404
UNIT 3
Organ Systems
Retina (Tunica Interna)
Fovea
centralis
Sclera
Choroid
Macula
lutea
Substantia propria
Episclera
A
Choroid
Choroid
Choriocapillaris
Choriocapillaris
Nerve fiber layer (9)
Ganglion cell layer (8)
Inner plexiform layer (7)
Inner nuclear layer (6)
Outer plexiform layer (5)
Outer limiting layer (3)
Photoreceptor layer (2)
Outer nuclear layer (4)
Pigment epithelium
layer (1)
Pigment
epithelial
cells
Inner limiting membrane (10)
Bruch
membrane
Choroid
Choriocapillaris
B
Choro
ro
id
id
Nerve fiber layer
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer limiting layer
Photoreceptor layer
Outer nuclear layer
Pigment epithelium
layer
Inner limiting membrane
Nuclei of cones
Nuclei of rods
Choroid
C
Figure 20-13A.
Overview of the retina; fovea.
H&E,
3
17
The
retina
is a multilayered sheet of neural tissue covering the
inner aspect of the posterior two thirds of the eyeball (Figs.
20-13B,C and 20-15A–C). There are regional variations in its
structure: the
macular
(
central
)
region
is generally thicker than
the
peripheral
region
, and cone receptors predominate in the cen-
tral retina, whereas rods are more numerous in the periphery. The
fovea centralis
is a small depression in the central retina caused by
the displacement of the super± cial layers, thereby allowing incom-
ing light more direct access to the photoreceptors in this area. The
photoreceptors here consist entirely of miniature cones, and there
are no blood vessels in this region. These characteristics permit
maximum visual acuity to be achieved in the fovea. The
macula
lutea
is the region immediately surrounding the fovea centralis.
Macula lutea
means “yellow spot”; this region appears yellow in
the living retina when viewed with an ophthalmoscope.
Figure 20-13B.
Macular region of the retina and the retinal
layers.
H&E,
3
184; inset
3
368
The
macular region of the retina
is relatively thick, and its
ganglion cell layer contains many layers of nuclei. However,
both the macular and peripheral regions of the retina have
the same histologic layers: (1) the
pigment epithelium layer
, a
layer of cuboidal cells that contain melanin granules; (2) the
photoreceptor layer
, external segments of rods and cones; (3) the
outer limiting layer
, a junction complex between the Müller cells
and photoreceptor cells; (4) the
outer nuclear layer
, containing
nuclei of the photoreceptor cells; (5) the
outer plexiform layer
,
containing processes of photoreceptor cells, bipolar cells, and
horizontal cells; (6) the
inner nuclear layer
, containing nuclei of
bipolar cells, horizontal cells, amacrine cells, and Müller cells;
(7) the
inner plexiform layer
, containing processes of the cells in
the adjacent layers; (8) the
ganglion cell layer
, containing nuclei
of the ganglion cells; (9) the
nerve ± ber layer
, containing axons
of the ganglion cells; and (10) the
inner limiting membrane
,
which is the basement membrane of the Müller cells.
Figure 20-13C.
Peripheral region of the retina and distribu-
tion of the rods and cones.
H&E,
3
184; inset
3
694
The
peripheral part of the retina
is thinner than the macular
region and its
ganglion cell layer
becomes a single layer of nuclei.
The
rod
photoreceptors
are more numerous in the peripheral
retina. The
nuclei of rod cells
are small and round and spread
throughout the depth of the outer nuclear layer. The
cone photo-
receptors
are present in both the center and periphery of the
retina but are most highly concentrated in the
fovea
and
macula
.
The
nuclei of cone cells
are large and ovoid in shape and often
located at the base of the
outer nuclear layer
.
Rod and cone cells
are both
photoreceptor
neurons
.
Rods
are specialized for motion
detection and vision in dim light.
Cones
are specialized for ±
ne
visual acuity and color vision. Note the difference in thickness
between the macular and peripheral regions of the retina and the
relatively low number of ganglion cells in the peripheral retina.
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