic lens capsule is under
tension, causing the lens constantly to attempt a globular rather than
a discoid shape.
• The pull of the radiating fibers of the zonule tends to keep the elastic
lens flattened, permitting the eye to focus on distant objects.

48. LOGO
CILIARY ZONULES
 Zonules of zinn or suspensory ligaments of lens
 Series of fibres which run from ciliary body and fuse in to
outerlayer of lens capsule around equatorial zone
 Holds lens in position and enable ciliary muscle to act
Structure
• Transparent stiff and non elastic
• Composed of glycoproteins and muco polysaccharides
• Susceptible to hydrolysis by alpha chymotrypsin,beneficial in
ECCE
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Types of zonules
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51. LOGO
Zonular spaces
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52. LOGO
BIOCHEMISTRY OF THE LENS
MAIN CONTENTS ARE
WATER (65%)
PROTEINS(34%)
LIPIDS,CHO AND TRACE
ELEMENTS(1%)

53. LOGO
WATER
relatively dehydrated organ
cortex more hydrated than nucleus
80% is free and 20% is bound
low water is natural consequence of need for
maintining refractive index
no significant alteration in hydration with age

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PROTEINS

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56. LOGO
Lens - Crystalline
Lens fibres contain high concentrations of
crystallins.
Crystallins represent the major proteins of the lens
(constitute 90% of total protein content of lens).
Crystallin has the following constituents:
Alpha
Beta and,
Gamma

57. LOGO
WATER INSOLUBLE PROTIENS
 UREA SOLUBLE
.CYTOSKELETAL PROTEINS
.VIMENTIN AND BEADED FILAMENTS
.GENETIC DISRUPTION OF BEADED FILAMENTS L/T
CATARACT FORMATION
• UREA INSOLUBLE
.MEMBRANE INHIBITORY PROTEINS
.MEMBRANE PROTEINS
.SERVES TO REDUCE LIGHT SCATTERING BETWEEN
CELLS

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LENS 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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WITH AGE
 lens proteins proteolyse  dissembly of fibres  aggregation
of water insoluble proteins  scatter light  opacification of
lens
 glutathione is essential for maintaining a reduced environment
any depletion cause cataract

60. LOGOOXIDATIVE DAMAGE AND
PROTECTIVE MECHANISMS
 FREE RADICALS-SCAVENGED BY GLUTATHIONE
• VITAMIN E AND ASCORBIC ACID IN LENS ALSO ACT AS
FREE RADICAL SCAVENGERS

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EXPOSURE TO LONG TERM
HYPERBARIC
OXYGENOPACIFICATION OF LENS

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Lens - Functions
 The lens serves two major functions:
– Focusing of visible light rays on the fovea
– Preventing damaging ultra-violet radiation from reaching
the retina

63. LOGO
PHYSIOLOGY OF LENS

64. LOGOMAINTANENCE OF LENS WATER AND
CATION BALANCE
 MOST IMPORTANT MECH FOR MAINTAINING LENS
TRANSPARENCY
 MAINLY BY ACTIVE AND PASSIVE TRANSPORT
MECHANISMS
ACTIVE- Amino acids,K,Na,INOSITOL ETC. 90% of energy in
the form of ATP utilised here
PASSIVE-Water,ions and waste products of metabolism

65. LOGO
WATER AND ELECTROLYTE TRANSPORT
pump leak mechanism

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67. LOGOTransport of amino acids
 Also included in pump leak concept
 Three types of pumps
 Inside the lens aa are utilised for protein formtion and energy
production or diffuse back in to aqueous by leak

68. LOGO
FACTORS MAINTAINING
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

69. LOGO
Changes during aging
1)Changes in structure
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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70. LOGOCATARCTOGENESIS
INCREASED AGE LEADS TO INCREASE WEIGHT AND THICKNESS OF LENS
LENS UNDERGO COMPRESSION AND HARDENING(NUCLEAR SCLEROSIS)
AGGREGATION OF PROTIENS ALSO CAUSES FLUCTUATIONS IN RI OF
LENS,LIGHT SCATTERING AND DECREASED TRANSPARENCY
CHEMICAL MODIFICATIONS ALSO INCREASES PIGMENTATION GIVING
RED YELLOW COLOURS TO LENS
K,GLUTATHIONE AND Ca AND Na

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RISK FACTORS
 Age >50
 Low SES
 Smoking and alcoholism
 Exposure to steroids and radiations
 Myopia,DM,HTN,renal failure etc.,

72. LOGOLOGO