Retina Anatomy

I. Overview
A. Location
1. Circular disc of approximately 42mm diameter
2. A neural layer between the choroids and the vitreous
3. Extends from the circular edge of the optic disc to the ora serrata
B. Structure
1. 7 cell types (see section II)
2. 10 layer (see section III)
C. Morphological development
1. Neural retina develops from the neural tube ectoderm
D. Local Specializations
1. Macula lutea- darkened region in the central retina (5.5mm dia.).
(May appear to have a yellow hew because of the pigment
a) Fovea- shallow depression in the center of the macular region
(1.5 mm dia.). The curved wall of the depression is known as the
clivus. High concentration of cones and is capillary free.
b) Foveola- contains the densest population of cones that have the
smallest cross-sectional diameter of all the photoreceptors
(0.35mm dia.). Red and Green sensitive cones only No rods or
Blue cones
c) Parafovea- contains the largest accumulation of retinal bipolar
and ganglion cells (0.5mm dia.).
d) Perifovea- area begins where the ganglion cell layer if four
cells thick & ends where it is one cell thick (1.5mm dia.).
2. Ora Serrata- the peripheral termination of the retina.
3. Optic Nerve head - site where ganglion cell axons accumulate & exit
the eye. Lacks all retinal elements except NFL and an internal limiting
membrane ~ because it contains no photoreceptors, it does not elicit a
response, and therefore represents the physiological blind spot.
4. Major blood vessels- The outer retinal layers receive their nutrition
from the choroidal capillary bed, the inner layer from the central retinal
artery (see section X).
E. Photoreceptor distribution
1. Cone peak @ fovea (Cones dominate the central retina.)
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2. Rod peak @ ~ 15 degrees temporal retina & 25degrees nasal retina.
(No rods located at the center of fovea or optic disc. Rods dominate at
the peripheral retina.)
F. Central & Peripheral retina
The periphery is designed for detecting gross form & motion, while the
central area is specialized for visual acuity. The periphery makes up most
of the retina, & rods dominate. Central retinal is dominated by cones, has
the most ganglion cells per area, & a fairly small area portion of entire
G. Role
The retina is the site of transformation of light energy into a neural signal
& contains the first three types of cells (photoreceptor, bipolar, &
ganglion) in the visual pathway, the route where visual information form
the environment reaches the central nervous system for interpretation.
II. Cell Types
A. Vertical/Direct Pathway
1. Photoreceptors: the rods and cones are special sense cells containing
photopigments that absorb photons of light. (Rods are more active in dim
illumination; cones are more active in well lit conditions.)
a) Outer segment—made up of a stack of membranous discs and
is enclosed by the plasmalemma of the cell. Visual pigment
molecules are located within the disc membranes.
b) Connecting cilium—extends from the innermost disc, joining
the outer segment with the inner segment. Acts as a conduit
between them.
c) Inner segment—composed of ellipsoids, closer to the outer
segment and contains mitochondria; and myoid, closer to the cell
body and is the site of protein synthesis.
d) Nucleus—contained within the cell body
e) Axon—an inner fiber containing microtubules running inward
from the cell body.
f) Synaptic terminal—ribbon synapses with bipolar and
horizontal cells
g) Phototransduction
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2. RPE
h) Structure of RPE—the outermost retinal layer, is a single cell
thick and consists of pigmented hexagonal cells. Basal surface is
next to choroids and apical surface faces neural retina.
i) Choriocapillaries
j) Bruch’s membrane—the basal aspect of the RPE contains
numerous infoldings and is adherent to its basement membrane,
which forms a part of Bruch’s membrane of the choroid.
Therefore, its attachment to the choroid is strong.
k) Apical processes—microvillus that extends into the layer of
photoreceptors, enveloping the specialized outer segment tips.
There are no intercellular junctions that connect RPE and
photoreceptor layers.
l) Adhering and occluding junctions—join the RPE cells near
their apices.
m) Photoreceptor renewal
2. Bipolar cells
a) Named from shape
b) Dendritic end—synapses with photoreceptor and horizontal
c) Nucleus—is large and contains little cell body cytoplasm
d) Axon terminal end: dyad synapse—synapses with ganglion and
amacrine cells.
e) Cell body lies between the dendritic end and the axon terminal
f) They relay information from photoreceptors to horizontal and
ganglion cells and receive extensive synaptic feedback from
amacrine cells.
g) Several types of bipolar cells:
(1) Rod (mop) bipolar cell: The only bipolar cell that
contacts rods.
(2) Midget bipolar cell: Flat midget bipolar cell or
invaginating midget bipolar cell.
