This document provides an overview of retinal anatomy. It begins with the embryological development of the retina from the optic vesicle and cup. The retina has distinct layers that develop between 5-10 weeks gestation, including the inner and outer nuclear layers and plexiform layers. Cell types in the retina include photoreceptors, bipolar cells, ganglion cells, horizontal cells and amacrine cells. The retina has distinct topographical regions including the macula, fovea, parafovea and ora serrata. Synaptic connections between neurons in the retina involve ribbon synapses, gap junctions and conventional synapses. The cellular anatomy and function of the different neuronal cell types are also described.

1. Retina
Dr. Samten Dorji
Resident
Department of Ophthalmology/KGUMSB

2. Outline
• Introduction
• Retinal embryogenesis
• General anatomy
• Cellular anatomy
• Anatomic layers of retina
• Circulation

3. Introduction
• The retina is the light-sensitive
tissue that lines the inside of the
eye.
• The optical elements within the
eye focus an image onto the
retina of the eye, initiating a
series of chemical and electrical
events within the retina.
• Nerve fibers within the retina
send electrical signals to the
brain, which then interprets
these signals as visual image

4. • First described by Herophilus
• Named by Rufus
• Observation of the similarity
between this tissue and the
brain by Galen
• Kepler introduced the concept
of the retina as the primary
photoreceptor tissue of the
eye.
• Treviranus first performed the
detailed microscopic studies of
retinal anatomy in 1835.

5. Retinal embryogenesis
• The eyeball starts to develop from
neuroectoderm around the third week of
fetal life.
• Formation of optic pits and then optic
vesicles to optic cup
• The optic cup forms a fold inferiorly and
ventrally to form the “embryonic fissure”
through which the mesenchymal and
vascular tissues enter the globe.
• The inner layer of the optic cup develops
into the retina, and the outer layer
develops into the retinal pigment
epithelium.

6. A. At 5th week, embryonic retina is separated from the future pigmented
epithelium.The embryonic or fetal fissure (arrowheads) provides the site of
entrance of blood vessels into the optic cup.
B. At 6th
week gestation, the hyaloid vessels extend from the embryonic fissure
through the vitreous cavity to reach the lens. The peripheral retina (arrowheads)
continues as a monolayer.

7. • C. At 5th
week gestation, the retina has the the inner neuroblastic layer (1) and the
outer neuroblastic layer (2). The wide space of the site of the primary optic vesicle (3).
The internal limiting membrane is formed and separates the neuroepithelium from the
vitreous (v)
• D. At 6th
week gestation, the neuroepithelium has distinct outer and inner neuroblastic
layers. The axons of the ganglion cells form the optic nerve (arrow). V, vitreous body
• E. At 10 weeks' gestation, the division into the two neuroblastic layers has extended
peripherally up to the future ora serrata (arrowhead). L, lens; V, vitreous body.

8. A. In 7th
week gestation, the developing retina is divided into two layers. inner
neuroblastic layer (1),the layer of Chievitz (2) outer neuroblastic layer (3). Mitoses
(arrowhead) occur in the outer layer. The underlying pigmented epithelium (PE)
B. In 10th week gestation cells of the inner neuroblastic layer (1) have started to
differentiate. Immature ganglion cells (arrowheads) are near the inner limiting
membrane. (2) The layer of Chievitz (3) Outer neuroblastic layer. The future cone
nuclei (arrows) are positioned near the monolayered pigmented epithelium.

9. •The inner limiting membrane-5th
week of gestation
•The nerve fiber layer-6th
week of gestation
•The ganglion cells layer-10th
week of gestation
•The inner plexiform layer-10th
week gestation
• The inner nuclear layer-10th week of gestation
•The outer plexiform layer- 10th
-12th
week of gestation
•The outer nuclear layer
•The outer limiting membrane
•The rod and cone layer-10th
week of gestation
•The pigment epithelium-7th
week of gestation

10. MACULA
• The formation of the vascular system in the internal retina by
the eighth month marks the end of retinal maturation except
for foveal development, which continues until 6 months after
birth.
• The foveal pit results from the gradual loss of overlying
ganglion cells as they spread slightly peripherally during
months 7 and 8.
• Until 4 months after birth, the macula continues to mature.
• The result is the disappearance of the layer of Chievitz and
the formation of the foveal pit.
This late maturation of the fovea accounts for the delay in
development of fixational ability until the postnatal age of 4
months.

