Retina : Anatomy and function of

different layers of retina

Suraj Chhetri

B.Optometry 16th

batch

Presentation layout

Introduction Embryology Topography & Major landmarks Anatomy & Functions of different retinal layers Clinical significance

Introduction

• Rete in Latin : Net

• Thin, transparent layer of nervous tissue

• Extends from optic disc to ora serrata

Embryology of Retina• Retina develops from neuroectoderm (3rd week of gestation) shows linear thickening on either side that gets depressed

Neural plate

Optic sulcus

Optic vesicle

deepens to form

( 4th wk ) invaginates

Optic vesicle

Optic cup

(4-5th wks)

Optic cup

Outer wall

Pigment epithelial

cells

Retinal pigment epithelium

Basement membrane

Bruch’s membrane

Inner wall

(4-5th wks)

(6-12 wks)

Inner neuroblastic

layer

Ganglion cells

Amacrine cells

Muller cells

Nerve fibre layer

Ganglion cell layer

Inner plexiform

layer

Outer neuroblastic

layer

Horizontalcells

Bipolar cellsRods & cones

Inner nuclear

layer

Outer plexiform

layerOuter

nuclear layer

Layers Of Rods& cones

(6-12th wks)

(12-28th wks)(12-28th wks)

Development of Macula

• Differentiation of the photoreceptor cell occurs early in macula.

• Foveal pit :7 months of gestation • Diameter of cones decreases

and there is continuous migration of cones towards the foveal pit

• Remodeling occurs till 4 years of age

TOPOGRAPHY OF RETINA

• Has a surface area of about 266 mm²

• Thickest near the optic disc (0.56mm)

• Thinner towards the periphery (0.18mm at equator and

0.1mm at ora serrata)

MAJOR LANDMARKS OF RETINA

• Optic disc • Area centralis

(including fovea and foveola)

• Peripheral retina• Ora serrata• Retinal blood vessels

Optic disc

• Pink well defined , round to oval

• 1.5 mm in diameter • All the retinal layers

terminate (except nerve fiber layer )

• Physiological cup : central retinal blood vessels emerges through the centre of the cup .

Area Centralis

• Located in the posterior pole

• Temporal to disc • Between arcuate and

temporal retinal vessels • Elliptical in shape • 5.5 mm in diameter • Corresponds to 15 degree

of visual field

Macula lutea

• Oval zone of yellow discoloration within the centre of retina

• 3 mm in diameter • Yellow colour is due to

xanthophyll • Histologically it shows

more than 1 layer of ganglion cells ( 6-8 layers)

Fovea

• Lies in the centre of area centralis

• 4 mm temporal to the disc and 0.8 mm below the horizontal meridian

• 1.85 mm in diameter

• 0.25 mm in thickness

• corresponds to 5 degree of visual field

Foveal avascular zone

• It is the capillary free zone in the center of fovea

• Diameter is variable 0.25mm-0.6mm

• It is important land mark in fluorescein angiography to localize the center of fovea

Foveola

• 0.35 mm in diameter and 0.13 mm in thickness

• Represents only 1.25 degree of visual field

• Highest visual acuity

• Umbo : Corresponds the foveolar reflex

Peripheral retina It is divided in four regions • Near periphery : 1.5 mm around the area centralis

• Mid periphery : 3 mm around near periphery

• Far periphery : extends from optic disc , 9-10 mm temporally and 16 mm nasally

• Ora serrata : it is the serrated peripheral margin where the retina ends and cilliary body starts

Ora serrata • Dentate process : teeth like

extensions • At ora ,sensory retina is firmly

attached to vitreous and retinal pigment epithelium

• Its distance from the limbus is 6 mm nasally and 7 mm temporally

• 2.1 mm wide temporally and 0.7 -0.8 mm wide nasally

• Located 6-8 mm away from equator and 25 mm away from the optic nerve on the nasal side

