CHAPTER 20
Eye
401
Figure 20-10A.
Anterior surface
of the lens.
EM,
3
3,600
A simple epithelium, the
lens epi-
thelium
, covers the
anterior surface
of the lens
. The heights of the cells
vary, ranging from squamous near
the center of the anterior surface to
columnar at the edge (
equator
) of
the lens. Because the lens develops
from a ball of
epithelial cells
(
lens
vesicle
) that invaginate from the
surface ectoderm (
lens placode
),
the basement membrane of the epi-
thelium covers the surface of the
lens as the
lens capsule
. The part
of the lens capsule at the equator
of the lens serves as the insertion
for
zonule ± bers
of the lens suspen-
sory ligament. Behind the lens epi-
thelium is an orderly array of lens
F bers. Each
lens ±
ber
is a highly
elongated,
crystallin-±
lled
remnant
of an epithelial cell that extends the
full thickness of the lens. The lens
F bers in this view are seen in cross
section.
A
Lens fiber filled with
Lens fiber filled with
epithelium
epithelium
Nuclei of the
Nuclei of the
lens epithelial cells
lens epithelial cells
Lens
Lens
Lens
Lens
capsule
capsule
Lens
cortex
cortex
Lens fiber filled with
protein crystallin
protein crystallin
Lens
capsule
Lens
Lens
epithelium
Nuclei of the
lens epithelial cells
Lens
protein crystallin
cortex
Myoid
Myoid
processes
processes
PE
PE
Posterior
Posterior
pigment
pigment
epithelial cells
epithelial cells
Anterior
Anterior
pigment
pigment
epithelial celsl
epithelial celsl
Nucleus
Nucleus
Nucleus
Nucleus
Melanin
Melanin
granules
granules
Junction boorder
Junction boorder
AE
AE
Posterior
iridial
epithelial cells
Anterior
iridial
epithelial cells
PE
Melanin
granules
Nucleus
Nucleus of
stromal cell
AE
Junction border
Myoid
processes
B
Figure 20-10B.
Posterior surface
of the iris.
EM,
3
4,600
The
posterior surface of the iris
is covered by double epithelium
derived from the inner and outer
layers of the lip of the original optic
cup. The cells of the
posterior irid-
ial epithelium
are tall and densely
packed with melanin granules. The
cells of the
anterior iridial epithe-
lium
are more complicated in shape.
Part of the cytoplasm of these cells
contains
melanin granules
, simi-
larly to the posterior epithelial cells;
however, these cells also extend con-
tractile processes into the adjacent
iridial stroma. These
myoid pro-
cesses
are F lled with
actin ± laments
,
and, because of their radial orien-
tation, the diameter of the pupil
is increased when they contract.
Therefore, the
myoepithelial cells
of the anterior iridial epithelium
collectively constitute the pupillary
dilator. At the border between the
posterior and anterior iridial epithe-
lia ([PE and AE, respectively]
inset
),
the apices of the epithelial cells con-
tact each other.
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