Dr.Lhacha Wangdi

Resident

Ophthalmology, JDWNRH

RETINA AND LAYERS

OBJECTIVES

1. Macroscopic anatomy of Retina

2. Retinal layer anatomy

3. Clinical correlates

GROSS ANATOMY-REVIEW

The retina proper is a thin and delicate layer of nervous tissue- covers ¾ of inner eye wall

Grossly retina is divided into two parts;Central retina

1. Foveala

2. Fovea

3. Para fovea

4. Peri

5. Macula

Rich in cones, has more ganglion cells per area than elsewhere, and is a relatively small portion of the entire retina.

Clinical function

• Designed for fine visual acuity,

• Photopic vision

• Stereopsis‘,

• Color vision

Peripheral retina

1. Near periphery

2. Mid periphery

3. Far periphery

4. Ora serrata

Makes up most of the retina, and rods dominate

Clinical function;

• Designed for gross vision and

• Scotopic/night vision

• Sensitive to motion and stimulates turning of eye/head

CENTRAL RETINA/POSTERIOR POLE- OVERVIEW

Macula-

-Demarcated by superior and inferior temporal arterial arcuate

-Elliptical shape horizontally

-Average diameter of about 5.5 mm/3.5DD

-Area centralis corresponds to approximately 15-18° of visual field

Fovea

-center of macula

-It has a diameter of 1.5-1.85mm/1DD

-Represents 5° of the visual field)

Foveola/fovea centralis

-Centre of fovea ( highest visual acuity)

-4 mm temporal to the centre of the optic disc

and about 0.8 mm below the horizontal meridian

-0.35mm in D/0.3-0.4DD

-corresponds to FAZ and 1° of visual field

-Umbo (Center of foveola) corresponds to light reflex

0.8mm4 mm

Parafovea

-0.5mm around the fovea

Perifovea

-1.5mm around the parafovea

MACULA Macula-Dark area in the central retina

Protects central vision by following characteristics;

1. Highly pigmented tall epithelial cells of RPE(highest pigmentation of entire retina)-which give dark macula

2. Dense pigmentation helps to reduce scattering of light

3. The choroidal capillary bed also is thickest in the macula

4. It has also yellowish hue due to highest concentration of xanthophyll pigments

Function of xanthophyll pigments;

• Act as filters, absorbs short wavelength visible light to reduce chromatic aberration

• Antioxidant effect,

• Protective role against UVR damage.

Clinical correlates;

• Lack of blood vessels and neural tissue in foveola allows light to pass unobstructed into the photoreceptor outer segment

• Chronic retinal oedema may result in the deposition of hard exudates around the fovea in the layer of Henle with a macular star configuration

A disc diameter central depression in macula- fovea

• Central depression foveola is formed by lateral displacement of retinal neurons leaving only photoreceptors in the center.

• The inner nuclear layer and ganglion cell layer are displaced laterally and accumulate on the curved walls of the fovea called Clivus

• The fovea has the highest concentration of cones (199,000-300,000 cones/mm)

• The photoreceptor fibers(cones) become longer as they deviate away from the center; these fibers are called Henle’s fibers

FOVEA

(Fovea)

Foveola

CTN…• Six layers present in the foveola are ;

(1) internal limiting membrane

(2)Henle’s fiber layers

(3) ONL (which contains about 10 rows of cone nuclei),

(4) external limiting membrane,

(5) photoreceptor layers

(6) RPE

Note;• Thin foveal layers and compact cone photoreceptor

helps to form sharpest vision and steoropsis

• Foveal reflex is caused by the parabolic shape formed by the clivus.