Layers of retina

Retinal pigment epithelium Layers 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

1) Retinal pigment epithelium

• Single layer of hexagonal shaped cells

• Contain pigments- melalin & lipofuscin

• Adjacent RPE cells are connected to each other by tight junctions (zonulae occludens and adherens) & constitute outer blood retinal barrier

• RPE cells at fovea are taller, thinner and contains more pigment granules, thereby giving a dark colour to this area

• The potential space between the sensory retina and RPE is called sub retinal space

Apical surface of human Retinal Pigment Epithelium (as seen through electron microscope)

Function of the RPE

Absorption of scattered lightBlood retinal barrier functionVisual pigment regeneration and synthesisSynthesis of growth factorsMaintenance of retinal adhesionPhagocytosisElectrical homeostasisRepair and regeneration after injury

Retinal Pigments

Melanin• Contain within the cytoplasmic granules : melanosomes

• In old age, these pigments fuse with lysosome and break down

• Absorb stray lights and minimize scatter within the eye

• Serves as a free radical stabilizer, bind toxins and retinotoxic drugs

Lipofuscin• Accumulates gradually with

age• Derived from - the outer

segment lipids that have been ingested and then digested by the RPE membrane fragments that have been damaged by the light or oxidation

Drusen

Control of fluid and nutrient in the subretinal space

• Cells Joined by the tight junctions ( zonulae occludens & zonulae adherens )

• Block the free passage of water and ions

• Equivalent to blood retinal barrier which are formed by the capillary endothelium of intrinsic retinal vasculature

Visual pigment regeneration

Cellular retinaldehyde binding protein(CRALBP)Lecithin: retinol acyltransferase (LRAT)Cellular retinol-binding proteins (CRBP )

Retinal Adhesion• Interphotoreceptor matrix (IPM) contain

glycosaminoglycans (GAGs) which surround rod and cones

• IPM functions: - physical support to the photoreceptors - transfer of nutrients and visual pigments - formation of an adhesive bond between retina and RPE• Other mechanism;

– Vitreous gel, intraocular fluid pressure, and RPE water transport

Phagocytosis

Photoreceptors

Light(radiant Source)

Oxygen(from the choroid)

Free RadicalsProduction

Damage Membranes Over time

Within the RPE the phagocytosed disc becomes encapsulated in vesicles called phagosomes

Merge with lysosome for digestion

Fatty acids are retained for recycling into outer segment

Damaged membrane material is egested across the basal RPE membrane

FACT: rods shed more vigorously in the morning whereas cones shed more in the darkness

Electrical homeostasis

• Selective ion channels and number of active or facilitative transport systems for ions and metabolites such as glucose and amino acids

• Different channels are present in both apical and basal membranes

• Movement of water across RPE is in apical to basal direction and the generation of voltage across the RPE

• The pump mechanism is very powerful and can pump fluid against a substantial gradient of hydrostatic or osmotic pressure

Repair And Regeneration

• Although of neural origin the RPE are the pluripotent tissues

• Capable of local repair and cell migration

Examples

1. Laser burns: RPE surrounding the burn begin to divide and fill the defect to form a new BRB within 1-2 weeks.

2. Retinitis pigmentosa: RPE migrate into the injured neural retina and comes to rest around vessels to contribute the characteristic bone spicule appearance

3. Macular degenerative process : vigorous RPE response can lead to duplicated layers of RPE cells and RPE scarring

• Specialized neurons in the retina

• End organs of vision that transform light energy into visual (nerve) impulse

• There are average 92 million of rods and 4.6 million of cones in human eye

2) Layers of rods & cones

Rod & Cone

RODS

• Contain photosensitive substance visual purple (rhodopsin)

• Rhodopsin has greatest sensitivity for blue-green light

• Serve for peripheral vision & vision of low illumination (scotopic vision)

• Each rod is 40-60 micron long

• Highest concentration is at nasal side( 20-25% more than temporal side) and superior side of optic disc

