Lens: Anatomy, Physiology

The lens is a transparent, biconvex, crystalline
structure placed between iris and the vitreous in
a saucer shaped depression the patellar fossa.
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).
Structure (Fig. 8.1)
1. Lens 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) and
thinnest 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 (Fig. 8.2).
i. Nucleus. It is the central part containing the
oldest fibres. It consists of different zones, which
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 and corresponds to the lens from 3
months of gestation till birth. Its fibres meet
around sutures which are anteriorly Y-shaped
and posteriorly inverted Y-shaped (Fig.8.2).
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.
ii. Cortex. It is the peripheral part which comprises
the youngest lens fibres.
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 and
enable the ciliary muscle to act on it. These fibres are
arranged in three groups:
i. The fibres arising from pars plana and anterior
part of ora serrata pass anteriorly to get inserted
anterior to the equator.
ii. The fibres originating from comparatively
anteriorly placed ciliary processes pass posteriorly
to be inserted posterior to the equator.
iii. The third group of fibres passes from the summits
of the ciliary processes almost directly inward to
be inserted at the equator.
APPLIED PHYSIOLOGY AND BIOCHEMISTRY
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
Accommodation (see page 39)
Lens transparency
Factors that play significant role in maintaining
outstanding 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 the
lens, maintaining relative dehydration and

Auto-oxidation and high concentration of reduced
glutathione in the lens maintains the lens proteins
in a reduced state and ensures the integrity of
the cell membrane pump.