11. General anatomy
• Transparent tissue
• Extends from the macula in the
posterior pole to the ora serrata,
where it becomes contiguous
with the nonpigmented
epithelium of the pars plana
ciliaris.
• Loosely adherent to the
underlying pigment epithelium.
• The only firm attachments of the
retina are at the margins of the
optic disc and at the ora serrata

12. CLINICAL ANATOMICAL DIAMETER
POSTERIOR POLE MACULA 5-6MM Histologically only region of the retina
with more than one layer of ganglion
cells.
MACULA FOVEA 1.5MM Presence of xanthophyll in the
ganglion and bipolar cells
FOVEA FOVEOLA O.35MM • capillary-free zone
• Pure cones

13. • At the equator, the retina has a vertical and
horizontal diameter of 24 mm.
• Measured as a cord, the distance from
optic disc to the equator is 14mm in the
superior meridian, 14mm inferiorly, 13mm
nasally, and 17mm temporally.
• From the equator, ora serrata lies 6mm
temporally, 5mmnasally, 5mm superiorly,
and 4mm inferiorly.
• The distance from the anterior limit of the
retina to Schwalbe's line is 6 mm
superiorly, 6 mm inferiorly, 5 mm nasally,
and 6mm temporally

14. Light micrograph of human peripheral
retina including portion of the choroid
(Richardson's methylene blue/azure II stain
mixture)
Inner limiting membrane (arrow)
(1)Nerve fiber layer
(2)Ganglion cell layer
(3)Inner plexiform layer
(4)Inner nuclear layer
(5)Outer plexiform layer
(6)Nuclei of photoreceptors (outer nuclear
layer),
(7)Rod cone inner segments
(8)Rod and cone outer segments
(9)Pigment epithelium.

15. Retina thickness map based on
histological studies.
•Thinnest at the fovea (0.10 mm)
•Thickest (0.23 mm) in the perifoveal
region.
•Between the equator and the ora, the
thickness is relatively constant (0.11 mm).

16. Neuronal connections in the retina
(1)Inner plexiform layer
(2)Inner nuclear layer
(3)Outer plexiform layer
(4)Outer nuclear layer
(5) external limiting membrane
(6)photoreceptor inner and outer
segments,
(7)pigment epithelium.
G -ganglion cells.

17. Topography
FOVEA(clinically macula)
• The entire fovea measures 1.5 mm
in diameter and contains 10% of the
cones.
• Highest cone density in the retina,
approximately 147,300 per square
millimeter.
• The central 0.40-mm zone is free of
capillaries and is nourished by the
choriocapillaris circulation.
• Clinically, this region is referred to
as the foveal avascular zone and
can be readily visualized on
fluorescein angiography

18. An arteriovenous-phase fluorescein
angiogram shows the narrower lumina of
the arterial circulation compared with the
venous circulation. Following bifurcation at
the disc, both artery and vein form
extensive branching networks throughout
the retina, except at the avascular fovea..

19. A. Light micrograph of human anatomic fovea and foveola stained
with Richardson's methylene blue/azure II mixture.
B. Immunofluorescence labeling of human cones (red with
monoclonal antibody against cone-specific enolase) and their
nuclei (pink) in the fovea.
• Fiber layer of Henle (HE)
• Outer nuclear layer (ONL),
• Inner nuclear layer(INL)
• Ganglion cell layer (GC)
• Retinal pigment epithelium (RPE)

20. • Clinically, this region has a yellow
discoloration on funduscopy because
of the presence of xanthophyll in the
ganglion and bipolar cells.
• In the human retina, two components
of the macular pigment have been
identified.They are zeaxanthin and
lutein.
• In general, zeaxanthin is confined
mostly to the foveal region, whereas
lutein is found more widely distributed
in the posterior pole.
• Among patients with age-related
macular degeneration, decreased
levels of these pigments have been
noted in the macula.

21. Parafovea and Perifovea

22. PARAFOVEA PERIFOVEAL REGION
Defined as the 0.5-mm-wide annular
zone surrounding the fovea
outermost ring of the anatomic area
centralis
The outer nuclear layer contains the
maximum density of rods
The outer nuclear layer contains the
maximum density of rods
The outer plexiform layer and the
inner nuclear layer are thicker
The outer plexiform layer and the
inner nuclear layer are reduced in
thickness
Inner plexiform layer is slightly thinner Inner plexiform layer is slightly
thickened,
The ganglion cells are somewhat
smaller
The ganglion cells are larger

23. ORA SERRATA
• Marks the junction between the
multilayered pars optica retinae and
the monolayered nonpigmented
epithelium of the ciliary body.
• Thin, lack of vascularity, and intimate
relationship to the vitreous base and
zonular fibers.
• Composed of elongations of the
retinal tissue into the nonpigmented
ciliary epithelium and are known as
dentate.
• Thickness=0.11mm and is the
thinnest part of the retina outside the
fovea

24. Ora serrata of newborn.
•The layers develop, in order, from the
photoreceptors(arrow) through the inner
nuclear layer to the ganglion cell (open
arrow)
•gradual loss of the nerve fiber layer,
ganglion cell layer, and plexiform layers.

25. Morphology of retinal neurons
RETINAL CELLS
Neuronal
primary function of the retina (i.e.,
conversion and transmission of
incoming light into an electrical
signal perceived by the brain)
Glial
Vascular
glial cells are important for their
barrier, trophic, sustentacular, and
insulating functions for the neurons

26. General structure of the synapse
• The retinal neurons form three major types of
intercellular connections:
1.The chemical synapse for neuronal signal
transmission,
2.The zonula adherens that maintains tissue
structure, and
3.The gap junction that provides electrical
transmission between developing retinal cells.

