Andersons pediatric cardiology 1287
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Introduction
Theso-calledRathkediagram,showingsixpairsofarteriesencirclingthe
tracheoesophagealpedicle,isstillfrequentlyusedtoprovideanunderstandingof
themorphogenesisofabnormalitiesofthebrachiocephalicarteriesinhuman
patientswithcongenitallymalformedhearts(Fig.48.1).Theexistenceofsixsets
ofarteriesinmammalianspecies,however,hasprovedcontroversialforwell
overacentury.Aslongagoas1907,thepotentialcandidatesforthepurported
fiftharcharterieshadbeenreviewed,withtheconclusionreachedthatnoknown
arrangementparalleledthismorphologyseeninloweranimals.1Subsequentto
aninvestigationofnormaldevelopment,itwasarguedthatfurtherstudieswere
requiredbeforeitcouldjustifiablybepresumedthatsucharteriesofthe
purportedfifthpharyngealarchwerepartofnormaldevelopment.2Asyet,
despiteseveralsuchfurtherstudies,evidencefortheexistenceoffiftharch
arteriesremainsscant.Extensivestudiesperformedindevelopingmouse
embryos,forexample,3,4havefailedtoprovideevidencenotonlyforthefifth
archarteriesbutalsoforthepresenceofsegmentsofpharyngealmesenchyme
thatmightcontaintheallegedchannels.Nonetheless,inuptohalfofthe
embryosstudied,collateralchannelswerefoundbetweenthedistalextentsofthe
fourthandsixtharcharteries.Thesecollateralchannelswerealsofoundbyother
groupsinvestigatingthedevelopingmouseheartbutinappropriatelydescribedas
fiftharcharteries.5,6Wediddiscoverinasolitaryhumanspecimenremnantsof
anarterythathadextendedthroughthepharyngealmesenchymefromtheaortic
sactothedorsalaorta,parallelingthecourseofthefourthandsixtharch
arteries.3Tothebestofourknowledge,thisistheonlychannelthusfar
describedduringnormaldevelopmentthattakesapathfromtheextrapericardial
systemicoutflowtracttothedescendingaortaandthepulmonaryarteries.
FIG.48.1 ClassicRathkediagram,whichpositsthepresenceofsixpairs
ofaorticarcharteries.Asdiscussedinthischapter,itisexceedinglyrareto
findevidenceoftheallegedfiftharcharteriesduringnormaldevelopment.
Notethelocationoftheseventhcervicalintersegmentalarteries,whichwill
eventuallyformthesubclavianarteries.
However,ahostofstructureshavebeeninterpretedbypediatriccardiologists
asrepresentingpersistentarteriesofthefifthpharyngealarch.Manyofthese
entitiesproducethelesionsknownasdouble-barreledaortas,withothervariants
producingsystemic-to-pulmonarycollateralchannelsorabnormalbranchingof
thebrachiocephalicarteries.Themajority,inouropinion,arebetterinterpreted
onthebasisofpersistenceofthecollateralchannelsorremodelingoftheaortic
sacandthearteriesofthethird,fourth,andsixtharches.Thischapterdescribes
thechangesthattakeplaceduringtheformationoftheextrapericardial
pulmonaryandaorticpathwaysinthemouseheart.Wealsoshowthevessel
discoveredinahumanembryothat,tothebestofourknowledge,istheonly
structurethusfarencounteredthatsatisfiesthecriteriaforanarteryofthefifth
pharyngealarch.Wethenexplainwhywebelievethatmanyoftheaccounts
providedintheworldliteraturearebetterexplainedonthebasisofpersistent
collateralchannelsorremodelingofthedevelopingbrachiocephalicand
pulmonaryarteries.Wenonethelessacceptthatsomeofthechannelsproviding
systemic-to-pulmonarycommunicationscouldwellrepresentpersistenceofthe
arteriesoftheenigmaticfiftharch,illustratingfeaturesthatwarrantsuch
descriptions.
