AnatomyandPhysiologyoftheRetina
RelevanttoInheritedRetinalDisease
atutorialfrom
StoneRounds.org
EdwinM.Stone,M.D.,Ph.D.Editor
©TheUniversityofIowa,2018
Page 2Thisistherighteyeofahumandonorviewedfromabove.Youcantellthatit’stherighteyebecausetheopticnerveexitstheeyeonthenasalsideofcenter.
Thetransparentfrontwalloftheeyeisknownasthecorneaandthecurvedsurfaceofthisstructureprovidesabout75%oftheconverginglenspowerneededtofocusimagesonthebackoftheeye.
Theremaining25%ofthefocusingpowercomesfromtheflexiblecrystallinelens,whichactuallybecomesthickerwhenapersonlooksanearobjectsduetocontractionofacircumferentialmuscleknownastheciliarybody.Thethincoloredstructurebetweenthecorneaandthelensistheiriswhichfunctionstocontroltheamountoflightenteringtheeye.
Theposteriortwothirdsoftheeyeislinedbyaverythinlayerofnervetissue–theretina.Inlife,theretinaiscompletelytransparentandthebrownishororangecolorthatyouseewhenlookingintotheeyewithanophthalmoscopeisduealmostentirelytothebloodfilledcapillariesandpigmentedstructuresbeneathit.
Thecentralportionoftheretina-shownhereinorange-isknownasthemacula.
Thisisacolorphotographoftheretinaoftherighteyeofanormalindividual.
Theopticnerveis1.5mmindiameterandthebloodsupplyoftheinneraspectoftheretinaemanatesfromthecenterofthenerveandsweepsaroundthemacula.
Thevenulescanbedistinguishedfromthearteriolesbecauseoftheirthickercaliberanddarkerpurplecolor.Theselargecalibervesselssupplyanddraincapillaryplexusesthatlieprimarilyintheganglioncellandinnernuclearlayers.
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Theretinahasaseconddistinctbloodsupplythatliesbetweentheretinalpigmentepitheliumandthesclera.ThefinecapillariesadjacenttotheRPEareknownasthechoriocapillarisandthesevesselsprovidethetremendousamountofoxygenneededbythephotoreceptorcells.
Opticalcoherencetomographyisaclinicalimagingtechniquethatcanprovideimagesoftheretinainlivingindividualswithnear-histologiclevelsofresolution.
Computerizedimageanalysisoftheseimagescan“segment”thetomogramintolayersthatcorrespondtothedifferentanatomicallayersoftheretina.Asinthegrossanatomyofthebrainthedarkergreylayerscorrespondtocollectionsofcellbodieswhilethelightergreyareascorrespondtothedenselypackedaxonsanddendritesthatconnecttheseneuronstoeachother.
ZoominginonasmallsectionoftheretinawecanillustrateourcurrentunderstandingoftheanatomiccorrelationswiththevaryingintensitiesoftheOCTimage.Withpropercontrols,onecandetectanatomicalchangeswitharesolutionofafewmicronsusingthisinstrument.
Hereisanartist’sconceptionofthemostimportantretinallayersthatmakestherelationshipsofthesecellsalittleeasiertovisualize.
Page 4Itisimportanttonotethatthereare20timesmorerodphotoreceptorsthanconesinthehumanretina,andthattheshortwavelengthsensitiveconesaretheleastnumerousofall.
Thelowestratioofphotoreceptorstoganglioncellsoccursinthefovea–approximately5toone--whichenablestheveryhighacuityvisionassociatedwiththisregion.Intheperiphery,asingleganglioncellcanreceiveinputfrommorethan1000photoreceptors.
Hereisanotherviewofthevariationindensityofvariousretinalneuronswithincreasingeccentricityfromthefovea.
Theveryhighdensityofconephotoreceptorsinthefoveahelpsexplainwhycone-selectivediseasesareoftenassociatedwithdecreasedacuity.
Therods’peakdensityoccursmorethan3millimetersfromthefovealcenterandislikelytheexplanationfortheringscotomasthatarecharacteristicoftherod-selectivediseaseretinitispigmentosa.
Theganglioncellsarethemostnumerousinthecentral5millimetersoftheretina.Oneofthereasonsthatourinitialphotoreceptorprecursortransplantswillbeplacedinthisregionisbecauseofthegreaterchancethesetransplantswillhavetoestablishsynapticconnectionwiththerelativelynumerousinnerretinalneuronsofthisregion.
RichardYoung’senfacephotomicrographsoftheprimateretinagiveusanotherviewofhowtheretinalmosaicchangeswithincreasingeccentricity.Thefovealcenterconsistsentirelyofunusualconeswithveryslenderoutersegments.
Onemillimeterfromthefovealcenter,aringofrodscanbeseensurroundingeachconeandoutsidethemacula,theratioofrodstoconesisevenhigher.
Ifwetakeaimmunohistochemicalsectionthroughthefoveaofahumandonoreyewecanseetheabsenceofrhodopsin-containingrodsnearthefovealcenteraswellasthewayinwhichthe
Page 5ganglioncellsthataresynapticallyrelatedtothefovealconesaredisplacedlaterally.Youcantellthatthissectiondoesnotpassthroughtheabsolutecenterofthefoveabecauseifitdid,theinnernuclearlayerwouldalsobedisplacedlaterally.