• Rods are absent at the fovea in an area of 0.35 mm

• Outer segment is cylindrical, transversely striated and composed of numerous lipid protein lamellar discs stacked one top of another

• Discs varies from 600-1000/rod & contain 90% of visual pigment, remaining are scattered in plasma lemma

• Inner segment – thicker than outer segment

• consists of two regions:

–Ellipsoid (outer portion): contains abundant mitochondria

–Myoid (inner portion): contains glycogen & free ribosome ( major site of protein synthesis )

• Outer rod fibre which pass through external limiting membrane swells into densely staining nucleus- (lies in ONL)

• inner rod fibre (lies in OPL) which at its end has an end bulb called rod spherules

Cones

• Cone cells is 40-80 micron long

• largest at fovea and shortest at periphery

• Highest density is at the fovea with an average of 199,000 cones/mm2

• Density of cones decreases as we move away from fovea

• Its density is 40-45% greater on nasal & slightly lower in superior retina

• Outer segment is conical in shape; shorter than that of rod & contains iodopsin

• The lamellar discs are narrower about 1000-1200 discs/cone.

• Inner segment – similar to that of rod however cone ellipsoid is very plump and contains large number of mitochondria

• It is in direct contact with nucleus & lies in O.N.L; cones inner fibres run from nucleus which end with cone pedicle in O.P.L

TYPES OF CONES

• L-cone: responds most to light of long wavelengths, peaking in the yellow region(564nm)

M-cone: responds most to light of medium-wavelength, peaking at green(533nm)

• S-cone: responds most to short-wavelength, peaking at violet color(437nm)s

• Night (scotopic) vision - rod photoreceptors

• Daylight (photopic) vision - cone photoreceptors

• Twilight (mesopic) vision - combination of cones and rods

Functions: Rods and Cones

VISUAL CYCLE

Phototransduction

In the dark:

• cGMP-gated Na+ channels kept open by high cGMP

• Na+ exiting the inner segment moves into the outer segment, through the open channel

Dark current / standing potential

Photoreceptor cells

Depolarized in the dark and Hyperpolarizes in Light

This 'switching off' that activates the next cell and sends the signal down the neural pathway depending on the type of cells

Switches off these cells

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• Photoreceptor depolarized approximately −40 mV membrane potential

• Voltage-gated Ca++ channels open

• Calcium ions facilitate glutamate release into the synaptic cleft

In the dark, photoreceptor terminal continuously release glutamate

3) External limiting membrane

• In low magnification, appears as fenestrated membrane extending from ora serrata to edge of optic disc

• Electron microscopy shows that it is formed by junctions (zonulae adherentes) between cell membrane of photoreceptor & Muller’s cell

• ELM is not a true membrane because small molecules

pass freely through junctional complex

Function of ELM

• Provides a selective barrier for nutrients that pass between adjacent Muller cells

• also involved in the stabilization of position of transducing portion of photoreceptor

4) Outer nuclear layer ( O.N.L )

• Is primarily formed by nuclei of rods and cones;

• Rod nuclei form the bulk of this multilayer except in cone dominated foveal region

• Cone nuclei are larger and lie in a single layer

• Number of rows of nuclei and thickness varies from region to region:

Nasal to disc – 8-9 layers of nuclei ; thickness is 45 µm

Temporal to disc – 4 rows of nuclei ; 22 µm Foveal region - 10 rows of nuclei (cone) ; 50

µmRest of retina except ora – 1 row of cones & 4

rows of rod; 27 µm

5) Outer plexiform layer

• Contains synapses between the rod spherules and cone pedicles with dendrites of bipolar cells and processes of horizontal cells

• Thickest at macula

• Contains predominantly of oblique fibres deviated from fovea also known as Henle’s layer

Clinical significance

Hard exudates- Formed in outer plexiform layer

by deposition of lipid and lipo-proteins

- results from abnormal vascular permeability of retinal or sub-retinal vessels

- Found in hypertension, diabetes, congenital telangiectasias

6) Inner nuclear layer

• Consists of 8-12 rows of closely packed nuclei

• Disappears at fovea

• Consists of the following :• Horizontal cells• Amacrine cells• The soma of Muller cells• Interplexiform cells.• Bipolar cells

Horizontal cells• Modulate and transform visual information

received from the photoreceptors.

