detailed

1. Anatomy and physiology of Lens
Presenter : Dr. Darshan S Maddur
Moderator : Dr. K Kanthamani

2. • The crystalline lens is a unique transparent,
biconvex, avascular intraocular structure with
slightly more curved posterior surface.
• The radius of curvature of anterior surface is
10 mm and that of posterior surface is 6 mm.

3. Aplied aspect
• Lens is a unique structure, which contains cells
of a single type, in various stages of
differentiation

4. Topography
• The center of the anterior surface is called
anterior pole and it is situated 3mm away from
the posterior (endothelial) surface of cornea.
• The center of the posterior surface is called
posterior pole.
• The distance between these poles is measured
as lens thickness. The thickness of lens is 3mm
at birth, which increases to 6mm in older age

5. • The marginal circumferences of the lens, where
anterior and posterior surface meet, are known
as equator.
• Equatorial or lens diameter is generally
measured in nasal to temporal dimension.
• The equatorial diameter of lens is 6.5 mm at
birth, which reaches to 9-10 mm in adult life

6. • The equator of the lens is encircled by the
cilliary processes of the cilliary body and held
in position by zonules (see later) laterally.

7. • lens equator has a serrated or ridged
appearance which is caused by the zonular
fibers
• and this serration or ridges disappears during
accommodation due to relaxation of the
zonular fibers.

9. Location:
• the Lens is situated behind the iris and in front of the
vitreous.
• The posterior lens surface is attached to anterior
vitreous in a circular fashion by Hyaloideo capsulare
(HC) which is also known as Weiger's ligament.
• It is not a true ligament and strength of the
attachment decreases with age.
• The potential space between hyaloid face of vitreous
and lens capsule which lies within the circular space of
hyaloideo capsulare is known as Burger's space or
retrolental space.

11. Refractive power:
• The diopteric power of human eye is
approximately 58 diopters.
• The refractive power of crystalline lens is about 15
diopters.
• Though lens has less refractive power than cornea,
it has the ability to change its shape with the help
of cilliary muscle, by which it can change its
diopteric power, allowing the distant and near
vision.
• However this property changes with age.
• Lens has a refractive index of 1.39
• 1.36 in periphery and 1.40 centrally - a property
which is termed as grading refractive index

13. Accomodative Power of Lens:
• The eye has the capacityto adjust its focus from
distance to near objects because of the ability of
the lens to change shape, a phenomenon known
as accommodation.
• The inherent elastic property of the lens allows it
to become more or less spherical depending on
the amount of tension exerted by the zonular
fibers on the lens capsule.
• Zonular tension is controlled by the action of the
parasympathetically innervated ciliary muscle

14. • When cilliary muscles contracts, relaxation of
zonular tension occurs. The lens then assumes a
more spherical shape, resulting in increased
dioptric power which helps to bring nearer
objects into focus.
• Ciliary muscle relaxation causes the zonular
tension to increase. As a result, lens flattens,
which helps in bringing more distant objects
into view.

16. Aplied aspect
• Presbyopia
• This condition may develop around the age of
40-50 yrs. With increasing age, with the
continuing appositional growth of lens fibers
and due to age related changes in lens capsule,
the elasticity of the lens diminishes.
• The contractility of the ciliary muscle also
diminishes due to the structural changes in the
muscle.
• As a result lens fails to change its shape
sufficiently during accommodation.

17. Structure of lens:
• lens is histologically composed of three
structures-
> lens capsule,
> lens epithelium
> and lens fibers

18. Lens Capsule
• Lens capsule is a transparent covering that
surround the entire lens.
• Histologically it is a basement membrane, secreted
by lens epithelium and lens fibers.
• The capsule is produced anteriorly by the lens
epithelium and posteriorly by the elongating fiber
cells.
• It is composed of type IV collagen fibers and
sulphated glycosaminoglycans.
• Though it has no elastic tissue, it is highly elastic in
nature because of lamellar or fibrillar arrangement
of fibers. This property of the lens gradually
decreases with age.

19. • Lens capsule is thickest near equator and
thinnest at posterior pole.
• Thickness of anterior lens capsule increases
with age, whereas thickness of posterior
capsule remains constant or changes slightly.
• This extreme thinness of the posterior capsule
makes it more vulnerable for posterior capsular
tear or rent during cataract surgery.