Fordescriptivepurposes,themaculahasbeendividedintosixconcentriczonesandthedimensionoftheouterboundaryofeachofthesezonesisshownhere.Clinically,thefoveolaandfovealavascularzoneareessentiallysynonymous.Thefoveaisthesamediameterastheopticnervehead–1.5millimeters.Theprefix“para”means“nextto”andtheterm“parafovea”ismostappropriatelyusedforfindingsthatlieinazonegreaterthanonebutabitlessthantwodiskdiameterscenteredonthefoveallightreflex.Theprefix“peri”means“around”andtheterm“perifovea”isusedforaringonediskdiameterinthicknessattheoutermostedgeofthemacula.
Thereasonthatweareinterestedintheunevendistributionsofthevariouscelltypesintheretinaisthatitislikelythattheregionaldifferencesinretinalanatomyplayanimportantroleinthediseasemechanismsofmanyoftheconditionswe’reinterestedinandanunderstandingofthesemechanismswillbeimportantinthesuccessfultreatmentoftheseconditions.
Oneothertypeofregionalspecializationweshouldtouchonbrieflyinvolvestheinnermostlayersoftheretina–theganglioncellsandtheiraxons.
Hogan’sclassicdrawingshowshowtheaxonsofganglioncellsnasaltothefoveaapproachtheopticnerveheadfairlydirectlywhilethosetemporaltothefoveasweepaboveandbelowthemaculatominimizetheamountoftissuelyingbetweentheincomingphotonsandthemacularphotoreceptors.
Oneofthepracticaleffectsofthisanatomicspecializationisthatganglioncellsthatarequiteclosetoeachotherinthetemporalretinacansendtheiraxonstotheopticnervebyquitedifferentroutes.Thisistheanatomicalexplanationforthe“nasalstep”observedinthevisualfieldsofmanyglaucomapatients.
Page 6Thelinealongwhichthissuperior/inferiorroutedecisionismadeisknownasthehorizontalraphe.
Becauseofthelowratioofphotoreceptorstoganglioncellsinthemacula,therearealargenumberofaxonsthatoriginatethereandgodirectlytotheopticnerve.Theseaxonsarerequiredforhighacuityvisionandareknownasthepapillomacularbundle.
Now,let’sspendafewminutesreviewingsomeofthephysiologyofthemoreimportantcelltypesintheretina.Thebasicwiringoftheretinaconsistsofjustthreeneuronsthephotoreceptorcellthebipolarcellandtheganglioncell.Theaxonsofthephotoreceptorsandbipolarcellstravelonlyafewdozenmicronsbeforesynapsingonthenextcellbuttheaxonsoftheganglioncellstravelover90millimeters–threeandahalfinches--togettothelateralgeniculatenucleusofthethalamus.
Allthreeoftheseneuronsuseglutamateastheirneurotransmitter.
Thephotoreceptorsandbipolarcellshavegradedinputsandoutputsbuttheganglioncellsconverttheirgradedinputsintotrainsofactionpotentialsthattraveldownmyelinatedneurons.Tomaintainthetransparencyofthenervefiberlayer,themyelindoesn’tbeginuntiltheaxonsexittheeye.
Somepeoplewonderwhyevolutionplacedthelightdetectingphotoreceptorsclosesttothescleraandalloftheotherneuronsbetweenthemandtheincominglight.
Thereasonisbecauseoftheessentialfunctionsperformedbytheretinalpigmentepithelium.TheRPEistheretina’sprimarysourceofthelightsensitivechromophore11-cisretinal.
Aswe’llseeinmoredetailinjustaminute,theprocessthatrecycles11-cis-retinalfromtheall-trans-retinalreleasedfromthephotoreceptorsisknownasthevisualcycleandittakesplaceoveraperiodoftensofminutes.Thisistheprocessthatyouexperiencewhenyoureyes“getusedtothedark”inamovietheatreorwalkingonatrailatnightonacampingtrip.
Theprocessthatconvertsastreamofphotonstoanincreaseinphotoreceptormembranepotentialisknownasphototransduction.
Thisprocessisfourordersofmagnitudefasterthanthevisualcycle–lessthan1/30thofasecondinconephotoreceptors.
Let’slookatthemechanismofphototransductioninabitmoredetail.
Page 7Todothiswe’ll“magup”ontheoutersegmentofarodphotoreceptor,showingaflattenedlipid-bilayerdiskcontainingatransmembraneproteinknownasrhodopsin.
Rhodopsinisa“G-protein-coupledreceptor”thatmediatesthefirststepofphototransduction.
ThelightsensitiveformofrhodopsincontainsamoleculeofvitaminAknownas11-cis-retinalcovalentlylinkedtoitatlysine296.
When11-cisretinalabsorbsaphoton,itisomerizestoalltransretinal.Thatisomerizationcausesaconformationalchangeinrhodopsinwhichsetsofftheremainderofthephototransductioncascade.
