EMBRYOLOGY , ANATOMY , PHYSIOLOGY, APPLIED ANATOMY OF LENS

BY Dr. RahulMODERATOR Dr. Monica

Embryology of lensFormation of crystalline lens begins very early in

embryogenesisAt about 25 days of gestation, 2 lateral out

pouching called optic vesicles form from forebrainAs optic vesicles enlarge,they become closely

apposed to surface ectodermLens plateCells of surface ectoderm that overlie optic

vesicles become columnar at about 27 days of gestation

This area of thickened cells is called lens plate or lens placode.

lens pit or fovea lentisAppears at 29 days of gestation As small indentation inferior to center of lens plateThe lens pit deepens by process of cellular

multiplication and invaginationLens vesicleAs the lens pit continue to invaginate ,the stalk of

cells that connects it to surface ectoderm consticts and eventually disappears

The resultant sphere is called lens vesicle

the lens vesicle was formed through a process of invagination of surface ectoderm , so the apices of cells oriented toward lumen of lens vesicle , with base at periphery

At same time optic vesicle is undergoing a process of invagination as it begins to form 2 layered optic cup

Primary lens fibers and embryonic nucleusThe posterior cells of lens vesicle become more

columnar and to elongate They obliterate lumen of lens vesicle completely

around 40 days.These cells are primary lens fibres

Lens capsule develop as basement membrane from lens epithelium anteriorly ,and lens fiber posteriorly

Secondary lens fiber At about 7 week of gestation ,lens epithelium in

area of equator begin to multiply and elongate rapidly to form secondary lens fibers

The new lens fibers are continully fomed,layer upon layer

The secondary lens fiber between 2 and 8 months of gestaion make up fetal nucleus

The nuclei of primary lens fiber moves from posterior to anterior position within lens fibre

And subsequently become pyknoticThese primary lens fiber make up embryonic

nucleus that will occupy central area of lens in adult life

Posterior layer of optic vesicle undergo marked differention to form primary lens fiberBut cells of anterior lens vesicle do not

change .this monolayer refer to as lens epithelium

Lens sutures and fetal nucleus Lens fiber grows anteriorly and

posteriorly ,these fibers meet with each other,this pattern known as sutures.

Y shaped are recognizable at about 8 weeks of gestation .

An erect Y suture appearing anteriorly and inverted y suture posteriorly

Only during fetal life Y sutures formed.Later on pattern of lens suture s becomes

increasingly complexAt birth lens weighs 90 mg and increases in

mass at rate of 2mg per yr

Y-shaped sutures of the fetal nuclear fibres

Tunica vasculosa lentisAs lens develops , nutritive support structure , the

tunica vasculosa lentis forms around it.At about 1 month of gestation ,hyloid artery gives rise

to small capillaries that form anastomtic net covering posterior aspect .

These small capillaries grows towards equator of lens where they anastomose with choroidal veins and form capsulopupillary portion of tunica vasculosa lentis

Branches of long ciliary arteries anastomos with branches of capsulopupillary portion to form anterior vascular capsule

It is fully developed at 9 weeks of gestation disapper shortly after birth

Congenital anomaliesCongenital aphakiaLenticonus and lentiglobusLens colobomaMittendorf dotEpicapsular starPeters anomaly

POSTERIOR Lenticonus

Lens coloboma

Epicapsular star

Lens Developmentlens placode in surface ectoderminvaginates as lens vesiclesupplied by hyaloid artery

LensThe lens is a transparent, biconvex, crystalline structure placed between iris and the vitreous in a saucer shaped depression the patellar fossa. Biconvex shape results from the anterior surface

being less convex then posterior surface.Its diameter is 9-10 mm and thickness varies with age from 3.5 mm (at birth) to 5 mm (at extreme of age). Its weight varies from 135 mg (0-9 years) to 255

mg (40-80 years of age).

It has got two surfaces: the anterior surface is less convex (radius of

curvature 10 mm) than the posterior (radius of curvature 6 mm).

These two surfaces meet at the equator.Its refractive index is 1.39 and total power is 15-

16 D. The accommodative power of lens varies with age, being 14-16 D (at birth); 7-8 D (at 25 years of age) and 1-2 D (at 50 years of age).

Lens is suspended in eye by Zonules which are inserted on anterior surface and equatorial lens capsule and attached to ciliary body. Zonular fibres are series of fibrillin rich fibre.