(3) Diffuse cone bipolar cell: Type a (flat bipolars) and
Type b (brush bipolars). These contact around 5
neighboring cones in central retina, and around 10-15
neighboring cones in the periphery.
(4) Blue cone bipolar cell: These contact widely spaced
rather than neighboring cones.
(5) Giant cone bipolar cell: Two types-diffuse and
bistratified. They differ in their axon terminations.
3. Ganglion cells
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a) Can be bipolar (single dendrite) or multipolar.
b) Dendrites—classified by branching pattern. These are either
stratified, with horizontal branches arranged in one to three layers,
or diffuse branching like a tree.
c) Cell body
d) Axons—each ganglion cell has a single axon
e) Types of ganglion cells:
(1) Midget ganglion cell (P1): Most common type. Has a
single dendrite. Named because it projects to the
parvocellular layer of the lateral geniculate body.
(2) P2 ganglion cell: Also terminates in the parvocellular
(3) M-type ganglion cell: Projects to the magnocellular
layer of the lateral geniculate body.
B. Lateral Pathways
1. Horizontal cells
a) Transfers information in a horizontal direction parallel to the
retinal surface.
b) Connections—synapse with photoreceptors, bipolar cells, and
other horizontal cells
2. Amacrine cells
a) Connections—forms complex synapses with axons of bipolar
cells, dendrites, and soma of ganglion cells, with processes of
interplexiform neurons, and with other amacrine processes.
b) They play an important role in modulating the information that
reaches the ganglion cells, owing to the extremely broad spread of
its process.
c) Numerous, distinguishable types
(1) Cell size
(2) Dendritic spread
(3) Branching level
(4) NTs/Nmodulators
(5) Effects
d.) Three main groups: small-field amacrines, medium-field
amacrines, and large-field amacrines.
III. Retina layers
A. Retinal pigment epithelium (RPE)
 Outermost layer of the retina; lies between the outer segments of the
photoreceptors and Bruch’s membrane of the choroid.
 arises from neural ectoderm
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single layer of hexagonal cells, progresses from columnar to cuboidal from
posterior pole to ora serrata, respectively
melanosomes contain pigment
liposfuscin granules contain degradation products of phagocytosis
microvilli found on apical surface
Gap junctions and tight junctions found at apicolateral cell surfaces
B. Photoreceptor cells (rods and cones)
 Outer segment contains visual pigments in stack of membranous discs
 Inner segment contains the cellular organelles
 Projections from the apical surface of Muller cells separate the inner segments.
 RPE microvilli and outer segments of PRs interdigitate
 PRs surrounded by adhesive interphotoreceptor matrix that may help orient outer
segments for optimum light capture
1. Rods
 more active in dim illumination
 discs are made in inner segment, displaced outward as more discs are made,
sloughed off and phagocytosed by RPE cells
2. Cones
 usually cone-shaped with discs wider at base than at tip, though not always
 outer segment shorter than that of rod and may not reach RPE
 discs are often shed at end of day
C. External limiting membrane
 consists of intercellular junctions b/w PR cells, and between PR cells and Müller
cells at the level of PR inner segments; not a true membrane.
 tight junctions provide barrier to passage of large molecules
 looks like a fenestrated sheet under light microscopy
D. Outer nuclear layer
 contains rod and cone cell bodies; Cone cell bodies and nucleus are larger than
those of the rod.
 cone nuclei arranged in single layer close to ELM
 rod nuclei in several rows inner to cone nuclei
E. Outer plexiform layer
1. External band - inner fibers of rods and cones
2. Inner band - synapses between PRs and cells of INL
 triads - two postsynaptic horizontal cell processes flanking bipolar dendrite at
cone pedicle
F. Inner nuclear layer
 Cell bodies of horizontal neurons, bipolar neurons, amacrine neurons,
interplexiform neurons, Müller cells, and some ganglion cells
 Horizontal cell nuclei found next to OPL
 Amacrine cell nuclei found next to IPL
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Bipolar cell dendrite in OPL, axon in IPL
G. Inner plexiform layer
 Inner synaptic layer
 synaptic connections between axons of bipolar cells and dendrites of ganglion
 Contains the synapse between the first-order and second-order neuron in the
visual pathway.
