Andersons pediatric cardiology 1289
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arteries.3Theinitiallybilaterallysymmetricsystemundergoesrapidremodeling
duringembryonicdays11.5and12toformthematurebranchesthattaketheir
originfromthedevelopingaorticarch.Theremodelingisfirstseenintheright
sixtharchartery,whichthinsandinvolutesduringembryonicday11.5.Thisis
accompaniedbysubsequentregressionoftheright-sideddorsalaorta(Fig.48.5).
Priortotheremodeling,thesegmentsofthedorsalaortasbetweentheinsertions
ofthethirdandfourtharcharteriesalongwiththeircranialcontinuationshave
beenrecognizableasthecarotidducts(seeFigs.48.2and48.3).Thesevessels
alsomarkedlyreduceinsizeduringembryonicday11.5(seeFig.48.4)andhave
disappearedbytheendofembryonicday12.5(seeFig.48.5,left).Thearteries
ofthethirdarchescontinuecraniallyintotheremainingpartsofthedorsal
aortas,thusformingtheinternalcarotidarteries.Withregressionoftherightsideddorsalaorta,thelefthornoftheaorticsacgivesrisetothearteryoftheleft
fourtharchandexpandstoformthedefinitiveaorticarch,withthearteryofthe
leftsixtharchrecognizableasthearterialduct.Therighthornoftheaorticsac
alsoundergoesremodelingtoformaconfluentchannelthatgivesrisetothe
arteriesoftherightfourthandthirdarches.Atthisstage,however,therehas
beennoformationofthesubclavianarteries.Thesevesselsarederivedfromthe
seventhcervicalintersegmentalarteries.Theywillbecomethesubclavian
arteries,butmajormovementisrequiredonboththerightandleftsidesbefore
theyreachtheirdefinitivelocation.Thisisbecausethesearteriesinitiallyoccupy
apositionthatismarkedlycaudaltotheireventualdefinitiveposition(seeFig.
48.5,left).Althoughitseemsthatthecervicalintersegmentalarteriesmigrate
cranially,itismorelikelythatthepharyngealmesenchymecontainingthearch
arteriesthemselves,alongwiththedevelopingheart,movescaudally.Regardless
oftheprecisemechanics,theendresultisthatthearteriesthemselvesremodel
suchthatthevesselontheleftsidecrossestheunionoftheleftsixtharchartery
withthedorsalaorta.Thismovement,whichcanbelikenedtothe“castling”
movementinchesswhenconsideredrelativetothearterialduct,dividestheleft
fourtharcharteryintothetransversesegmentofthedefinitiveaorticarchandthe
aorticisthmus(seeFig.48.5,right).Evenmoremarkedrelativemovementtakes
placeontherightside,suchthattheintersegmentalarteryachievesunionwith
therighthornoftheaorticsac,whichbecomesthebrachiocephalictrunk(see
Fig.48.5,right).However,noneofthemouseembryosthatwehavestudiedthus
farhasshownanyevidencefortheexistenceofafifthpharyngealpouchoraset
offifthpharyngealarcharteries.
FIG.48.4 Reconstructionsofthearcharteriesfromanepiscopicdataset
fromadevelopingmousesacrificedduringembryonicday11.5.Theleft
panelshowsafrontalviewofthereconstruction.Bynowthe
intrapericardialoutflowtracthasseparatedintotheaortaandthe
pulmonarytrunk(PT).Therightpanelshowsaviewoftheleft-sided
channels.Notethecollateralchannelformedbetweentheconnectionsof
thefourthandsixtharcharterieswiththedorsalaorta.Suchcollateral
channelsarefoundinuptohalfofallnormalmouseembryos.Notealso
theregressingrightsixtharchartery.
FIG.48.5 Reconstructionsfromepiscopicdatasetsderivedfrom
developingmiceshowingthemajorremodelingthattransformstheaortic
sacanditsrightandlefthornsintotheaorticarchandthebrachiocephalic
arteries.Thisinvolvesmarkedmigrationoftheseventhcervical
intersegmentalartery,whichbecomesthesubclavianarteryonbothsides.
Althoughitseemsthattheseventhsegmentalarterymigratescranially,this
structureisfixed.Itismorelikely,therefore,thatthepharyngeal
mesenchymemovescaudally.Ontheleftside,therelativemovements
takethesegmentalarteryacrossthejunctionofthesixtharchartery,which
becomesthearterialductandthedorsalaorta.Initseventualposition,it
dividesthefourtharcharteryintothetransverseaorticarchandtheaortic
isthmus.Comparablemovementstakeplaceontherightside.Theleft
panel,showninfrontalprojection,isfromanembryosacrificedat
embryonicday12.5,whereastherightpanel,viewedobliquelyfromtheleft
side,isfromanembryosacrificedatembryonicday13.5.PT,Pulmonary
trunk.
Wehavestudiedamuchsmallernumberofdevelopinghumanembryos.3In
oneoftheseembryoswefoundaspecimenwithremnantsofachannelthat
qualifiesasanarteryofthefifthpharyngealarch.Thisartery,whichwasleftsided,notonlyparalleledthecourseofthedevelopingfourthandsixtharch
arteries(Fig.48.6A)butwasalsocontainedwithinadiscretesegmentof
pharyngealmesenchyme(Fig.48.6B).Hadthearterypersistedinitsentirety,it
wouldhaveextendedfromanoriginintheaorticsactoaninsertioninthesixth
archarteryandthencetothedescendingaorta(seeFig.48.6).Wesuggestthat
thecourseofthisvesselshouldserveastheexemplarforcandidatestobe
consideredaspersistentfiftharcharteries.Tojustifytheirexistenceasarteriesof
thefiftharch,thechannelsshouldthereforerunanextrapericardialcourse,
havingtakentheiroriginfromtheascendingaorta.Iftheyarefeedingthe
pulmonarycirculation,thisshouldbethroughadorsalserpentinecontinuation
withthesixtharch.
