Anatomi Fis Lensa

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Literature review

dr. Alie Solahuddin, SpM

DEPARTMEN OF OPHTHALMOLOGY

MEDICAL FACULTY OF SRIWIJAYA UNIVERSITY/

DR. MOHD HOESIN HOSPITAL PALEMBANG

2007

Anatomy and Physiology of the Lens


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BACKGROUND

Lens is a transparent, avascular, biconvex

structure.

It lies between the aqueous humor at the

anterior of it and the vitreous humor at the

posterior of it.


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BACKGROUND

Lens has a composition of 65% water and

35% protein which consists of water

soluble and water insoluble protein.

Main function of the lens is to focus rays

toward retina.


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EMBRYOLOGY

Indentations at left and right of diencephalons (starting at22nd day of gestation)

optic vesicles

ectoderm

lens plate/ lens placode


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EMBRYOLOGY

Lens vesicle

Invagination

Lens vesicle (33rd day)


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EMBRYOLOGY

Cells of posterior wall elongates to anterior, filling the vesicles

lumen, so that the lumen is gone (40th day) primary lens fiber.


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EMBRYOLOGY

In 7th week, equatorial lens epithelial cells multiply secondary lens fiber

to the posterior and anterior of the lens sutures


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EMBRYOLOGY

Vascularization of lens in its embryonic days

Tunica vasculosa lentis grows as the lens grows


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ANATOMY

Lens is an avascular, transparant,biconvex structure with a 4 mm thicknessand a diameter of 9 mm

Lens is suspended in its position byzonula zinni (lig. suspensorium lentis)which attached to equator of the lens andconnects it to ciliary body.

It is a curved plate posterior>anterior


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ANATOMY

Differences Birth Adult

Diameter 6.4 mm 9 mm

Thickness 3.5 mm 5 mm

Weight 90 mg 255 mg

Consistency Soft and pliable Stiff


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ANATOMY

Substructures of thelens

1. Capsule is a basalmembrane formed bylens epithelial cellswhich covers the

whole lens.


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ANATOMY

Lens epithelium

isa layer of cellswhich cover theanterior part of lens.

At the equator ofthe lens, these cells

elongate into ameridian columnarcells.


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ANATOMY

3. Nucleus andcortex: Nucleus is the center of

the lens, made of theoldest lens fibers;

Cortex is formed fromthe peripheral fibersrecently formed.

The sutures formed

from the lens fibers isseen for the differingoptical densitiesbetween apical andbasal cell processes.


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ANATOMY


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HISTOLOGY

Composed of epithelial cells with several

junctions between them

Some of the important structures are:

1. Plasma membrane

2. Lens capsule

3. Epithelial cells4. Lens fibers


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HISTOLOGY

Plasma membrane

Is a selective permeable membrane which

consists of a bilayer phospholipid

Has some junctions, such desmosomes,

gap and tight junctions


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HISTOLOGY

Lens capsule

Is derived from the basal membrane of

epithelial cell. This membrane shows a

laminar structure. Each lamina consists ofseveral layer of tiny parallel collagen

filaments

Its anterior part is growing with time, unlikeits posterior part


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HISTOLOGY

Epithelial cells

Forms a single layer of cells beneath anterior

and equator capsule

These cells are cuboid at vertical cross

section and hexagonal at transversal cross

section


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HISTOLOGY

Lens fibers

Forms the nucleus and cortex of the lens

Has some unique junctions to maintain the

lens structure


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HISTOLOGY

Light micrograph of superficial human lens, close to the anterior equator showing the lenscapsule, epithelium and layers of cortical lens fibres

Zonular fibres close to

the capsule CapsuleEpithelium (transition zone)

Lens fibres


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BIOCHEMISTRY

Protein of the lens

Water soluble Water insoluble

Alpha Beta, gamma Soluble in 8molar urea

Insoluble in 8molar urea

Molecular biology of lens


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BIOCHEMISTRY

Metabolism of lens

The most important metabolism in the lens

is the metabolism of glucose

2 main pathway of glucose metabolism:

1. HMP shunt (provides 5% of ATP)

2. Glycolysis (provides the rest of ATP)

Oxidative damage and protectivemechanism (antioxidants)


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GLUCOSE METABOLISM


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PHYSIOLOGY

1. Maintenance of lens water and cation balancea. Active transport

b. Leak-pump theory

A process of maintaining the clarity of lens by the

combination of epithelial active transport andpermeability

2. AccommodationAn ability of lens to change its shape and thus itsretractile index by the help of ciliary body and

zonular fibres


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PHYSIOLOGY

Pump-leak theory


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ANOMALIES

Congenital Aphakia

Lenticonus and Lentiglobus

Lens Coloboma Mittendorf Dot

Epicapsular Star

Peters Anomaly

Microspherophakia Aniridia

Congenital and Infantile Cataract


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ANOMALIES

Acquired anomalies can be caused by UV

radiation or lens composition change

(cataract)


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LENS EXAMINATION

One of the examination of the lens byflashlight is the shadow test.

This test is used to know the opacity of the

lens Positive result shows low opacity of the

lens (immature cataract), while negative

result shows high opacity of the lens(mature cataract)


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CONCLUSION

Lens main function is to refract light and to

provide accommodation

To maintain its functions, its clarity must be

preserved The most important metabolism to maintain the

clarity of lens is the glucose metabolism

Radiation and aging can reduce the clarity oflens and promote cataract


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