Optometry: LENS :- Definition, Function

*lens is transparent, biconvex, crystalline structure .
*It is placed between iris and vitreous humour.
*The lens capsule is a small cavity or potential space are called as Retrolental.
*Equatorial diameter of the lens at birth about 6.5 mm, increase to 9-10 mm in the second decade.
*Thickness varies with age between 3.5 mm at birth and 5 mm at extreme of age.
*Weight of the lens is about 65 mg at birth.
*Lens has two surface : the anterior surface less convex than the posterior
*The anterior pole is about 3 mm from the back of the cornea.
*Refractive index of the lens is 1.39
*Refractive power of the lens is 16-17 D.
*Accommodative power varies with age , 14-16 D at birth ; 7-8 at 25 yrs of age and 1-2 D at 50 yrs of age.
*Color of the lens is also change with age , transparent lens in Infants and Young adults is colorless.

STRUCTURE OF THE LENS :-

*it consist of three distinct parts-

Lens capsule
Anterior lens epithelium , and
Lens fibres

1- Lens Capsule-

it is the component of the globe of the eye
it is thin, transparent, hyaline collagenous membrane.
the lens capsule is highly elastic.
it does not contain any elastic tissue.
it is secreted by the basal cell area of the lens epithelium
lens is composed of type IV collagen and 10% glycosaminoglycans.
it contains enzyme, ATP and glycolytic intermediates.
capsule thickness varies according to the age.

2- Anterior Lens Epithelium-

It is a single layer of cuboidal nucleated epithelial cells.
ALE lies deep to the anterior capsule extending upto equatorial lens bow.
it contain three cytoskeletal elements, microfilament, intermediate filaments, and microtubules.
the anterior lens epithelium can be divided into three zones :
Central Zone - it consist of cuboidal cells are polygonal in flat section. these cells approx. 10 µm high and 15 µm wide . their nuclei are round and located slightly apically.
Intermediate Zone - it consist of comparatively smaller and more cylindrical cells located peripheral to the central zone. their nuclei are round and central.
Germinative Zone- it consist of columnar cells which are most peripheral and located just pre-equatorial. Nuclei of these cells are flattened and lie in the plane of cell axis.

3- Lens Fibres-

lens fibres from the bulk of the lens.
they are long, thin, transparent, firmly packed.
Diameters between 4-7 mm and length upto 12 mm.
fibres are almost hexagonal in shape and are bound together by the ground substance.
the cytoplasm of the cells of the superficial bow region and the newly formed lens fibres contain a Nucleus, polysomes, mitochondria, ER, Golgi apparatus.
The nuclei of the lens fibres are present temporarily and disappear later on.

the initial fibres forming the fetal nucleus just surrounding the embryonic nucleus are arranged in such a way that they terminate with two Y-shaped sutures on the anterior and the posterior surface of the lens.

Lens fibres are arranged compactly as nucleus and cortex of the lens.

A- Nucleus- it is the central part containing the oldest fibres.

it consist of different zones-

Embryonic nucleus- is its innermost (formed at 1 to 3 months of gestation).
Fetal nucleus- corresponding to lens from 3 months of gestation till birth.
Infantile nucleus- corresponding to lens from birth to puberty.
Adult nucleus- corresponding to the lens in adult life.

B- Cortex- it is the peripheral part of the lens substance which lies just outside the adult nucleus.it comprises the youngest (most recent formed) lens fibres.

FUNCTIONS :-