398
UNIT 3
Organ Systems
Refractive Media of the Eye
Posterior
Ciliary
body
Cornea
Lens
Iris
Lens cortex
Lens cortex
Lens
Lens
nucleus
nucleus
Anterior chamber
Posterior chamber
Fig. 20-7C
Fig. 20-7B
Fig. 20-7D
A
Lens cortex
Lens
nucleus
Figure 20-7A.
Overview of the lens.
H&E,
3
11
The
lens
is a biconvex, avascular, fl exible, and colorless transparent
structure. Anterior to the lens is aqueous humor and posterior to it is
the vitreous body. The lens is composed of the
lens
capsule
,
subcap-
sular
epithelium
, and
lens
± bers
. The peripheral region of each lens
F ber contains a nucleus and organelles, which form the bulk of the
lens called the
lens cortex
. In the central part of the lens, the F bers
lose their nuclei and organelles; this region is F lled with crystalline
proteins and is called the
lens
nucleus
. The lens is held in place by
zonular F bers (position indicated by
dashed lines
; see ±igs. 20-8A
and 20-11C). The
anterior chamber
is the space between the cornea
and the iris. The
posterior chamber
is a narrow space between the
iris and the posterior zonular F bers of the lens. These chambers are
F lled with aqueous humor and communicate through the pupil.
Figure 20-7B.
The anterior region of the lens.
H&E,
3
272; inset
3
680
The
anterior
lens
is covered by a
thickened
lens
capsule
(see ±igs.
20-8B and 20-10A), a transparent basement membrane that envel-
ops the entire lens. Beneath it is a single layer of fl
at, squamous cells
(
subcapsular epithelium
).
Figure 20-7C.
The posterior region of the lens.
H&E,
3
272;
inset
3
680
The
posterior
lens
is covered by a
thin lens capsule
(see ±ig. 20-8B).
No subcapsular epithelium lies beneath the capsule in this region.
The
posterior lens surface
is in contact with the
vitreous
body
, a
transparent gel, which contains water, collagen, and hyaluronic acid
and F
lls the interior of the globe posterior to the lens.
Figure 20-7D.
The equatorial region of the lens.
H&E,
3
272;
inset
3
544
The cells of the
subcapsular epithelium
in the
equatorial region of
the lens
are increased in height, and most cells are cuboidal in shape.
The sizes of the
lens ±
bers
and their
nuclei
are increased in this
region. The equatorial surface of the capsule is connected to the
zonular F
bers, which hold the lens in place (±ig. 20-8A).
C
Posterior lens surface
Posterior lens
Posterior lens
suface
suface
Lens cortex
Lens cortex
Posterior
Posterior lens
suface
Lens cortex
D
Lens capsule
Subcapsular
epithelium
(cuboidal cells)
Subcapsular
epithelium
Lens fibers
Nuclei of
peripheral
lens fibers
Anterior
Lens capsule
L
e
n
s
c
c
o
rte
rte
x
Subcapsular epithelium
(anterior lens epithelium)
Anterior
Anterior
lens epithelium
lens epithelium
B
Lens cortex
Anterior
lens epithelium
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