Andersons pediatric cardiology 99
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arterialrootsdisappears,leavingthearterialvalvarsinusesformingtheexternal
wallsofthepulmonaryandaorticroots.
FIG.3.31 Imagefromanepiscopicdatasetpreparedfromadeveloping
mouseeuthanizedatembryonicday13.5.Thetuberclesfromthe
ventricularaspectoftheatrioventricularcushionshaveclosedthe
persistingembryonicinterventricularcommunication(star).Thedistalparts
oftheoutflowcushionsintheintermediatepartoftheoutflowtracthave
nowseparatedtoformtheaorticandpulmonaryroots.Theproximal
cushionshavefused,withtheirrightventricularsurfacebeginningto
muscularizetoformthesubpulmonaryinfundibulum.Theattenuationofthe
centralcoreofthecushionmassisbeginningtoattenuate.Thisareawill
becomethefibroadiposetissuethat,inthepostnatalheart,separatesthe
infundibulumfromtheaorticroot.
FormationofValves
Thevalvarstructuresseenatthesinuatrialjunctionaremostconspicuousduring
theearlierstagesofdevelopment.Whenfirstseen,aspaceisseenbetweenthe
left-sidedvalveandtherightsideofthedevelopingatrialseptum.Thisisthe
bodyoftherightatrium,butitssitecanonlyrarelyberecognizedinthemature
heart,sincetheleftvenousvalveusuallyregressesinitsentirety.Therightvalve
ofthesystemicvenoussinus,incontrast,doesremainrecognizable,beingseen
tovariousextentspostnatallyindifferentindividuals.Itsventralportionpersists
toguardtheorificeofthecoronarysinus,andisknownasthethebesianvalve.
Thedorsalpartpersistsastheeustachianvalve,whichguardstheorificeofthe
inferiorcavalvein.Iflargerpartsofthevalvepersist,however,theycandivide
therightatrialchamber,orformaChiarinetwork.
Thecushionsformingwithintheatrioventricularcanalbytheprocessof
epithelial-to-mesenchymaltransformationserveasvalvesintheearlystagesof
development.Theythenprovidethescaffoldforformationofthedefinitive
valvarleaflets.Theleftventricularcomponentsfusetoformtheaorticleafletof
themitralvalve,withtherightventricularpartsprovidingtheseptalleafletofthe
tricuspidvalve.Additionalcushionsdevelopwithinthelateralpartsofthe
atrioventricularcanaltoprovidetheprimordiumsoftheothervalvarleaflets.
Althoughthereisinitialdelaminationofventricularmyocardiumatthesitesof
thecushions,analysisoflineageshowsthatthedefinitiveleafletsdonothavea
myocardialheritage.19Itfollowsthatthedelaminatingmyocardiummust
subsequentlydisappear.Theinitialtrabecularcomponentoftheventricularwalls
compactstoformthepapillarymusclesofthevalves.Notalltheleafletsmature
atthesametime.Formationoftheseptalleafletofthetricuspidvalve,in
particular,isanextremelylateevent.Itisthefinalstepsofunderminingofthis
leafletthatseparatestheatrioventricularandinterventricularpartsofthe
membranousseptum.10Theleafletsofthearterialvalvesareformedby
excavationofthedistalendsofthemajorandintercalatedcushionswithinthe
intermediatepartoftheoutflowtract,withthevalvarsinusesformedbyongoing
migrationofnonmyocardialtissuesfromthesecondheartfield.
ConductionSystem
Itisrelativelylateindevelopmentthatareasofmyocardiumcanberecognized
assatisfyingthehistologiccriterionsoftheso-calledconductingtissues.20Long
beforethesestages,however,itispossibletorecordanelectrocardiogramfrom
thedevelopingembryo.Thisisbecauseallmyocardialcellswithintheheart
havetheabilitytoconductthecardiacimpulse.Theelectrocardiogramisseenas
soonasthehearttubehasdevelopedtocreateareaspermittingfastasopposedto
slowconduction.Thisisseenconcomitantwiththeappearanceofthechamber,
orsecondary,myocardiumthatballoonsfromthelinearhearttube(seeFig.3.5).
Thechambermyocardiumconductsrapidly,itscellsbeinglinkedbymultiple
connexinsthatareabsentfromtheslowlyconductingprimarymyocardiumof
thelinearhearttube.Attheearlystages,theprimarymyocardiumformsthe
atrioventricularcanalalongwiththeoutflowtract.Italsopersistsasacorridorof
myocardiumextendingfromtheatrioventricularcanaltoincorporatetheorifices
ofthesystemicvenoustributaries.Inthedevelopingmurineheart,this
myocardiumcanberecognizedbyitscontentoftheTbx3gene.Thisgenemarks
theentiretyoftheatrioventricularcanalatanearlystage,alongwitharingof
cellssurroundingtheembryonicinterventricularcommunication.Inthehuman
heart,thisringismarkedbyanantibodytothenodoseganglionofthechick(see
Fig.3.18).Thepartofthisringonthecrestofthemuscularventricularseptum
willbecometheatrioventricularconductionaxis.Withongoingdevelopment,
mostoftheatrialtissuesinitiallypositivefortheTbx3genefailtoretainthis
feature.Eventually,onlytheatrioventricularnodeandthesinusnoderemain
Tbx3positive.ThelocationofthetissuesinitiallypositiveforTbx3withinthe
atrialvestibules,aroundthemouthofthecoronarysinus,andalongtheterminal
crestofferssomeexplanationfortheoriginsofarrhythmicactivityinpatients
withatrialarrhythmias.21ThetractsoftissuevisualizedbycontentofTbx3
extendingbetweenthesiteofthesinusnodeandtheatrioventricularcanalis
indicativethat,earlyindevelopment,allofthisareawasmadeupofprimary
myocardium.Withongoinggrowthandmaturation,theinternodalareabecomes
convertedintoworkingatrialmyocardium.Tissueswithintherightatrial
vestibule,incontrast,persistasrecognizablenode-likestructures.Theseentities
wererecognizedbyKentattheturnofthe19thcentury,buterroneously
interpretedbyhimasprovidingmultiplemuscularconnectionsacrossthe