21. Capsulorhexis:
• Diameter of the adult lens is approximately 10mm and an
area of diameter 6mm is the zonule free area in anterior
capsule.
• In cataract surgery, a circular opening is made
(capsulorhexis) with in this area.
• To overcome the elastic strength of the capsule, while
performing continuous curvilinear capsulorhexis, two types
of forces are applied
> -tearing by stretching (force is applied perpendicular to
the desired direction of tear, which is uncontrolled)
> and tearing by shearing (force is applied perpendicular to
the capsular plane and it is more controlled).
• As the capsule in children is highly elastic than in adults, it
becomes very difficult to perform continuous curvilinear
capsulorhexis in such patients.

22. Lens epithelium:
• The lens epithelium is a simple cuboidal
epithelium and is found only in the anterior
surface of the lens.
• Theses cells (A cells) secrete the anterior lens
capsule throughout the life

23. • Near the equator the cells of the anterior lens
epithelium, elongate and becomes columnar(E cells).
• As they elongate their apical parts lies deeper to
other cells which are placed more anteriorly. Thus
these elongated epithelial cells trasformed to lens
fibres at the equator.
• The band consisting of preequatorial and equatorial
cells is called germinal zone. Here the mitotic
capacity of the cells are at maximum. Cells in the
germinal zone divides constantly. The newly formed
cells are forced into the transitional zone where they
elongate and differentiate to form the fiber mass of
the lens.

25. Aplied aspect
• After cataract surgery, residual epithelial cells may
cause posterior capsule opacification.
• E cells migrate posteriorly along the posterior
capsule and often forms large balloon like bladder
cells, known as Wedl cells.
• These are clinically termed as Elschnig pearls.
Each pearl represents the failed attempt of a
epithelial cell to differentiate into a new lens fiber.
• E cells are also responsible for a dumb bell
dough-nut-shaped opacification, known as
Soemmering’s ring.

26. • Remnant cells on the anterior capsule after
cataract surgery differentiate into spindle-
shaped, fibroblast-like cells, which are known
as myofibroblasts.

27. Lens Fibers:
• As the transitional zone cells continue to elongate
and differentiate, they turn meridionally.
• The apical end of these cells pass anteriorly
towards the anterior pole and the basal end are
pushed posteriorly towards the posterior pole.
• These processes of newly formed cells, pushing
towards the center of the lens substance continues
throughout the life.
• In this way with the growth of the lens, new
superficial lens fibers are added in a concentrically
arranged lamina, like the layers of an onion.

28. Zones of Lens:
• Approximately nucleus occupies 84% of the lens
and cortex occupies 16%.
• The nucleus is further subdivided into embryonic,
fetal, infantile, and adult nuclei.
• Primary lens fiber cells, formed in the lens vesicle
during embryogenesis forms the embryonic nucleus
and the fibers laid down around the embryonic
nucleus before birth forms the foetal nucleus.
• After birth, new fibers formed before puberty give
rise to infantile nucleus and adult nucleus is formed
after puberty. -

29. • the cortex consists of recently formed nucleated
fibers which lie outside the adult nucleus of the
lens.
• The fibers of the cortex are loosely arranged
whereas fibers of the nucleus are arranged in
more compact fashion, the reason for which the
nucleus is harder in consistency than cortex.
• Epinucleus is formed by the zone between foetal
nucleus and cortex.

31. Hydroseparations
• Hydroseparation
techniques are used in
cataract surgery.
• Hydrodissection is the
separation of lens from
its capsule by injecting
fluid in between the
two .

32. • whereas hydrodelineation is
achieved by injecting fluid
between epinucleus and
nucleus.

33. Sutures of the lens:
• junction of the lens fibres with other lens fibres of
the same plane at the poles are known as suture of
the lens.
• Thus anterior suture is formed by the apical parts
of the lens fibres and posterior suture by the basal
parts.
• During embryogenesis, the lens fibres meet in
three branches, there by forming a Y shape. The
resultant anterior suture is an upright Y and the
posterior one is an inverted Y.
• The sutures formed after birth has multiple
branches like 6 to 9 or 9 to 15 and are of dendrtic
pattern.

34. Zonules of Lens
• Zonules or suspensory ligament of lens are a
group of radially arranged, thread like fibres
which helps the lens to be held in position.
• Most of the zonules arise from posterior part
of parsplana, about 1.5 mm from ora serrate -

37. • Pars orbicularis: The part of the zonules which lie over
pars plana.
• Zonular plexus: part of the zonules that lie between the
cilliary processes.
• Zonular fork: the point of angulation of the zonule, which
lies at the mid zone of cilliary valleys.
• Zonular limbs: consists of
> Anterior zonular limb: passes from pars plana to
preequatorial part of the lens.
> Posterior zonular limb: passes from pars plicata to
postequatorial part of the lens.
> Equatorial Zonular limb: passes from pars pliacata to
lens equator