Let’sreturntothediagramoftheoutersegmentdiskandcellmembranetoseethemainstepsinthecascade.
Inthedark,acyclicnucleotidegatedcationchannelisopenandsodiumionsflowinfreely,therebyloweringthemembranepotential.
WhenlightstrikesavitaminAmoleculeboundtorhodopsin,itisomerizesthevitaminAtoall-trans,whichcausesaconformationalchangeintheprotein.
Thisconformationalchangedissociatesrhodopsin’strimericGprotein,knownas
Page 8transducin.ThealphasubunitoftransducinactivatesaphosphodiesterasewhichcleavesthecyclicGMPthathasbeenkeepingthecationchannelopen.Whenthechannelcloses,therestingmembranepotentialrises–diminishingthereleaseofneurotransmitterfromthesynapticterminalsofthephotoreceptoraxon.
Sotoreviewthemainproteinsinthephototransductioncascade:lightfallingonthe11-cisretinalboundtorhodopsincausesittochangeconformation,thisreleasesalphatransducinwhichactivatesphosphodiesterase,loweringcyclicGMP,closingthecationchannel,elevatingthemembranepotential,andreducingneurotransmitterrelease.
Theevolutionarypurposeofthiselaboratecascadeislow-noiseamplification–asinglephotonfallingonarodphotoreceptorcanincreaseitsmembranepotentialbyamillivolt–thisrepresentsatleastaonemillionfoldamplificationoftheinputsignal.
Theby-productofphototransductionisall-trans-retinal.Forthismoleculetobeabletodetectanotherphoton,itneedstobere-isomerizedto11-cis-retinal.
Thisprocessisknownasthevisualcycleandasyouwillsee,someofthestepstakeplaceoutsidethephotoreceptors.Also,theprocessisabitdifferentforrodsandcones.
Theprocessbeginswhenall-trans-retinalisreleasedfromrhodopsinintothelipidbilayeroftheoutersegmentdisks.There,thealdehydegroupoftheretinalisexposedtotheaminogroupsofthepolarheadsofsomeofthemembranelipidsandareversiblecovalentbondformscreatingamoleculeknownasN-retinylidenephosphatidylethanolamineorN-ret-PE.
TheproteinencodedbythegeneABCA4isanATP-bindingcassettetransporterthatflipsN-Ret-PEtotheouterleafletofthediskmembrane.
There,thealltransretinalismetbyadehydrogenase--RDH8--thatremovesitfromthephospholipidandconvertsittoalessreactivealcohol.
Page 9Itisthisall-trans-retinolformofvitaminAthatleavesthephotoreceptorfortransporttotheretinalpigmentepitheliumorMuellercellsboundtoaproteinknownastheinterphotoreceptorretinoidbindingproteinorIRBP.
Aftercrossingtheplasmamembraneoftheretinalpigmentepitheliumthealltransretinolmovestotheendoplasmicreticulumwhereitisesterifiedtoalltrans-retinylesterbythelecithinretinolacyltransferase,commonlyreferredtoasLRAT.
Theseall-trans-retinylestersareinturnacteduponbyRPE65,whichde-esterifiestheretinolandre-isomerizesittothelightsensitive11-cisisomerofvitaminA.
The11-cisretinolisthenoxidizedto11-cisretinalbymembraneboundretinoldehydrogenasessuchasRDH5.The11-cis-retinalthenmovestothecellsurface,crossestheplasmamembraneandisreturnedtothephotoreceptorcellbyIRBP.
Thevisualcycleiscompletewhenthe11-cisretinalreturnstothebindingpocketofrhodopsin,formsacovalentbondwithlysine296,andrestoresrhodopsintothelightsensitivestate.
Tosummarizethekeystepsofthevisualcycle:
Lightisomerizes11-cisretinaltoall-trans-retinal,whichthenbindstothediskmembraneasN-retPE.
ABCA4flipstheN-retPEtothecytoplasmicleafletofthediskmembrane.
RDH8reducesthe11-cis-retinaltoall-trans-retinol.
IRBPcarriesall-trans-retinoltotheretinalpigmentepitheliumwhereRPE65re-isomerizesitto11-cis-retinolandRHD5oxidizesitto11-cis-retinal.
IRBPthenreturnsthe11-cis-retinaltothephotoreceptor.
AutosomalrecessiveStargardtdisease-iscausedbyadeficiencyoftheATP-bindingcassettetransporterA4,whichaswehavejustseenusuallyflipsN-retinylidenephosphatidylethanolaminefromtheinnertotheouterleafletoftheoutersegmentdiskmembrane.
Page 10BecauseofthedysfunctionofABCA4,theN-ret-PEgraduallyaccumulatesontheinnerleafletuntiltheconcentrationreachesapointthatasecond-retinylidenemoleculebindstoasingleethanolaminecreatinganirreversibleandinsolublebis-retinoidknownasA2PE.
A2-PEisdirectlytoxictothephotoreceptorsbutalsoaccumulateswithinandbeneaththeretinalpigmentepitheliumintheformoftheophthalmoscopicallyvisiblepisciformflecksthatarecharacteristicofStargardtdisease.