prepared during lens session

1. ANATOMY AND
PHYSIOLOGY OF LENS
Bipin Bista
Resident
Ophthalmology

2. INTRODUCTION
Emanation theory of vision
Celsus (25BC-AD 50) drew the lens in the
center of the eyeball with an empty
space called locus vacuus anterior to it
True position of the lens was illustrated
by the Italian anatomist Fabricus ab
Aquapendente in1600 and Swiss
physician Felix Plater (1536-1614)
postulated to be the part responsible
for sight
20 million of people (51%) are suffering
from cataract (WHO 2010 A.D)
Prevalence of lens disorder and
continuing developments in their
management make the basic and
clinical science about lens an important
subject.

3. EMBRYOLOGY
At 25 days of gestation , 2 lateral evaginations (Optic vesicles) form
the forebrain, or diencephalon .
As the OV enlarges and extend laterally, they become closely apposed
and adherent to surface ectoderm (single layer of cuboidal cell) in 2
patches in either side of the head.
Lens Placode : ectodermal cell overlie the optic vesicle become
columnar at 27 days and are called LP. For subsequent formation
Bone Morphogenetic Protein are required.
Lens Pit : appear at 29 days as an indentation (infolding) of the Lens
Placode, this pit deepens and invaginates to form the lens vesicle.

5. EMBRYOLOGY
Lens Vesicle : stalks of cells connecting to the surface ectoderm
degenerates by apoptosis separating the lens cells from the surface
ectoderm. This resultant sphere, a single layer of cuboidal cells
encased in BM (lens capsule) is LV.
As the lens vesicle was formed through invagination the apices of the
cuboidal cells are oriented toward the lumen of the vesicle , with the
base of the cell attached to the capsule around the periphery of the
vesicle.
At the same time, the optic vesicle is invaginating to form the 2
layered optic cup.

6. EMBRYOLOGY
Primary lens fibers and the
embryonic nucleus : Cells in
posterior layer of the lens vesicle
stop dividing and begin to elongate
they begin to fill the lumen of the
lens vesicle, and approximately 40
days of gestation the lumen of the
lens vesicle is obliterated .these
elongated cells are primary lens
fibers which make up the embryonic
nucleus which will ultimately the
central area. Cells of the anterior
lens vesicle remain as a monolayer
of cuboidal cells – Lens epithelium
responsible for subsequent growth
of the lens. Lens capsule develops as
a BM elaborated by the lens
epithelium anteriorly and by lens

7. EMBRYOLOGY
Secondary lens fibers : with the
proliferation, the epithelial cells
near the lens equator elongate
and form secondary lens fibers.
Layer upon layer the lens fibers
are formed anteriorly beneath
the epithelium and posteriorly
toward the posterior pole. These
fibers formed between 2 and 8
months make up fetal nucleus.

8. EMBRYOLOGY
Lens sutures and fetal nucleus : As the fibers grow anteriorly and
posteriorly, and the ends of the fibers meet and interdigitate with the
ends of fibers arising of the lens make a pattern – Sutures which are
recognisable at about 8 weeks . As the lens continues to grow the
pattern of the lens continues to grow the pattern of the lens sutures
become complex, resulting in 12 or more sutures.

9. TUNICA VASCULOSA LENTIS
At about 1 month of gestation , the hyaloid artery enters eye and
forms network of capillaries (TVL) on the posterior surface of the lens
capsule. These capillaries grow toward the equator of the lens , where
they anastomose with a second network of capillaries, Anterior
pupillary membrane derived from the ciliary veins and which covers
the anterior surface of the lens
After complete development at 9 weeks it disappears by an orderly
process of Apoptosis.

10. ZONULES OF ZINN
Secreted by the ciliary epithelium, although insertion into lens
capsule is not known. Develops at the end of the third month of
gestation.
Arrangements :
•Main zonular fibers : Pars orbicularis, zonular plexuses, Zonular fork,
Zonular limbs
•Zonular limbs : Anterior zonular limbs, equatorial limbs, posterior
limbs.
•Hyaloid zonule
•Hyalocapsular zonule
•Circumferential zonular girdle : anterior and posterior.

11. ANATOMY
Transparent, biconvex structure which has no blood supply or
innervation after fetal development, depends on aqueous humor for
its metabolic wastes.
Lens is suspended in position by the zonules of zinn.
Lens is composed of the capsule, lens epithelium, cortex, and the
nucleus.
Anterior and posterior pole are joined by imaginary line called as the
optical axis. Line passing from one pole to other are meridians. The
greatest circumference comes from the equator of the lens.

13. ANATOMY
At birth, it measures 6.4 mm equatorially and 3.5 mm
anteroposteriorly and weighs 90 mg, adult lens typically measures 9
mm equatorially and 5 mm anteroposteriorly and weighs 255 mg.
Capsule : elastic, transparent BM composed of type IV collagen,
capable of molding it during accommodative changes. Zonular
lamella is the outer layer of capsule and is the point of attachment
for zonular fibers. Anterior > posterior
Zonular fibers / zonules of fibers : consist of microfibrils composed
of elastic tissue, originate from basal laminae of non pigmented
epithelium of pars plana and pars plicata. Gets inserted in a
continuous fashion, on the capsule in the equatorial region. 1.5 mm
onto anterior capsule and 1.25 mm onto posterior. 5-30 µm in
diameter shows eosinophilic structures that have positive PAS
reaction.