• Responsible for the surrounding receptive field of Bipolar cells.

• Concentration is highest at fovea and declines towards periphery.

• Forms a network of fibers that integrate activity of photoreceptors cells horizontally.

Bipolar cells

• 2nd order neuron( approximately 35 million )

• Most of midget and diffuse bipolar cells are GLUTAMATERGIC

• The glutamate terminal establish contact with Amacrine cell process and Ganglion cell dendrites.

Excitatory Action InhibitoryAction

Synapses @ stratum 1 of IPL

Synapses @ stratum 5 of IPL

• On the basis of morphology and synaptic relationship there are 9 types of bipolar cells-

1. Rod bipolar cells2. Invaginating midget bipolar cells3. Flat midget bipolar cells4. Flat diffuse bipolar cells5. Invaginating diffuse bipolar cells6. ON- center blue cone bipolar cells7. OFF- center blue cone bipolar cells8. Giant bistraitified9. Giant diffuse invaginating.

MULLER CELLS• Inner half cells contain rough and smooth ER, golgi

apparatus, mitochondria free ribosomes protein synthesis.• Outer half of cells adapted to absorption and intracellular

transport.• Important role in orientation, displacement and

positioning of developing neurons.• Acts as principle glial cells of retina conserve structural alignment of neuronal elements.• Also produce retinaldehyde binding proteins for retinal

metabolism.

Amacrine cells• Soma are flask or urn shaped with diameter of 12 micron.

• Has single process but branches extensively.

• Nucleus is round and has prominent infoldings

• Most Amacrine cells have GABA(gamma-aminobutyric acid) and glycine as neurotransmitters having inhibitory action on ganglion cells.

Functions of inner nuclear layer

Bipolar cells• First cells in the visual pathway to respond with

an action potential• Once a threshold is reached, the ganglion cell

responds and a signal is sent to higher CNS locations

• Rod bipolars do not synapse with ganglion cells directly but with amacrine cells

OFF bipolar

• Depolarizes in dark

• Hyperpolarizes in light

The ON bipolar

• Depolarizes in light and

• Hyperpolarizes in dark

ON or OFF designation does not imply that the bipolar itself is responding to the light condition; only photoreceptors do that

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Sublamina b ≈ GCL

Sublamina a ≈ INL

Amacrine Cells

• Receive information at the synapse of the bipolar cell axons with the ganglion cell dendrites

• Bipolar cells project onto both ganglion and amacrine cells

• Negative feedback• Reciprocal inhibition

7) Inner plexiform layer

• Formed by the synapse between the axons of bipolar cells, dendrites of ganglion cell and the processes of amacrine cells.

• Absent in foveola.

8) Ganglion cell layer• Contains cell bodies of the third-order neurons of

visual pathway.

• single layered in peripheral retina, two layers on temporal side of optic disc , 6-8 layered at edge of foveola.

• 1.2millon ganglion cells are present in the retina

• absent in foveola.

