anatomy and embryology of crystalline lens drbp

EMBRYOLOGY AND ANATOMY OF CRYSTALLINE LENS

Dr. Bhushan Patil

It is a highly organised system of specialsed cells.

It constitutes an important component of optical system of the eye.

Introduction

1. It allows the passage of incident light to the retina as it is transparent.

2. It enables the eye to focus the images of the objects on the retina lying at distances from near to infinity ( accomodation)

3. Lens helps in refraction of light and it constitutes one fourth of total diopteric power of the eye.

Anatomy It is a transperent, biconvex, crystalline

structure placed between iris and vitreous in the patellar fossa.

It is attached posteriorly to vitreous in a circular manner with ligamentum hyaloideocapsulare also called Wiegerts`s ligament.

Between the hyaloid face and lens capsule there there is a small potential space called retrolental or Bergers space.

It is supported in its position by zonules of zinn.

Dimensions

1. Anterior surface of lens- radius of 10 mm (8-14mm)

2. Posterior surface- radius of 6 mm (4.5- 7.5mm)

3. Anterior pole- 3 mm from the back of cornea.4. Equatorial diamter- at birth- around 6.5mm of adult- 9- 10 mm

6. Axial width ( AP width) at birth- 3.5-4 mm of adult- 4.75- 5 mm7. Refractive index of lens as a whole – 1.39 of nucleus – 1.42 of cortex- 1.388. Refractive power – 16-17 D9. Weight of lens at birth- 65 mg at extreme of age- 258 mg

10. Accomodative power at birth - 14-16 D at 25 yrs- 7-8 D

at 50 yrs- 1-2 D11. Colour of lens at birth, of infant and adults- Colourless

at about 30 yrs- yellowish tinge old age- amber colour12. Consistency of lens- semisolid cortex is softer as compare to nucleus

It is composed of 64% water,35% proteins and 1 % lipids, carbohydrates and other trace elements.

Structure of lens:1. Lens capsule2. Lens epithelium3. Lens Fibres

Lens capsule It is a thin , transperent, hyaline

collagenous membrane which surrounds lens completely.

It is secreted by lens epithelium anteriorly and by elongating lens fibers posteriorly.

It is the thickest basement membrane of body.

It is more thick anteriorly than posteriorly and at equator than poles.

It is thinnest at the posterior pole. It is mainly composed of type-IV collagen.

Anterior lens epithelium

It is a single layer of cuboidal nucleated epithelial cells.

Almost all metabolic, synthetic and transport process of lens occurs here.

zones of lens epitheliuma) central zoneb) Intermediate zonec) Preequatortial or germinative zone

Central zone :- It consists of cuboidal cells. These cells are stable and their number

reduces with the age. Normally, these cells do not mitose. But can do so in response to various insults

like uveitis, atopic dermatitis.

Intermidiate zone :- It contains small but more cylindrical cells. They can mitose occasionally.

Germinative zone :- It consists of coloumnar cells. These are actively dividing and elongating to

form new lens fibres. Very susceptible to irradiation.

Lens fibres

Anterior lens epithelium elongate to form the lens fibres.

Superficial new fibres are nucleated with elongation of the fibres.

Anterior shifted nucleus of the newly formed cells form the line convex forward at equator, called as nuclear or lens bow.

Initial fibres forming the foetal nucleus terminate with two Y-shaped sutures, anteriorly upright Y and posteriorly inverted Y.

But later, growth of the lens sutures is irregular so forming dendritic pattern.

In adults continuously forming fibres are arranged compactly as nucleus and cortex.

Nucleus : Central part containing the oldest fibres. Parts of nucleus- 1) embryonic nucleus 2) fetal nucleus 3) infantile nucleus 4) adult nucleus.

Cortex : Peripheral part It has the youngest fibres. Histologically ,section through the equator

shows the hexagonal structure of lens fibres. and bound together by ground substance.

There are interlocking processes between cells as ball-and-socket and tongue-and-groove interdigitations.

Interdigitations are less complicated in superficial zone to permit moulding of the lens in

accomodation.

Applied aspect

1. Capsule2. Superficial cortex :

a) C1α- subcapsular clear zone

b) C1β- first zone of disjunction

c) C2 – second cortical clear zone.3. Deep cortex : a) C3 – bright light scattering zone b) C4- clear zone of cortex.4. Nucleus.

Embryology Development of eye starts around day 22 of

gestation. At this stage, embryo has 8 pairs of somites

and around 2mm in length.Formation of optic vesicle

Neural plate shows linear thickened area on either side of it.

It depresses to form the optic sulcus. It then forms the optic vesicle. Proximal part of optic vesicle constricts &

elongate to form the optic stalk.

Formation of lens vesicle At around 27 day of gestation, area of surface

ectoderm overlying optic vesicle thickens, called as lens placode.

It sinks gradually to form the lens vesicle.

It gets seperated from the surface ectoderm by 33rd day of gestation.

Lens vesicle is lined by single layer of cells covered by basal lamina.

Cells from the posterior wall of vesicle elongate rapidly to form the primary lens fibers.

Their base remain anchored to the basal lamina posteriorly and their apices grow towards the anterior lens epithelium.

Primary lens fibres are formed up to the 3rd month of gestation.

These fibres are preserved as the compact core of lens, called embryonic nucleus.

Hence, posterior aspect of the lens become devoid of epithelium.

Equatorial cells of the anterior epithelium forms the secondary lens fibres.

Tips of the these fibres extend around the primary fibres and they meet at the Y-shaped anterior and posterior lens sutures.

Tunica vasculosa lentis During embryonic and fetal development lens

receives nourishment from this structure. It completely encloses lens by aprox. 9 weeks. It is formed by mesenchyme sorrounding

lens. In the early stage, it receives abundant blood

supply from hyaloid artery.

malformations Rubella-congenital cataract

Abnormalities in lens epithelium and capsule, leads to anterior or posterior lentiglobus.

Lenticulocorneal fusion occurs due to failure of seperation lens vesicle from surface ectoderm.

Lens coloboma- failure of closure fetal or choroidal fissure.

Ciliary zonules These are series of fibres which run from the

ciliary body and fuse into outer layer of lens capsule around the equator.

These are transperent,stiff fibres. Diameter is about 0.35 to 1.0 μ. It is composed of glycoproteins and

mucopolysaccharides.

Structurally , 3 different types of fibres- 1) First type fibres- thick,wavy and lie near

vitreous. 2) second type fibres-thin and flat. 3) third type fibres- very fine & run circular

course.

Arrangement of zonular fibres

1) orbiculo-posterior capsular fibres- - most posterior and innermost fibres - origin- ora serrata - insertion- on posterior capsule of lens

alonwith Weigerts ligament. - second type fibres.

2) orbiculo-anterior capsular fibres- -thickest and strongest - arise from pars plana - inserts on anterior to equator of lens - first type fibres.3) Cilio-posterior capsular fibres- -most numerous zonular fibres - arise mainly from the valleys - inserts on posterior capsule

4) cilio-equatorial capsular fibres- -- arise from valleys of ciliary process - directly inserts on the equator of lens - third type of fibres - abundent in youthful eyes and reduces in

number with advancing age.

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