Andersons pediatric cardiology 1347
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thesecondheartfield.Thenewlyaddedtissuesarenonmyocardial(seeFig.
51.2B).Thesenonmyocardialwallsextendinitiallyasaringwithtwoparietal
tongues.Theadditionofthenonmyocardialtissuesdistallyservestoproducean
effectiveregressionofthedistalmyocardialborderawayfromthemarginsofthe
pericardialcavity.1–3Concomitantwiththisproximalmovementofthedistal
myocardialborderofthetube,thejellythatinitiallysurroundedthecommon
lumenischanged,bytheprocessofendothelial-to-mesenchymaltransformation,
intopairedcushions.Theyspiralwhentracedtowardtheventricularoriginofthe
tube.Wheninitiallyformed,theirdistalmarginsareconfluentwiththe
myocardialborder,whichhasamarked“fishmouth”appearance.As
developmentproceeds,thecushionsbegintofuseindistaltoproximaldirection.
Thisdividesthepartsofthetubewithmyocardialwallsintoseparatechannels
thateventuallyexitfromtherightandleftventricles.However,atthestartof
fusionofthecushions,theproximalborderofthetubeissupportedexclusively
bytherightventricle(seeFig.51.2B).Withcontinuingfusionofthecushions,
thereisongoingproximalregressionofthedistalmyocardialborder.Thisis
accompaniedbyadditionofstillfurthernonmyocardialtissuestothedistalpart
oftheoutflowtract.Consequenttothesechanges,byCarnegiestage15in
humansandduringembryonicday11.5inthemouse,itbecomespossibleto
recognizeadiscretenonmyocardialdistalcomponentofthetube.The
componentthathasretaineditsmyocardialwallscanitselfnowbeidentifiedas
havingdistalandproximalparts,withformationoftwoadditionalcushions,
knownastheintercalatedcushions,initsdistalpart(Fig.51.3A).
FIG.51.3 Imagestakenfromepiscopicdatasetspreparedfrom
developingmice.(A)Long-axissectioncomingfromanembryosacrificed
atembryonicday11.5.(B)Short-axissectionfromamousesacrificedat
embryonicday12.5.Theintercalatedcushions(stars)areformedinthe
intermediatepartoftheoveralloutflowtract.However,thispartisthedistal
componentofthetractthatretainsitsmyocardialwalls.PanelBshowshow
theinterdigitationsoftheintercalatedcushionswiththeunfusedparietal
partsofthemajorcushionsproducestheprimordiaofthedeveloping
arterialroots.Atthesestages,therootshaveyettoseparatefromeach
other.
Thedistalpartthathasretaineditsmyocardialwallsnowoccupiesthe
intermediatecomponentoftheoveralloutflowtract(seeFig.51.3A).Itisthe
nonmyocardialdistalpartoftheoveralloutflowtractthatnowbecomes
separatedintotheintrapericardialarterialtrunks.Thisisachievedbygrowthinto
theoutflowtractofanobliqueprotrusionfromthedorsalwalloftheaorticsac.
Theprotrusiongrowsfrombetweentheoriginsofthearteriesofthefourthand
sixthpharyngealarches,whichwillbecomethesystemicandPAs,respectively.
Therefore,asitgrowsintothepericardialcavity,theprotrusionisanembryonic
APseptum.Concomitantwithitsgrowthtowardthedistalmarginsofthemajor
outflowcushions,thecushionsthemselveshavefusedtoseparatethe
intermediatepartoftheoutflowtractintothearterialroots.Thespacebetween
theleadingedgeoftheprotrusionandthedistalmarginsofthefusedoutflow
cushionsisanembryonicAPforamen(seeFig.51.3A).Ifdevelopmentproceeds
normally,theprotrusionfuseswiththecushions,obliteratingtheAPforamen,
andtheintrapericardialarterialtrunksrapidlydeveloptheirownwalls.Itisthen
nolongerpossibletorecognizeanyseptaltissuesbetweenthem.Failuretoclose
theembryonicforamenaccountsforpersistenceofAPwindows.4However,itis
amistaketoconsiderthelesionsasaortopulmonaryseptaldefects.Abnormal
developmentofthedistaloutflowtractalsoprovidesarationalexplanationfor
directaorticoriginofoneofthePAs.Astheprotrusionfromthedorsalwallof
theaorticsacgrowsintothecavityofthedistaloutflowtract,itdoessoin
markedlyobliquefashion.ItisfrequentlystatedthatthePAsarederivedfrom
thesixtharcharteries;thisisnotthecase.TherightandleftPAsdevelopwithin
thepharyngealmesenchyme,takingoriginfromthesixtharcharteriescloseto
theirownoriginfromtheaorticsac(Fig.51.4).Thereforeitiseasytoenvisage
thatincreasingordecreasingobliquityoftheprotrusioncouldleaveoneorother,
usuallytheright,PAincontinuitywiththesystemiccomponentoftheaorticsac.
FIG.51.4 Imagesmadefromepiscopicdatasetspreparedfrom
developingmicesacrificedatembryonicday11.5.(A)Reconstructionof
thelumenoftheaorticsac,whichgivesrisetothearteriesofthethird,
fourth,andsixthpharyngealpouches.Thepulmonaryarteriestaketheir
originfromthesixtharcharteriesclosetothesac.(B)Theyextend
caudallywithinthepharyngealmesenchyme,whichisanobliquecut
throughtheleftsideofthesac.
Asexplainedearlier,bythetimetheembryonicAPforamenhasclosed,
producingtheseparateintrapericardialarterialtrunks,thedistalendsofthe
majoroutflowcushionshavethemselvesseparatedtheintermediatepartofthe
outflowtractintotheputativeaorticandpulmonaryroots.Theinterdigitationof
themajorandthenewlyformedintercalatedcushionswithinthispartofthe