Function of Ganglion Cell layer• Electrical response of bipolar cells after modification by the

amacrine cells

• Transmit the information by means of action potential

• Two types depending upon their response upon illumination of the centre of receptive

• 1. on center and 2. off center

RECEPTIVE FIELDS

• Consists of the area in the visual field or the area of the retina that, when stimulated, elicits a response in a retinal neuron

• All the photoreceptors and horizontal cells can influence it

• Receptive field is enlarged beyond its dendritic tree

• When light activates cells in the center of field, a given response occurs

• When light falls on the surround , an antagonistic response occurs

Arranged in a center-surround pattern

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The center-surround response occurs in part due to

• Lateral inhibition by horizontal cells

• Amacrine cell activity on bipolar axon terminal

The center-surround configuration allows a neuron

• To respond to a direct message

• To gather information from neighboring areas

• Provide details e.g. detection of edges

• Maximizes retinal contrast sensitivity

Single ganglion cell

250 amacrine cells

5000 rod bipolar cells

75,000 rods

Summarizing………

9) Nerve fibre layer

• Consists of unmyelinated axons of ganglion cells

• Fibers converge at the optic nerve head, pass through lamina cribrosa and become ensheathed by myelin posteriorly.

Clinical significanceCotton wool spots• white fluffy patches that lie in

nerve fibre layer• Occur in diseases causing micro

vascular ischemia like: Diabetes, HTN, SLE & venous infarcts.

Nerve fibre layer hemorrhages linear, flame shaped due to tracking along

the axons

Arrangement of nerve fibres in the retina

• From nasal half of retina come directly to optic disc as superior & inferior radiating fibres

• From macula pass straight to temporal part of disc as papillomacular bundle

• Fibres from temporal retina arch above and below the macula and papillomacular bundle as superior and inferior arcuate fibres.

Arrangement of nerve fibres of optic nerve head

Fibres from the peripheral part of retina lie deep in retina but occupy the most peripheral (superficial) part of optic disc

Fibres closer to optic nerve head lie superficially in retina and occupy a more central (deep) portion of optic disc.

Thickness of nerve fibre layer at disc:

• Thickness of N.F.L around different quadrants of optic disc margin progressively increases in following orders:

–Most lateral quadrant ( thinnest )–Upper and lower temporal quadrant–Most medial –Upper and lower nasal quadrant ( thickest )

Clinical significancePapilloedema appears first in the thickest quadrant -

upper & lower nasal and last in lateral.

Arcuate fibres which occupy superior and inferior temporal quadrant of optic nerve head are most sensitive to glaucomatous damage, accounting for early loss in corresponding region of visual field.

Macular fibres occupying lateral quadrant are most resistant to glaucomatous damage; retention of central vision till end.

10) Internal limiting membrane– It is basement membrane that forms the interface

between the retina and vitreous.

• It consists of four elements:–Collagen fibrils–Proteoglycans (mostly hyaluronic acid)–Basement membrane–Plasma membrane of Muller cell.

• It is thickest at the fovea but absent at edge of optic disc.

Blood supply of retina

• Outer four layers – get their supply from choriocapillaries ( RPE-ONL)

• Inner six layers – get their supply from central retinal artery (OPL-ILM)

• Fovea is avascular mainly supplied by the choriocapillaries

Central retinal artery

• It arises from ophthalmic artery near optic foramen.

• In retina there are four terminal branches which divide as they proceed towards ora serrata where they end without anastomosis.

• Superior nasal• Inferior nasal• Superior temporal• Inferior temporal

Retinal capillaries

Two capillary networks :

Superficial – lies at the level of nerve fibre layer

Deep – lies between inner nuclear and outer plexiform layer

Blood-Retinal Barrier

• Outer – tight junction between RPE cells.

• Inner – endothelial cells of retinal capillaries are bound closely by intercellular junction of zonula occludens - thus prohibiting free flow of fluid and solutes.

• These endothelial cells are encircled by basement membrane around is a layer of pericytes.

Clinical significance

Diabetic retinopathy - there is loss of pericyte,

increase in thickness of basement membrane & damage to endothelial cell.

REFERENCES

Anatomy and physiology of eye by A.K. Khurana Ophthalmology by Myron Yanoff & Jay S. Duker Wolff’s anatomy of the eye & orbit by Bron, Tripathi &

Tripathi Internet

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