Lens - AnatomyHistologically lens consists of three major

components:1. Capsule – It is a thin, transparent, hyaline membrane

surrounding the lens which is thicker over the anterior than the

posterior surface. The lens capsule is thickest at pre-equator

regions (14 μ) andthinnest at the posterior pole (3 μ).

2. Anterior epithelium. It is a single layer of cuboidal cells which lies

deep to the anterior capsule. In the equatorial region these cells become

columnar, are actively dividing and elongating to form new lens fibres throughout the life.

There is no posterior epithelium, as these cells are used up in filling the central cavity of lens vesicle during development of the lens.

3. Lens fibres. The epithelial cells elongate to form lens

fibres which have a complicated structural form.

Mature lens fibres are cells which have lost their nuclei.

As the lens fibres are formed throughout the life, these are arranged compactly as nucleus and cortex of the lens

i. Nucleus. It is the central part containing the oldest fibres. It consists of different zonesWhich are laid down successively as the

development proceeds. In the beam of slit-lamp these are seen as zones of discontinuity.

Depending upon the period of development, the different zones of the lens nucleus include

Embryonic nucleus. It is the innermost part of nucleus which

corresponds to the lens upto the first 3 months of gestation.

It consists of the primary lens fibres which are formed by elongation of the cells of posterior wall of lens vesicle.

Fetal nucleus. It lies around the embryonic nucleus, corresponds

to lens from 3 months of gestation till birth. Its fibres meet around sutures which are

anteriorly Y-shapedand posteriorly inverted Y-shaped . Infantile nucleus corresponds to the lens from birth to puberty, and Adult nucleus corresponds to the lens fibres formed after puberty to rest of the life.

Lens Cortex

Cortex. It is the peripheral part which comprises

the youngest lens fibresIt is located peripherally and is composed of

secondary fibres formed continuously after sexual maturation. It is further divided into: Deep cortexIntermediate cortexSuperficial cortex

Lens CortexThe region between embryonic and fetal

nuclear core and soft cortex i.e. infantile and adult nucleus is sometimes referred to as epinucleus. The region between deep cortex and adult nucleus is sometimes referred to as Perinuclear region.

Lens fibres are held together by interlocking of lateral plasma membranes of adjacent fibres to form ball-and-socket and tongue-and-groove joints.

4. Suspensory ligaments of lens (Zonules of Zinn).

Also called as ciliary zonules,

these consist essentially of a series of fibres passing from ciliary body to the lens.

These hold the lens in position andenable the ciliary muscle to act on it

These fibres are arranged in three groups:i. The fibres arising from pars plana and anteriorpart of ora serrata pass anteriorly to get insertedanterior to the equator.ii. The fibres originating from comparativelyanteriorly placed ciliary processes pass posteriorlyto be inserted posterior to the equator.iii. The third group of fibres passes from the summitsof the ciliary processes almost directly inward tobe inserted at the equator

Lens - CrystallineLens fibres contain high concentrations of

crystallins.Crystallins represent the major proteins of the

lens (constitute 90% of total protein content of lens). Crystallin has the following constituents:Alpha Beta and,Gamma

Lens - FunctionsThe lens serves two major functions:

Focusing of visible light rays on the fovea Preventing damaging ultra-violet radiation

from reaching the retina

Lens - PhysiologyLens function is dependent on the

metabolism of glucose to produce energy , protein synthesis and a complex antioxidant system. Glutathione is found in high concentration in lens and helps protect its structure from oxidative damage.

The crystalline lens is a transparent structure playing main role in the focussing mechanism for vision.

Its physiological aspects include : Lens transparency, Metabolic activities of the lens, and AccommodationLens transparencyFactors that play significant role in maintaining

out standing clarity and transparency of lens are

Avascularity, Tightly-packed nature of lens cells, The arrangement of lens proteins, Semipermeable character of lens capsule, Pump mechanism of lens fibre membranes that regulate the electrolyte and water balance in thelens, maintaining relative dehydration Auto-

oxidation and high concentration of reducedglutathione in the lens maintains the lens proteinsin a reduced state and ensures the integrity ofthe cell membrane pump.

Pump leak theory

ACCOMMODATIONAs we know that in an emmetropic eye, parallel

rays of light coming from infinity are brought to focus on the retina, with accommodation being at rest.

APPLIED ANATOMY ANOMALIES OF ACCOMMODATION

Anomalies of accommodation are not uncommon.These include: (1) Presbyopia, (2) Insufficiency of accommodation, (3) Paralysis of accommodation(4) Spasm of accommodation

Thank You