Andersons pediatric cardiology 692
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FIG.26.19 Productionofisomericatrialappendagesbygenetic
manipulationofdevelopingmice.Thetoppanel,whichshows
unequivocallyrightatrialappendagesbilaterallyasrevealedbyscanning
electronmicroscopy,wasproducedbyknockingoutthePitx2gene.The
bottompanelshowsafrontalhistologicsectionfromamouseinwhich
Lefty-1wasknockedout,producingisomericleftatrialappendagesanda
commonatrioventricular(AV)junction.(Bottompanel,CourtesyProfessor
C.Meno,KyushuUniversity,Japan.)
IsomerismandConjoinedTwinning
Thereisafascinatingassociationbetweenconjoinedtwinsanddisruptionof
normalleft-rightasymmetry.Complexcardiacmalformationsandabsenceofthe
spleeninoneofthetwinshavebeenwelldescribed.27,30–32Themostcomplex
cardiacanomaliesareusually,butnotalways,observedintheright-handtwin
(seeChapter48).Featuresofleftisomerism,incontrast,arerarelyobserved.
Morphogenesis
Mostearlystudiesofthedevelopmentofthe“splenicsyndromes”concentrated
theirattentionuponthespleen.3,9Whileofundoubtedvalueintermsof
knowledgeofsplenicdevelopment,thisapproachdidlittletoclarifythegrossly
abnormalcardiacdevelopmentassociatedwithisomerism.Here,aswiththe
analysisoftheheartitself,thesignificantfeatureistheisomericnatureofthe
atrialappendages.Thesignificantpointfromthestanceofdevelopmentisthat
theapicalpartsoftheventricles,whichconfermorphologicrightnessorleftness,
ballooninseriesfromtheinletandoutletpartsoftheventricularcomponentof
theprimaryhearttube,whiletheatrialappendagesballooninparallelfromthe
atrialcomponent(seeChapter3).Thereforeitisnotsurprisingthattheisomeric
malformationshouldproduceduplicationoftheatrialappendageswithout
producingsymmetryoftheventricles.Thevenousmalformationsareequally
wellexplained.Withbilateralmorphologicallyrightatrialappendages,therewill
benofocusforincorporationofthepulmonaryvenouscomponent.Innormal
development,thisoccursbylumenizationofthepulmonaryveininthe
developingdorsalmesocardialconnectionbetweenthehearttubeandthebody
(seeChapter3).Theseconnectionsaregrosslyabnormalwhenbothappendages
developwithrightmorphology.Thereforetheintrapulmonaryvenousplexuses
developinginthemediastinumwilljoinupwithsuitablesystemicchannelsto
formtotallyanomalousconnections,orelsewillconnectdirectlybut
anomalouslytotheroofoftheatrialchambers.Persistenceoftheinitialbilateral
symmetryofthesystemicvenoustributariesaccountsfortheusualfindingof
bilateralsuperiorcavalveinsandabsenceofthecoronarysinus.Thebilateral
formationoftheterminalcrestsaccountsforthedevelopmentofbilateralsinus
nodes.Sincetheinferiorcavalveinisabletodraininnormalfashiontotheatrial
chamber,thehepaticveinswillalsodevelopnormally.
Whenboththeatrialappendagesdevelopmorphologicallyleftcharacteristics,
thepulmonaryvenouschannelscanbeincorporatedintoeithersideofthe
atrium,accountingforthecommonfindingofbilateralpulmonaryvenous
connections.Becausetheterminalcrestdevelopsfromthemorphologicallyright
side,thesuperiorcavalvenouschannelswillneverdraininnormalfashionin
heartswithleftisomerism,andtherewillbenopotentialforformationofa
normalsinusnode.Sincetheventriclesdevelopinseries,theycanforminany
fashionasoccursinmalformationswithoutisomerism.Thisisreflectedinthe