• Loss of foveal reflux implies disruption of neural layers

Around the optic cup is neuroretinal rim

• Pink and sharp peripheral margin

• Contain nerve axon

• Broad inferior rim followed by superior> nasal>temporal ( ISNT rule)

• Optic disc is important landmark in retina

• The site where ganglion cell axons accumulate and exit the eye

• The optic disc lacks all retinal elements except the nerve fiber layer and an internal limiting membrane

• Vertically oval Vertical diameter-1.8mm Horizontal diameter- 1.5 mm

• Anterior posterior width- 0.7-1mm in length

Pale center area-physiological cup

• Horizontally oval

• Free of nerve axon(cup)-0.3mm

• Normal CDR= 0.4

OPTIC DISC- SURFACE ANATOMY

Inferior

superior

Nasal temporal

• Optic disc contains no photoreceptor cells light incident on the disc does not elicit a response; thus it represents the physiologic blind spot.

• It is paler than the surrounding retina because there is no RPE.

• The pale-yellow/salmon color of disc is due to combination of the scleral lamina cribrosa and the capillary network.

PAPILLEDEMA is edema of the optic disc secondary to an increased in intracranial pressure (ICP).

• pressure within the meningeal sheaths causes fluid to accumulate within the fibers so that they swell leading to blurring of the disc margins

• Edema of the optic disc from any other cause is referred to as simply “edema of the optic disc.”

CLINICAL CORRELATES- OPTIC DISC

PR is divide into four parts

1. Near periphery

• (1.5mm around the macula )

2.Mid-periphery

• (3mm around the near periphery )

3.Far periphery

• Extends 9-10 mm from optic disc on the temporal side and

• 16 mm on the nasal side in the horizontal meridian

4.Ora serrata.

PERIPHERAL RETINA (PR)

5.5mm

Macula 1.5mm

3mm

9-10mm16mm

• Anatomical equator is located approximately 2DD from the entrance of vortex vein/about 24mm from the center of optic disc

ORA SERRATA

Normal variant of Ora serrata

• Oral bays- scalloped edges of pars plana between dentate processes

• Dentate processes- teeth like extension of retina into pars plana

• Enclosed oral bays- island of pars plna surrounded by retinal tissue, can be mistaken for retinal hole

• Granular tissue- white opacity withion the vitreous base(yellow circle), can be mistaken for peipheral opercula

• Meridional fold- thickened retinal tissue

It is transition zone between retina and par plana

Description Length

Width of ora serrata 2.1mm temporally0.7-0.8mm nasallly

Location from limbus 6mm nasally7mm temporally

From equator 6-8mm

From optic disc 25 mm nasally

Implication-

1. Posterior Vitreous detachment may case tractional retinal detachment

2. Blunt trauma may cause avulsion of thin ora serrata and vitreous base and tearing of pars plana and anterior border of retina

• Vitreous base is 3-4mm wide stradding the ora serrata

• Vitreous is strongly adhered at vitreous base

CTN….

RETINAL MICROANATOMYRetina is composed of 10 micro layers (Sclerad to vitread)

Retinal pigment epithelium;

Photoreceptor layer of rods and cones;

External limiting membrane;

Outer nuclear layer;

Outer plexiform layer;

Inner nuclear layer;

Inner plexiform layer;

Ganglion cell layer;

Nerve fibre layer;

Internal limiting membrane.

Neuron 1

( perception elements )

Neuron 11

(conductive and associative elements)

Neuron 111

(conductive elements )

• In far periphery it continues forward as the pigmented layer of the ciliary epithelium

• A single layer of outermost cuboidal epithelial cells of the retina

• Located between the highly vascular choriocapillaris and the outer segments of photoreceptor cells

• 4- 6 million RPE cells per eye.

• Extends from the optic disc to the ora serrata,

RPE

RPE

Bruch’s membraneChoriocapillaries

Photoreceptor layers

RPE are Cuboidal cells with three Cell Surfaces characteristics;

Clinical coorelates;

• No specialized junctional complex between RPE and photoreceptors- loosely adhered

• Creates potential space( subretinal space- prone for RD)

RPE- ULTRASTRUCTURE

Apical surface

• Has microvillous processes- Increases Surface area

• Interdigitate with outer segments of photoreceptor cells

• Contains melanin granules – more in macular region

1.Apical surface –inner surface 2.Paracellular surface-intercellular surfaces 3.Basal surface – outer surface

Paracellular surface;

• Contains tight junction( Zonula Occluden & adheran, gap junctions)

• Junctional complex form blood-retinal barrier

• Maintains retinal homeostasis and prevents from toxic damages

RPE- ULTRA STRUCTURE

Basal surface;

• Attached to its basal lamina, the lamina vitrea of Bruch's membrane.

