The Crystalline Lens

OPTOMETRY · SEMESTER 2

Ocular Anatomy and Physiology

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The Crystalline Lens

CHAPTER 10. THE CRYSTALLINE LENS

BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE

FOLLOWING ASPECTS OF THE TOPIC:

DEFINITION

THE STRUCTURE OF THE CRYSTALLINE LENS

BIOCHEMISTRY AND METABOLISM OF THE LENS

THE FUNCTIONS OF THE LENS

SOME DISORDERS OF THE CRYSTALLINE LENS

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1. DEFINITION.
The crystalline lens is a biconvex, transparent structure located directly
behind the pupil.
It lies posterior to the iris and anterior to the vitreous, Suspended in position
by the
The equatorial diameter of the adult lens is 9- 10mm.
The anteroposterior length (approximately 4-5mm ), varies with
accommodation.
Fig. 10.1. Diagram of the anterior segment of the eye showing the lens
held by the zonules to the ciliary body.
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2. THE STRUCTURE OF THE CRYSTALLINE LENS.
The lens is composed of :
e capsule;
e lens epithelium;
° cortex;
e nucleus.
2.1. The capsule is a transparent elastic membrane . Anteriorly is
considerably thicker than posteriorly.
2.2. The lens epithelium jis a single layer of metabolically active epithelial
cells, situated under the anterior capsule. Continuously , new cell are formed
The newly formed cells migrate posteriorly and differentiate into fibres. This
change is associated with increase in protein content and loss of organelles
(mitochondria, nucleus).
The fibres are laid down in a concentric manner, their tips meeting to form
sutures.
The fibres crowd and compact the previously formed ones, with the oldest
being the most central.
2.3. The cortex is made -up by the outermost fibres, most recently laid.
The fibres have a high content of crystalline proteins which are responsible
for the high refractive index of the crystalline lens.
2.4. The nucleus consists of all the fibres laid down before birth.
There is no distinct morphologic differentiation between cortex and nucleus;
rather, the transition between these regions is gradual.

Anterior Pole

Surrounding Germinative

Capsule ry, Zone

Epithelial Cells . Equator

Bow Region

Cortical Fibers Nuclear Fibers

Posterior Pole

Fig. 10.2. Diagram showing the structure of the Crystalline Lens.
3. BIOCHEMISTRY AND METABOLISM OF THE CRYSTALLINE LENS.
The human lens has a protein concentration of 33% , highest of any
structure in the body; lens proteins can be divided in two groups based on
their water solubility :
a. Crystallins / water soluble (alpha, beta, gamma) &
b. Water insoluble
The crystallins are the major lens proteins.
The crystalline lens lacks innervation and has no vascular system, it
depends totally on the aqueous and vitreous for its nourishment.
4. THE FUNCTIONS OF THE CRYSTALLINE LENS.
The functions of the lens are:
e to maintain its own clarity (transparency);
e torefract the light;
e to provide accommodation.
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4.1. The most important aspect of lens physiology is the maintenance of
transparency.
Lens transparency is highly dependent on cellular hydration; perturbation of
cellular hydration can lead to lens opacification
Responsible for the transparency is , also the homogenous structure of
fibres , the small size of the extracellular space and the stable structure of
the major lens proteins, the crystallins.
As such, the lens is remarkable, unique, for its ability to preserve
transparency throughout the human life span.
4.2. Refraction of light.
The lens has a high refractive index (1.41 ) achieved by the high protein
content of the fibres.
The refractive power of the lens is 15 dioptres.
4.3 Accommodation , the mechanism by which the eye changes focus from
distant to near images, is produced by a change in the lens shape, resulting
from the action of the ciliary muscle on the zonules.
When the ciliary muscle contracts , the zonular fibres relax allowing the lens
to become more spherical .
Thus, the axial thickness of the lens increases , its diameter decreases, and
its dioptric power increases , producing accomodation.
Accommodation is controlled by the parasympathetic system, via cranial
nerve Ill (oculomotor ).
5. CLINICAL CONSIDERATIONS.
Accomodation diminishes with age; clinically evident loss of accommodation
is called presbyopia. It can be corrected with spectacles.
The opacification (loss of transparency )of the lens is called cataract.
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Opaque lens
Fig. 10.3. A. Slit lamp photograph of an eye with cataract.
B. Diagrams of cataract.
When the opacity of the lens interferes with the patient’s daily activities , the
cataract can be surgically removed.
The absence of the crystalline lens is called aphakia.
The most common method of replacing the lost optical power after cataract
removal, is the implantation of an intraocular lens.
Correction with an intraocular lens is called pseudophakia.
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Cataract is the leading cause of blindness in the world.
17 million people are blind from cataract.
The most common type of cataract is the senile cataract.
Cataracts may be congenital ( eg. rubella syndrome) ; can also result from
injury or disease: secondary cataracts.
In all cases , treatment consists of surgical removal of the opacified lens.