14. ANATOMY

15. ANATOMY
Lens epithelium : single layer , metabolically active and carry
biosynthesis of DNA, RNA, protein, and lipid. Also generate ATP to
meet energy demands of the lens. These cells are mitotic , premitotic
activity occurring the ring around the anterior lens K/A germinative
zone. Newly formed cells migrate toward the equator where they
differentiate into fibers. With advancement, epithelial cells elongate
to form lens epithelial cells, which leads to increase in the mass of
cellular proteins. These cells start losing cell organelles which leds
the lens to be less absorbant and will have less scattering of light but
gets dependent on glycolysis for energy production.
Nucleus and Cortex : outermost fibers are recently made and make
up cortex, whereas the older fibers get crowded and are usually
located in the center. There is no morphological distinctions between
the nucleus, epinucleus, and the cortex.

16. CONGENITAL ANOMALIES
AND ABNORMALITIES
Congenital aphakia
Lenticonus and lentiglobus
Lens coloboma
Mittendorf dot
Epicapsular star
Peters anomaly
Microspherophakia
Aniridia
Cataract

17. DEVELOPMENTAL DEFECTS
Ectopia lentis
Marfan syndrome
Homocystinuria
Hyperlysinemia
Ectopia lentis et pupillae
Persistent Fetal Vasculature

18. PHYSIOLOGY
Continual growth of lens
Epithelium and outer cortex : site for highest metabolism
Utilises glucose and oxygen for the active transport of electrolytes,
carbohydrates and aminoacids into the lens.
Communications through gap junction between newer and older cells
and also through Major Intrinsic Protein (MIP)
Minimise the extracellular space between fiber cells .

19. PHYSIOLOGY –
MAINTENANCE OF LENS
WATER AND CATION
BALANCECritical for lens transparency
Imbalance in cellular hydration may lead to opacification
Normally 66% water and 33% protein
Cortex > nucleus (Hydrated)
5% is found between the fibers
20 mM and 120 Mm are the concentrations of sodium and potassium
respectively.

20. LENS EPITHELIUM : SITE OF
ACTIVE TRANSPORT
Lens is dehydrated and has higher level of potassium ions and amino
acids than surrounding aqueous and vitreous and lower level of
sodium and chloride ion
Cation balance is result of permeability of lens cell membrane and
activity of the sodium pumps that reside within the cell membranes of
lens epithelium and each lens fiber
Sodium pumps functions by pumping sodium out and potassium in
which is dependent on the breakdown of ATP is regulated by the
enzyme Na+ ,K+ -ATPase.
Inhibition of this enzyme may lead to loss of cation balance and
elevated water content in water.

21. PUMP LEAK THEORY
Referred to combination of active
transport and membrane
permeability
Potassium and various other
molecules like AA are actively
transported into the lens via the
epithelium anteriorly, then diffuse
out with concentration gradient
through the back of the lens (no
active transport mechanism)
Sodium flows in through the back of
the lens with concentration gradient
and then is actively exchanged for
potassium by the epithelium.
Most of the Na+ , K+ -ATPase are
found in the epithelium and the
superficial cortical fiber cells

22. PUMP LEAK THEORY
Electrical potential is generated d/t
unequal distribution of electrolytes
across the lens cell membranes.
Inside of lens is electronegative,
approximately -70 mV
Potential difference between anterior
and posterior surface of lens is -
23mV.
Calcium pump plays critical role to
lens, lens epithelial cells calcium
level is 100 nanomolars maintained
by Ca2+ -ATPase
Glucose enters through facilated
diffusion and waste product leave by
simple dissusion.

23. ACCOMODATION AND
PRESBYOPIA

24. ACCOMMODATION
Mechanism by which the eye changes focus from distant to near
images, is produced by a change in lens shape resulting from the
action of the ciliary muscle on the zonular fibers
According to the Helmholtz theory of accommodation, most of the
accommodative changes in the lens occurs at the central anterior lens
surface.
There is central anterior bulge with accommodation as the anterior
zonular fibers are inserted slightly closer to the visual axis
Ciliary muscle is a ring shaped muscle that on contraction has the
opposite effect as the muscle contracts the diameter of the muscle
ring is reduced , thereby relaxing the tension on the zonular fibers
and allowing the lens to become more spherical.
When CM relaxes zonular tension increases, zonular tension increases
,lens flattens and diopteric power decreases.

25. ACCOMMODATION
Stimulated by the known or apparent size and distant of an object or
blur, chromatic aberration , or a continual oscillation of ciliary tone.
Mediated by parasympathetic fibers of CN III
Amplitude of accommodation : amount of change in the eye’s
refractive power that is produced by accommodation. Diminishes with
age , medications and diseases.
Adolescents have accommodative power have 12-16 D of
accommodation, whereas adults at age have 4-8 D. After age 50,
accommodation decreases to less than 2D.

26. PRESBYOPIA
Loss of accommodation d/t
hardening of the lens .
With the age above 40 the
rigidity of the lens reduces
rigidity, as contraction of ciliary
muscle would no longer result in
increased convexity and diopteric
power of the anterior surface of
the lens.
- Changes in the elastic property
of lens capsule
- Sclerosis or hardening of the
lens
- Weakening of the ciliary

27. REFERENCES :
- Ophthalmology : Yanoff and Duker : 4th Edition
-American academy of ophthalmology : section 11 : 2014-2015

28. THANK YOU