ANATOMY OF THE LENS

LENS EMBROLOGY

The development of the eyeball starts around day 22 of gestation and it is around 2mm in length.

First, a linear thickening develops on either side of the neural plate which is going to form the prosencephalon

It becomes depressed to form the optic sulcus.

Walls of prosencephalon overlying the sulcus bulge out to form the optic vesicle

Proximal part of the optic vesicle becomes constricted to form the optic stalk

Optic vesicle grows laterally and comes in relation with the surface ectoderm

At 27th day the overlying surface ectoderm becomes thickened to form the lens placode

It slowly sinks below the surface and is converted into lens vesicle

Seperates from the surface at 33rd day.

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.

The neural tube is surrounded by mesenchyme which condenses to form the meninges

The mesenchyme around the optic vesicles forms 2 layers-

Superficial fibrous layer- sclera and cornea Deep vascular layer- uvea

Part of the inner vascular layer is carried into the cup through the fissure. When fissure closes, it forms the hyaloid vessels

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.

ANATOMY OF THE LENS Lens is a tranparent, avascular,

biconvex crystalline structure placed between the iris and vitreous in the patellar fossa

It has 2 surfaces- anterior surface and posterior surface.

These 2 surfaces meet at the equator.

The centre of the anterior and posterior surfaces are known as anterior and posterior pole

The posterior surface is attached to the vitreous by Wiegert’s ligament.

Between the lens capsule and the hyaloid face is a small cavity k.a. retrolental space or berger’s space.

DIMENSIONS--

1. Equatorial diameter- 6.5 mm at birth, 10 mm in adults

2. AP diameter- 3.5 mm to 5mm3. Radius of anterior surface- 10mm4. Radius of posterior surface- 6 mm5. Wieght- 150 mg- 250 mg

Refractive index- 1.38 ( cortex) 1.42

( nucleus) Refractive power- 16-17 D Accomodative power- 14-16 D at

birth 7-8 D

at 25 yrs 1-2 D

at 50 yrs

STRUCTURE OF LENS

Lens capsule It is a thin , transparent elastic membrane

which surrounds lens completely Secreted by basal cells of anterior lens

epithelium and basal area of elongating fibres posteriorly.

Thicker anteriorly than posteriorly Thicker at the equator than the poles,

thinnest at the posterior poles

It is made up of type 4 collagen and 10% glycosaminoglycans

Anterior lens epithelium

Single layer of cuboidal nucleated epithelial cells.

Cells contain all the organelles found in a normal epithelial cell since all the metabolic, synthetic and transport processes of lens occur in this layer

3 zones Central- cuboidal cells which reduce

with age. Do not mitose normally. Can undergo mitosis in response to

injury like in uveitis. Metaplasia of these cells into

spindle shaped myofibroblast like cells can lead to ASC- eg. Shield cataract in atopic dermatitis,

Glaucomflecken after ACG

Intermediate zone Smaller and more cylindrical. Undergo mitosis occasionally

Germinative zone Columnar cells located pre- equatorially.

Actively dividing to form new cells which migrate posteriorly to become lens fibres.

Extremely susceptible to radiation Dysplasia can lead to PSC in radiation

exposure, myotonic dystrophy and NF-2.

Lens fibres Form the main bulk of the lens. They

are long thin transparent cells firmly packed with diameter of 4-7 micron and length upto 12mm

The cells of the newly formed fibres contain all the organelles and as the fibres mature the nucleas disappears.

The cells are linked by gap junctions and interdigitations that look like ball and socket. These are fewer in the superficial fibres.

FORMATION OF LENS FIBRES

EARLY LENS VESICLE- consists of a single layer of cells covered by a basal lamina.

LATE LENS VESICLE- cells of posterior

wall elongate and get filled with crystallins which make them transparent

EMBRYONIC NUCLEUS- the apices of these cells

grow towards anterior lens epithelium. These are PRIMARY LENS FIBRES.

The nuclei of these cells are present anteriorly forming nuclear bow.

When these cells become attached to the anterior lens epithelium the nuclei of the cells disappears. These fibres are formed upto 3 months of gestation.

LAYERS OF LENS NUCLEUS

1. EMBRYONIC NUCLEUS- formed by the primary lens fibres upto 3rd month of gestation

2. FETAL NUCLEUS- 3rd month to 8th month

3. INFANTILE NUCLEUS- 8TH month till puberty

4. ADULT NUCLEUS- after puberty

Primary lens fibres forms the embryonic nucleus.

Rest of the nuclei are formed from the secondary lens fibres. These are the fibres formed from the anterior epithelium throughout life .

Initially the fibres of the fetal nucleas reach both the anterior and posterior pole.

Later they are not able to extend all the way and instead meet at radiating lines which appear as an erect Y anteriorly and inverted Y posteriorly

Later the fibres grow asymmetrically giving a complicated dendritic pattern in the infantile and adult nucleus

The most newly formed (youngest) lens fibres are present most peripherally outside the adult nucleus.

These form the cortex of the lens.

Layers of lens on slit lamp

Surgical anatomy