• Nutrients from choriocapillaries difusess to RPE through basal lamina

RPE- FUNCTION • Visual pigment regeneration• Phagocytosis of shed photoreceptor outer-segment discs

• Transport of necessary nutrients and ions to photoreceptor cells and removal of waste products from photoreceptors

• Absorption of scattered and out-of-focus light via pigmentation

• Adhesion of the retina• Synthesis and remodeling of the interphotoreceptor matrix,

• Formation of the blood-retina barrier,

• Elaboration of humoral and growth factors.

Clinical notes

• Failure of the RPE to process cellular debris associated with outer segment turnover cause deposition of Drusen in ARMD

• Disruption of blood retinal barriers causes retinal edema eg. Macular edema

PHOTORECEPTOR LAYERS Density and distribution of photoreceptorsCones- Rods - 4.6 million - 92 million - density is maximum at fovea - maximum density in 20°(3mm) from the fovea (199 000 cones/mm2) (160,000 rods/mm2) -minimum density- periphery - rod-free at the fovea ; 0.35 mm -minimum density- periphery - light sensitive molecule- Iodopsin - light sensitive molecule- rhodopsin ( color vision-photopic vision) ( night vision- scotopic vision)( 3 types of iodopsin- trichromatic pigments - rhodopsin is sensitive to blue-green light- 493nm 1. S wavelength cones- 440nm blue light) 2.M wavelenght cone- 540nm green 3. L wavelenght cone- 577nm redClinical notes-• Mutation of rhodopsin in retinitis pigmentosa causes maximum pigmentation 3mm around the fovea• With advanced of age there is progressive loss of photoreceptors (rods are affected more than cone)- poor

night vision in elderly

MORPHOLOGY OF PHOTORECEPTORS Rods and cones are composed of several parts- six main parts

1. The outer segment, containing the visual pigment molecules for the conversion of light into a neural signal;

2. A connecting stalk, the cilium(cytoplasmic isthmus)

3. The inner segment, containing the metabolic apparatus;

4. The outer fiber;

5. The cell body- forms outer nucleated layers

6. The inner fiber, which ends in a synaptic terminal- outer plexiform layer

STRUCTURE OF ROD CELL:1. 40-60 µm long.

2. Outer segment is cylindrical- contains visual pigments

3. Pigments are located in flattened double lamellae in the form of discs.

4. Discs various between 600 to 1000/rod cell. There are no special attachments bet. discs or bet. discs and plasma membrane.

5. Discs contain 90% of the visual pigment remaining is scattered on plasma membrane.

6. Inner segment of the rod is thicker than the outer. It has two regions.

a. Outer eosinophilic ellipsoid which contains more mitocondria.

b. Myoid which contains glycogen as well as usual organelles

Clinical notes;

• Rod need great sensitivity to detect the small amount of light available

• Rod are numerous and contains a bout a million rhodopsin molecule in each sac/disc

CTN…• The rhodopsin molecule is

composed of protein called opsin and chromophore (aldehyde of Vitamin A- 11-cis-retinal)

• Opsin protein is embedded in the lipid membrane lamellar discs

• Opsin is made up of amino acids

• It has 7 helical loops.

• At 7th helical loop 11-cis-retinal is attached lysine by a protonated Schiff base linkage- molecular antena!

