Andersons pediatric cardiology 1348
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outflowtractprovidesthescaffoldforformationofthearterialvalves(seeFig.
51.3B).However,atthisstage,whichoccursduringembryonicdays11.5and
12.5inthemouseandbyCarnegiestage15inhumans,therehasbeenno
formationofthearterialvalvarsinuses.
Thewallsofthearterialvalvarsinusesareformedbystillfurtherproximal
growthofthenonmyocardialtissuesderivedfromthesecondheartfield.This
additionalgrowthisaccompaniedbyexcavationofthedistalmarginsofthe
cushionsthemselvestoformtheleafletsandtheirsemilunarhinges.5Duringthe
processofendothelial-to-mesenchymaltransformation,thecushionsthemselves
havebeeninvadedbycellsmigratingfromtheneuralcrest.1,3Theneuralcrest
cellsthenformcolumnsofcondensedmesenchymeinthecentralpartsofthe
unfusedproximalcushions.Someinvestigatorshavedescribedthesestructures
asproducingan“aortopulmonaryseptalcomplex.”2However,asdescribedin
Chapter3,thecolumnscannotbeidentifieduntilaftertheformationofthe
intrapericardialarterialtrunks.Theneuralcrestcells,nonetheless,doplayan
importantroleinseparatingthearterialtrunksfromeachother.Theprotrusion
fromthedorsalwalloftheaorticsacisitselfcoveredbycellsderivedfromthe
crest,althoughthecoreoftheprotrusionisderivedfromthesecondheartfield.1
Itistheongoinggrowthofthetissuesfromthesecondheartfieldthatformthe
wallsofthearterialvalvarsinuses,withthecellsderivedfromtheneuralcrest
beingconfinedtothevalvarleafletsandtheirhinges,albeitwithdifferential
contributionstotheintercalatedcushions.Subsequenttofusionofthecentral
partsofthemajoroutflowcushions,thecoreofthecushionmassattenuates.
Thisareabecomesconvertedintothefibroadiposetissuethat,inthepostnatal
heart,separatestheaorticandpulmonaryroots.Asimilarprocesstakesplacein
theproximalpartoftheoutflowtract(Fig.51.5).
FIG.51.5 Imagestakenfromepiscopicdatasetspreparedfrom
developingmice.Thesesectionsreplicatetheobliquesubcostal
echocardiographiccutandaretakenfrommicesacrificedatembryonicday
14.5(A)and15.5(B).Theimagesshowthechangesthattakeplaceduring
remodelingoftheoutflowcushionsintothehingesandleafletsofthe
arterialvalves,alongwiththefreestandingsubpulmonaryinfundibular
sleeve.Abnormalremodelingofthehingesoftheaorticvalvecancreate
thepotentialforchannelseitherleadingbackintotheleftventricleorinto
therightventricle.Suchabnormalremodelingproducestheaortoventricular
tunnels.
Whenthemajorcushionsbegantheirfusion,theoutflowtractwassupported
exclusivelybythedevelopingrightventricle.Asthefusioncontinues,thecaudal
partoftheproximaloutflowtractistransferredacrossthecrestofthemuscular
ventricularseptumtoarisefromthedevelopingleftventricle.Thisbringsthe
fusedcushionsthemselvesintolinewiththeventricularseptalcrest.Atthesame
time,thesurfaceofthefusedcushionsbeginstomuscularize,whilethecoreof
thecushionsmassbeginstoattenuate(seeFig.51.5A).Bythebeginningof
embryonicday14.5inthemouseandbyCarnegiestage18inhumans,the
persistingpartoftheembryonicinterventricularcommunicationhasbeenclosed
bygrowthoftheso-calledtuberclesderivedfromtheventricularsurfacesofthe
majoratrioventricularcushions(seeFig.51.5A).Duringthesestagesthedistal
partsofthecushionsremodeltoformthearterialvalvarleaflets,with
myocardiumderivedfromtheproximaloutflowtractincorporatedintothebasal
partsofthedevelopingleftventricularmyocardialcone.Abnormaldevelopment
ofthecushionmassanditsmyocardialsupportgivesthepotentialforabnormal
communicationsaroundthehingesofthedevelopingaorticvalvarleaflets(see
Fig.51.5B).Suchcommunicationsbypassingthevalvarleafletsaretheessence
ofthevariousformsofaortoventriculartunnel.6Itislikelythatabnormal
developmentofthejunctionbetweenthemyocardialandnonmyocardial
componentsoftheintermediatepartoftheoutflowtractalsosetsthescenefor
eventualaneurysmalformationofthevalvarsinuses.Itisunlikelytobea
coincidencethatsuchaneurysmalformationisassociatedbothwithformationof
thetunnelsandwithdoublycommittedandjuxtaarterialventricularseptal
defects(VSDs),thelatterdefectsformedconsequenttofailureof
muscularizationoftheproximaloutflowcushions.
Itiscurrentlymuchhardertoprovidearationalexplanationastowhythere
shouldbeformationofthreearterialrootsorwhytherightandleftPAsshould
haveseparateventricularoutlets.Itshouldberemembered,nonetheless,thatitis
anormalfindinginreptilesandcrocodilianstofindthreearterialtrunksarising
fromtheheart.7,8Furthercomparisonsbetweendevelopmentinmammalsand
theseotherphylamaywellcastfurtherlightontheseproblems.9
AortopulmonaryWindows
APwindowsprovidecommunicationsbetweenthecavitiesofthe
intrapericardialarterialtrunksbutinthepresenceofseparateaorticand
pulmonaryroots(Fig.51.6).10Theycanbefoundwithatresiaofoneorother
arterialvalve,thenprovidingaccesstotheotherwiseblind-endingcirculation.
Thepresenceofseparateaorticandpulmonaryrootsservestodistinguishthe
windowsfromcommonarterialtrunk,asitdoesfromsolitaryaortictrunk.The
latterentityisfoundwhenthereiscompleteabsenceoftheintrapericardialPAs.
Theseparatenatureofthewallsoftheintrapericardialarterialtrunksmeansthat
itisincorrecttodescribethelesionsas“aortopulmonaryseptaldefects.”Aswe
haveshown,thelesionsareduetofailureofclosureoftheembryonicAP
foramen.4Smallwindowscanbefoundadjacenttothesinutubularjunctions
(Fig.51.7A).