 midget bipolar cell axon, inner half of IPL, flat midget bipolar cell axon outer
 ribbon synapses among bipolar axon, amacrine process, ganglion dendrite
H. Ganglion cell layer
 usually a single cell thick except at macula (8-10 thick) and temporal side of
optic disk (2 thick)
 number of ganglion cells diminishes toward ora serrata
 contains nuclei of ganglion cells and of some displaced amacrine cells
I. Nerve fiber layer
 ganglion cell axons (run parallel to retinal surface)
 fibers proceed to optic disc, turn 90° and exit through lamina cribosa as optic
 papillomacular bundle radiates to disc from the macular area, and carries the
information that determines visual activity
 superficial capillary network
 Müller cell processes
J. Internal limiting membrane
 Innermost boundary of the retina.
 terminations of Müller cells (footplates) covered by basement membrane form
uneven outer surface of ILM
 vitreal fibers incorporated at periphery
 continuous with ILM of ciliary body at anterior
 present over the macula but undergoes modification at the optic disc, where
processes from astrocytes replace those of the Muller cells
VI. Sampling Unit = the smallest retinal region containing at least one representative
from each type of ganglion cell.
 Photoreceptors—green, red, blue cones; rods for dark
 Bipolar cells—a class of retinal neurons that receive inputs from photoreceptors
in the outer plexiform layer and have outputs to amacrine and ganglion cells in
the inner plexiform layer (least 12 types)
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Midget bipolar cells—type of bipolar cell that receives input from either red or
green cones; in the central retina, the input is from a single cone
 Diffuse bipolar cells—receive input from several red and green cones
 ON bipolar cell—cone bipolar cell that depolarizes in response to increasing
light intensity (ON) and terminates in the inner half of the inner plexiform layer
 OFF bipolar cell—cone bipolar cell that depolarizes in response to decreasing
light intensity (OFF) and terminates in the outer half of the inner plexiform layer
 Midget ganglion cells—receives excitatory inputs from midget bipolar cells; in
central retina, the input is from a single midget bipolar cell
 Parasol ganglion cells—a unistratified ganglion cell that receives excitatory
inputs from diffuse bipolar cells
 Horizontal cells—a class of retinal interneurons that receive inputs from
photoreceptors and have inhibitory outputs back onto photoreceptors. Blue cones
are a major source of input onto H2 horizontal cells.
 Amacrine cells—a class of retinal interneurons that make connections in the
inner plexiform layer among bipolar, ganglion and other ganglion cells (30 types)
AII amacrine cells-- a small amacrine cell that receives inputs from rod bipolar cells
and passes those signals to both ON & OFF cone bipolar cells.
The sampling unit is defined in terms of information output, but its structural
basis is determined by the ganglion cells. The ganglion cell type with the largest tiles
and lowest density will have a larger sampling unit (i.e. in the periphery).
The smallest sampling units are located at the fovea and are dominated by cone
signals (Figure 15.35) from red and green cones. (pg 692) A parasol cell of the fovea
contains about 35 midget ganglion cell dendritic fields, the highest ratio of midget to
parasol cells in the retina. In typical sampling unit of this area, there will be 35 cones
(22-25 red, 10 to 13 green). Each cone will synapse onto one ON and one OFF midget
bipolar cell, for a total of 70 midget bipolar cells in a sampling unit. This level also
contains approximately 18-24 diffuse bipolar cells. Each midget bipolar cell directly
inputs onto a midget ganglion cells (therefore there are 70 midget ganglion cells). The
diffuse bipolar cells integrate onto 2 parasol ganglion cells.
X. Retinal Circulation
A. The retina in vivo is thin and delicate. It cannot be seen if healthy. Only
vascular system is visible.
B. Components of retinal circulation
1. superior temporal vein
2. superior temporal artery
3. Nerve Head
4. Inferior Temporal vein
5. Inferior Temporal artery
Fovea indicated because region is darker; vessels also arc around it. Anatomical
evaluation of the fundus is concerned with the uniformity of the background, the
characteristics of the vessels, and appearance of optic nerve head.
C. Central Retinal Artery-end arterial system
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1. no connection with any other arterial system
2. no anastosomes between major and minor branches
3. obstruction of any branch, large or small, will deprive the inner retina of
D. Arterial and Venous branches extend radially into the retina
1. interconnected by capillaries
2. each capillary bed is supply by one venule
3. deepest capillaries extend farthest into the fovea
4. most superficial capillaries enter fovea last
The foveal center lacks capillaries. The foveal cones are adequately supplied by
the choriocapillaries behind them. Capillaries do appear along the foveal slope.
Retinal capillaries are specialized to create a blood-retinal barrier. This is
accomplished by the presence of overlapping endothelial cells and tight junctions.