Clinical notes

• Chromophore and opsin forms molecular antenna which transform single quantum of light into electrical energy ( action potential ) process called phototransduction

• it is accomplished within the outer segments of the photoreceptors − the rods and cones

• Lack of vitamin A causes night blindness due to defective photo pigments

CONES- MORPHOLOGY 1. Conical in shape

2. 40 TO 80 µm long

3. Cone at periphery is short but in central fovea it is tall and resembles rod

4. Outer segment contains photo pigments called iodopsin.

5. The cone outer segment have more discs (1000-1200 per cone) than do rod outer segments

6. Lamellar disc are attached to the membrane

7. Inner segment is similar to rod structures.

8. Ellipsoid contains a large number of mitochondria.

• Not a true membrane• Composed of the terminal bars

(zonulae adherentes) between Muller cells and photoreceptors

• Fenestrated membrane• Extents from the ora serrata to

the edge of optic disc. Main function-• Selective barrier for nutrients• Stabilization of transducing

portion of the photoreceptors.

OUTER NUCLEI LAYER(ONL)• Formed by nuclei of rods and cones.

• Rod nuclei form the bulk of this layer.

RPE

ELM

EXTERNAL LIMITING MEMBRANE .

ONL

Muller cells

OUTER PLEXIFORM LAYER:• It marks the synaptic junction of

photoreceptor and second order neurons in retina( between ONL and INL)

• The outer plexiform layer is thickest at the macula

• In the fovea there is no synaptic terminals, because cone pedicles are displaced laterally to the extrafoveal region.

Function-

• Transmission and amplification of electrical potential• The presence of numerous junctions-Aids in the homeostasis of the retina.• Act as a functional barrier to diffusion of fluids and metabolites- eg.dark maroon dot and blot

hemorrhage in deep retinal hemorrhage

Cone pedicle are pyramidal in shape with three type of neuronal axon/processes ( triad arrangement)

1. Central axons of midget bipolar cell that may contact the same cone at 10-25 different points.

2. Two dendrites on either side of the triad which originates from different horizontal cells

• Have total synaptic complex of 25 per pedicle which helps to spread light excitation over a large portion of the connections in the outer plexiform layer

• Synaptic terminal are formed by the photoreceptor terminal and the dendritic processes of INL cells

• The rods has a round or oval cytoplasmic expansions called spherules with few synaptic terminals

• Cone have larger cytoplasmic expansions known as pedicles with multiple synaptic terminals

Two type of dendritic processes contact with spherules

1. One Deep horizontal cell axon

2. 1-4 Shallow bipolar cell axon

(this form total synaptic complex of 2-7 per spherules)

PHOTORECEPTOR SYNAPTIC TERMINAL

Located betweeen OPL and IPL

Consists of following 8-12 rows of cells:

• Bipolar cells- 9 types

• Horizontal- 3 types(H1,H11,H111)

• Amacrine

• Supportive Muller’s cells

Four layers can be distinguished by light microscopy

1. Outermost layer -horizontal cell nuclei

2. Outer intermediate layer- bipolar cells

3. Inner intermediate layer -Muller cell

4. Innermost layer-amacrine and interplexiform cell nuclei.

INNER NUCLEATED LAYER

Numerous cells and extensive cellular connection of INL is essential for transduction and amplification of light signals.

Horizontal cells

• Flat cells, • Has numerous neuronal

interconnections between photo receptor and bipolar cells in the outer plexiform layer.

• highest concentration in fovea• Their processes branch extensively as one

proceeds from the central retina towards the ora serrata.

• This arrangement expands the receptive field of the photoreceptors beyond their dendritic field.

HORIZONTAL CELLS

Function-

• Modulate and transform visual information received from the photoreceptors

HORIZONTAL CELLS Divided into 3 types

1. HI

• Has stout dendrites that connects only cones at traid and rod spherules

• The HI cells connects more with L-cone and M-cones

2.HII

• The HII cells have slim overlapping dendrites and a short curved axon

• Connects all types of cones

3.HIII

• The Hlll cells are 30% larger than the HI cells and contact more cones, but are otherwise similar with H1

Specific Functions-

• HI cells connects longer wavelength cones and functions as luminosity cells- enhances visual clarity

• HII types connects shorter wavelength cones and act as chromaticity cells- enhances color vision

BIPOLAR CELLS- 2ND ORDER NEURON

• Oriented radially in the retina

• Located in the inner nuclear layers and their processes extend to the outer and inner plexiform layers

• Receive extensive synaptic feedback from amacrine cells

Function-

• Bipolar cells relay information from photoreceptors to horizontal, amacrine, and ganglion cells .

