Download 19.11 STRUCTURE AND FUNCTION OF THE

19.11 STRUCTURE AND FUNCTION OF THE RETINA
The retina, in the posterior segment of the eye, has two parts derived from separate layers of the
embryonic optic cup. The outer is the RPE. The inner, stratified layer—the neural retina—contains
three sets of modified neurons (photoreceptors, bipolar cells, and ganglion cells) that are linked in
series by synapses. They are cross-linked by association neurons (amacrine and horizontal cells) and
supported by glial cells (Müller cells and astrocytes). Photoreceptors (rods and cones) are polarized,
primary sensory cells. Their light-sensitive parts—the outer segments— face the RPE, and light must
cross all layers of the retina before reaching rods and cones. These cells in turn synapse with bipolar
neurons, which synapse with multipolar ganglion cells. Nerve fibers emanating from them form the
optic nerve (cranial nerve II) and leave the eyeball. They conduct impulses that stimulate the visual
(occipital) cortex of the brain. Rods and cones are structurally similar—each has outer and inner
segments connected by a slender stalk—but their outer segments differ in shape and the type of visual
pigment. Outer segments consist of parallel membranous discs. Visual pigments are incorporated in
disc membranes, which undergo a steady daily turnover, being progressively shed at the outer
segment tips and then phagocytosed by adjacent RPE cells. Rods are narrow, cylindrical cells for dim
light perception that produce visual images in shades of gray. Cones are larger, shorter, conical cells
that are used for color perception and fine visual acuity. The retina has more than 12 × 106
photoreceptors. Rods outnumber cones by a ratio of about 15:1.
CLINICAL POINT
In retinal detachment, a common cause of blindness, neural retina separates from RPE. During fetal
development, the space between the two layers disappears when they become apposed, but they may
separate if fluid (such as vitreous, blood, or exudate) accumulates in the potential space.
Photoreceptors and RPE usually act as a unit, but if they separate, oxygen and nutrients reaching the
outer retina from the choroid must diffuse across a greater distance. Photoreceptor degeneration
results. Retinal detachment may occur in diabetic retinopathy and intraocular infection. Laser
treatment has greatly improved the prognosis for this condition.
19.12 HISTOLOGY OF THE RETINA
The retina has an outer surface next to the highly vascular choroid and an inner surface in contact
with the vitreous body. A highly ordered, multilayered structure, it is about 0.5 mm thick. From
outside to inside, 10 distinct layers are usually seen in histologic sections. 1) The RPE consists of
one layer of melanin-rich cuboidal cells. Separated from the choroid by Bruch basement membrane,
they are between the choroid and outer tips of photoreceptors. 2) A layer of rods and cones, arranged
in parallel, is a prominent, fibrillar layer that comprises the outer photoreceptor segments. 3) An
external limiting membrane is the line formed by junctional complexes between photoreceptors and
supportive (Müller) cells. 4) An outer nuclear layer marks the middle nucleated parts of rods and
cones, which are arranged in palisade manner. 5) The outer plexiform layer, a lightly stained zone,
represents synaptic areas between photoreceptors and dendrites of bipolar cells. 6) An inner nuclear
layer contains mostly cell bodies of bipolar cells, other associated neurons, and nuclei of Müller cells.
7) An inner plexiform layer is a relatively thick synaptic region, mostly between bipolar cells and
ganglion cells, that also holds amacrine cell processes spreading laterally as interconnecting neurons.
8) The ganglion cell layer contains cell bodies of multipolar ganglion cells. Their dendrites branch
in the inner plexiform layer, and their axons enter the next layer. 9) The nerve fiber layer comprises
ganglion cell axons, about 1 million in each retina, that course radially toward the optic nerve. 10)
An internal limiting membrane, the thin basal lamina of Müller cells, marks the boundary between
neural retina and vitreous body.
LMs comparing retinal layers of human (Above, Right) and monkey (Below, Right) eyes.
Regardless of species, the retinas appear remarkably uniform and similar. The retina is between the
vascular choroid (Ch) and vitreous body (Vi). The layers are easily distinguished: 1) RPE; 2) rods
and cones; 3) external limiting membrane; 4) outer nuclear layer; 5) outer plexiform layer; 6) inner
nuclear layer; 7) inner plexiform layer; 8) ganglion cells; 9) nerve fibers; 10) internal limiting
membrane. Above: 250×. Below: 300×. H&E.
LM of part of a monkey retina. Rods and cones are seen at higher magnification. The retinal
pigment epithelium (RPE) is indicated. 560×. H&E.