27. ZONULA ADHERENS
• Present between neuronal cells in the retina.
• These nonsynaptic junctions do not serve the
purpose of electrical communication between cells.
• Act as binding sites to maintain tissue cohesion.
GAP JUNCTION
• There is close apposition between
presynaptic and postsynaptic
membranes, and current is allowed
to pass directly between adjacent
cells. Thus, it is classified as an
electrical synapse.
• The rate of signal transmission is
faster in these junctions than in
typical chemical synapses..
Electron micrograph of a gap junction
between two horizontal cells in a teleost
retina

28. CHEMICAL SYNAPSE
• Intercellular communication is
achieved when a chemical substance,
the neurotransmitter, synthesized
from the presynaptic axon terminal, is
released into the synaptic cleft, where
it binds to specific receptors on the
postsynaptic membrane.
• In the retina, three different classes of
chemical synapses have been
described based on the appearance
of the presynaptic element:
conventional synapse, ribbon
synapse, and flat or basal junction

29. Conventional synapse
• Aggregates of synaptic vesicles clustered
close to the presynaptic membrane.
• These vesicles are membrane-bound
organelles that contain neurotransmitter
substances and enzymes involved in
neurotransmitter metabolism.
• These synapses are found in the presynaptic
terminals of horizontal cells, amacrine cells,
and interplexiform cells.
• They typically have an inhibitory effect in the
postsynaptic neuron.

30. RIBBON SYNAPSE
• Found in the presynaptic terminals of photoreceptors and bipolar cells in the retina.
Two ribbon synapses in a rod spherule (S) are surrounded
by synaptic vesicles: (1) central bipolar dendrite terminal
and (2) terminal buds of horizontal cells.
• Basal junctions are found in the presynaptic terminal of photoreceptors only.

31. Synaptic cleft
• The synaptic cleft of a typical chemical
synapse is 20- to 30-nm wide.
• It houses material that binds the
presynaptic and postsynaptic
membranes together in a strong
adhesion.
• This synaptic cleft substance may be
important for formation, localization, and
proper function of the synapse.
• Macromolecules spanning the gap
between the presynaptic and
postsynaptic membranes may ensure
rapid transport of the transmitter
molecule through the cleft and may
prevent the transmitter from diffusing
away from the synapse

32. Post synaptic membrane
• In the postsynaptic
membrane, filaments project
out toward the cytoplasm.
• Minute bumps within the
specialized zone of the
postsynaptic membrane that
may represent the
neurotransmitter receptor
molecule.

33. Cellular anatomy
• Divided into a three basic cell types
1.Photoreceptor cells
2.Neuronal cells
3.Glial cells

34. Photoreceptor cells
• These are the primary neurons in
the visual pathway.
• lie at the outer edge of the retina. .
• The photoreceptor cells differentiate
longitudinally into four major
regions:
1. The inner segment containing the
metabolic apparatus,
2. The outer segment containing the
visual pigment for the conversion of
light into neuroelectrical energy,
3. A perikaryal region containing the
cell nucleus, and
4. A synaptic terminal.

35. Photoreceptor cells
RODS CONES
Illuminated conditions and provide color
vision
Dim light and provide black-and-white
vision
120 million rods 6 million cone photoreceptors
Projection called a pedicle at the
termination of the axon
Projection called a spherule
Outer segment contains the pigment
rhodopsin
Iodopsin pigment is contained within
cones
Peripheral retina is rod dominated Central retina is cone dominated
The highest density of cones is at the
center of the fovea
There are no rods in the center of the
fovea

36. Neural cells
• Neural cells (nerve cells)
1.Bipolar cells,
2.Ganglion cells
3.Horizontal cells
4.Amacrine cells

37. Bipolar cells
• Connecting the
photoreceptors to the
ganglion cells.
• Vertically oriented
(perpendicular to the
retinal surface).
• Nine types of bipolar cells.
• Postsynaptic to rods and
cones.

38. Ganglion cells
• Have dendrites that synapse
with bipolar cells.
• The axons of ganglion cells
become the nerve fiber layer
within the retina and then
become optic nerve fibers
terminating within the brain.

39. Horizontal cells
• Connect bipolar cells with
each other.
• Laterally interconnecting
neurons in the outer
plexiform layer of the retina.
• Responsible for allowing
eyes to adjust to see well
under both bright-light and
dim-light conditions.
• These are horizontally
oriented (parallel to the
retinal surface).

40. Amacrine cells
• Connect bipolar and ganglion
cells with each other.
• Function within the inner
plexiform layer, the second
synaptic retinal layer where
bipolar cells and retinal
ganglion cells form synapses.
• 40 different types of amacrine
cells, most lacking axons.
• Horizontally oriented and work
laterally, affecting the output
from bipolar cells

41. Summary
• Introduction
• Embryology
• Gross anatomy
• Cellular anatomy

42. Next session
• Layers of retina
• Circulation

43. Thank you