Under light microscopy nine types

a. Rod bipolar cells(RB)

b. Invaginating midget bipolar(7) –smallest(MB)

c. Flat midget bipolar

d. Invaginating diffuse bipolar

e. Flat diffuse bipolar

f. On-centre blue cone bipolar

g. Off-centre blue cone bipolar

h. Giant bistratified bipolarGBB)

i. Giant diffuse invaginating bipolar

Rod bipolar interacts only with rod photoreceptor rest contacts with cones

There are about 35.68 million Bipolar cells in retina with highest concentration at fovea

Number & Axonal complexity for- Amplification and modulation of visual perception

BIPOLAR CELLS – 2ND ORDER NEURON

CNT…

Clinical application;

• The OFF bipolar depolarizes in dark and hyperpolarizes in light- activated by cones

• The ON bipolar depolarizes in light and hyperpolarizes in dark- activated by rods

• Bipolar cells transfer information to retinal ganglion cells, which are the first cells in the visual pathway to respond with an action potential

• Bipolars cells which respond with depolarization(inactivation) are OFF bipolars

• Bipolar cells which respond with hyperpolarization (activated state) called are ON bipolars.

• OFF bipolars synapse in the outer part of the IPL

• ON bipolars synapse in the inner tier, closest to the ganglion cell layer

AMACRINE CELLSAmacrine cells are flusk/urn shape cell located in inner nucleated layer close to the inner plexiform layers

Upto 24 varieties has been described but two major types are based on their characteristics;

1. Diffuse Amacrine cells (A11)

• processes extend throughout the entire inner plexiform layer

• Further subdivided into narrow, medium, or wide field types, depending on the width covered by their axonal fields

2. Stratified Amacrine cells(A)

• fibers run in the outer half of the inner plexiform layer

The remarkable feature of amacrine cells is the broad distribution of their axonal processes through all strata of the inner plexiform layer

• Amacrine cell processes indicates that these cells play an important role in the modulation of electrical information reaching the ganglion cells

• Consists of synapses bet. Axons of bipolar cells and dendrites of ganglion and amacrine cells.

• Two elements are present at synapse in an arrangement that is known as a 'dyad‘

• In this type of synapse, the bipolar cell contracts two processes, one from a ganglion cell and the other from an amacrine cell

INNER PLEXIFORM LAYERS

Two distinct synapses are unique to the amacrine cells in IPL:

1. The reciprocal synapse

• Connecting amacrine cell synapse back to the nearby bipolar cell terminal, suggesting a local feedback mechanism between these cells.

2.The serial synapse-

• Amacrine cell process synapsing with an adjacent amacrine cell process

The reciprocal synapseSerial synapse

Ganglion cells transmits signal from the bipolar cell to the lateral geniculate body

1.2 million ganglion cells are present in the retina each with a single axon

• 3rd order neuron of ganglion cells lie in this layer.

• Single row in Peripheral retina

• At the adge of foveola (macula) it is multi layer(6-8 layered) and on temporal side of disc it has two layers.

• It is absent in foveola and optic disc

GANGLION CELL LAYERS-

CTN..

1.P ganglion cells- The midget cells

• Monosynaptic ganglion cells, show dendrites that synapse exclusively with axon terminals of midget bipolar cells and amacrine cell processes

• P cells are concentrated in central retina,

• Constitute 80% of the ganglion cell population.

2.M ganglion/Paasol cells-polysynaptic because they make �synapses over a wide area.