Optic Nerve and Nerve Head
All ganglion cell axons and all branches of the central retinal artery and vein
converge at the optic nerve head
-Nerve head is slightly elliptical, elongated vertically
-Typically there are 4 arterial and 4 venous branches from the nerve head,
creating 2 superior and 2 inferior branches of both the central retinal artery
and central retinal vein
The optic disc, or optic nerve head, is the site where ganglion cell axons
accumulate and exit the eye
The nerve head and optic nerve consist primarily of axon bundles separated by
sheaths of glial cells and connective tissue
-Nerve head can be divided into 3 different parts: 1) laminar portion, 2)
prelaminar portion, 3) postlaminar portion
Laminar portion- defined by scleral fibers that intersect the axons as they
exit the eye at the lamina cribosa
Prelaminar portion- the nerve head proper, lying between the lamina
cribosa and the vitreous
Postlaminar portion- the 1st millimeter or so of the optic nerve just behind
the eye
-Meniscus of Kuhnt- at the center of the nerve head and is the thickest
part of the internal limiting membrane of Elschnig; looks like a cup or
depression in the optic disc
Ganglion cell axons form a stereotyped pattern as they cross the retina to the optic
nerve head
-Axons from ganglion cells at retinal locations nasal, superior, and inferior
to the optic nerve head follow direct, almost straight-line paths to the
nerve. Ganglion cells on the nasal side of the fovea also have axons
running directly to the nerve, as the papillomacular bundle. Axons from
all other ganglion cells follow paths around the fovea. Temporal to fovea,
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the horizontal raphe divides the ganglion cells whose axons run below
the fovea from those whose axons run above it
Axons from many widely separated ganglion cells are collected in bundles in the
nerve fiber layer
Axon bundles have an orderly arrangement in the nerve head
-Nerve head has been divided into sectors and annuli
-The organization of axon bundles in the nerve head is chronotopic
ordering which depends on the timing of axon outgrowth during
-blind spot- the counter part of the nerve head. It is temporal to the fovea
The lamina cribosa is weaker than the rest of the sclera
Behind the lamina cribosa, the optic nerve has two sources of blood supply: 1)
central artery branches 2) plexus derived from short posterior ciliary nerves
Two theories in origin of field defects in glaucoma 1) vascular defects at the optic
nerve head or 2) structural changes in the lamina cribosa
Field defects in glaucoma may be due to blockage of axonal transport secondary
to deformation of the lamina cribosa
-There is progressive deepening of the cup in the nerve head from normal
to advanced glaucoma. As the cup deepens and the lamina cribosa
becomes more curved, axons passing through are subject to kinking and
pressure as they make their way through the lamina cribosa.
1. Age Related Macular Degeneration (ARMD) - #1 cause of vision loss in the
US. Occurs in two forms, wet and dry. Dry is more common than the wet form.
ARMD is scarring of macula which distorts or obscures part of the central image
that they eye transmits to the brain. Drusen (tiny yellow deposits) build up in
Bruch’s membrane signaling degeneration and thinning of nerve tissue.
2. Glaucoma- a group of eye diseases in which a specific pattern of damage to the
optic nerve results in loss of eyesight. Three of the most common types are open
angle, closed angle, and congenital glaucoma. In most cases, damage to the nerve
is thought to be a consequence of increased pressure in the eye. However,
damage often occurs without increased IOP. Glaucoma is better thought of as a
neurodegenerative disease caused by damage to and loss of nerve cells rather than
just a disease of high IOP.
3. Retinitis Pigmentosa-a primary degeneration of the neuroepithelium of the
retina, its pigment, and its blood vessels. Main symptoms are night blindness
with progressive loss of the visual field (peripheral vision lost first, with central
vision lost in the final stages of the disease). Its cause is unknown.
4. Diabetic Retinopathy-eye disease caused by diabetes resulting in damage to
small blood vessels in the retina leading to vision loss. Two stages: 1)
nonproliferative retinopathy 2) proliferative retinopathy. Nonproliferative is
characterized by microaneurysms that develop in small vessels which burst and
leak into the retina and vitreous. Proliferative retinopathy’s main feature is the
growth of fragile new blood vessels on the surface of the retina. These abnormal
blood vessels may break easily, bleeding into the middle of the eye and clouding
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vision. They also form scar tissue that can pull on the retina, leading to retinal
5. Retinal Detachment-occurs when the retina becomes separated (detached) from
the RPE and the wall of the eye. Fluid from the eye flows into the space between
the two layers, further separating them. Injury to the eye, a blow to the head, eye
disease, and conditions such as diabetes can lead to retinal detachment at any age.
The direct causes of retinal detachment are: tears or holes in the retina, traction on
the retina, and fluid buildup in the potential space under the retina.
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