• They synapse with all types of bipolar cells except the midget bipolars

• M cells constitute 5% of the total ganglion cell population at the fovea and 20% at the periphery of the retina

18 types of ganglion cells described- two main types are

CTN… P Ganglion cells

• The P ganglion cells project to the parvocellular layers

• P cells seem to be specialized cells in primates for color vision

M ganglion cells

• M ganglion cells projects to Magnocellular layers of LGN

• M cells have expansive processes and cover a large area of retina, they can respond rapidly to moving or changing stimuli.

• Sensitive to luminance changes in dim illumination (scotopic conditions)

NERVE FIBER LAYERS • The nerve fibre layer contains the

axons of the ganglion cells

• Optic nerve consists of approximately 1.2- 1.5 million axons of retinal ganglion cells

• Their course runs parallel to the retinal surface

• The fibers proceed to the optic disc at a right angle, and exit the eye through the lamina cribrosa as the optic nerve.

• The fibers generally are unmyelinated within the retina

ARRANGEMENT OF NERVE FIBRES IN THE RETINA1. Fibres from the nasal half of the retina come

directly to the optic disc as superior and inferior radiating fibres (srf and irf).

2. Fibres from the macular region pass straight in the temporal part of the disck as papillomacular bundle (pmb).

3. Fibres from the temporal retina arch as superior and inferior arcuate fibres (saf and iaf).

• The nerve fibre layer is thickest at the nasal edge of the disc, where it measures 20-30 microns

• The thickness decreased with increasing distance from the disc margin, becoming 8 to 11 microns just posterior to the ora serrata

• The papillomacular bundle represents the thinnest portion of the nerve fibre layer around the optic disc

ARRANGEMENT OF NERVE FIBRES OF THE OPTIC NERVE HEAD:• Fibres form the peripheral part of the retina lie deep in

the retina but occupy the most peripheral part of the optic disc.

• While the fibres originating closer to the optic nerve head lie superficially in the retina and occupy a more central (deep) portion of the disc.

THICKENSS OF NERVE FIBRE LAYER AT THE DISC:

• Thickness of the nerve fibre layer around the different quadrants of the optic disc margin progressively increases in the following order:

1. Most lateral quadrant (thinnest)

2. Upper temporal and lower temporal quadrant

3. Most medial quadrant

4. Upper nasal and lower nasal quadrant (thickest)

CLINICAL SIGNIFICANCE OF DISTRIBUTION AND THICKNESS OF NERVE FIBRES AT THE OPTIC DISC MARGIN:

1. Papilloedema appears first of all in the thickest quadrant (upper nasal and lower nasal) and last of all in the thinnest quadrant (most lateral).

2. Arcuate nerve fibres are most sensitive to glaucomatous damage, accounting for an early temporal arcuate visual scotoma in glaucoma

3. Macular fibres occupying the lateral quadrant are most resistant to glaucomatous damage and explain the retention of the central vision till end.

The loss of tissue seems to be associated with compaction and fusion of the laminar plates . It is most pronounced at the superior and inferior poles of the disc

INTERNAL LIMITING MEMBRANE • The internal limiting membrane forms the innermost layer

of the retina and the outer boundary of the vitreous

• Both the retina and the vitreous contribute to the formation of this membrane.

• Consists of four elements:

(1) collagen fibrils and

(2) proteoglycans (mostly hyaluronic acid) of the vitreous;

(3) the basement membrane;

(4) the plasma membrane of the Muller cells and possibly other glial cells of the retina

• In the posterior retina the internal limiting membrane attains a thickness of 0.5-2.0 Um.

• It continues uninterrupted at the fovea where it is thickest

• At the periphery of the retina, the membrane is continuous with the basal lamina of the ciliary epithelium

• It gives the posterior retina a characteristic sheen when observed with the ophthalmoscope

THANK YOU

REFERENCES • Wolff's ANATOMY OF THE-EYE AND ORBIT , 8th edition

• AAO, section 2 fundamental and section 12

• LEE and Remington- clinical anatomy and physiology of visual system-3 rd edition

• Yanoff and Duker- Ophthalmology- 3 rd edition

• Images and graphics- internet sources