Andersons pediatric cardiology 97
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protrudesinobliquefashionintothedistalpartoftheoutflowtract,thusforming
anembryonicaortopulmonaryseptum(Fig.3.26).Duringthisperiodof
ingrowthofthenonmyocardialcomponentsofthedistaloutflowtract,thereis
accompanyingmarkeddiminutioninsizeoftheright-sidedarteriesthatrunfrom
theaorticsactojointhedescendingaorta.Asthedorsalpartsofthearteries
runningwithintheright-sidedpharyngealarchesbegintoinvolute,itbecomes
possibletorecognizethedevelopingpulmonaryarteries.Theytaketheirorigin
fromtheflooroftheaorticsac,extendingcaudallywithintheventral
mesenchymeofthemediastinumtofeedtherapidlygrowinglungbuds.Asthe
arteryrunningwithintheright-sidedsixtharchinvolutes,theorificeoftheleftsidedsixtharcharterybecomesappreciablylarger,extendingtojointhe
descendingaortaasthearterialduct(Fig.3.27).
FIG.3.26 Imagetakenfromanepiscopicdatasetpreparedfroma
developingmouseatembryonicday11.5showingashort-axissection
throughthejunctionbetweentheaorticsacandthedistaloutflowtract.The
shortarrowsshowhowthedistalmyocardialborderhasregressedrelative
tothemarginsofthepericardialcavity,formingtheparietalwallsofthe
intrapericardialaortaandpulmonarytrunk.Theventralprotrusionofthe
dorsalwalloftheaorticsacisseparatingthecavityintothepulmonaryand
aorticchannels,withtheprotrusionforminganembryonicaortopulmonary
septum.Thespacebetweentheleadingedgeoftheprotrusionandthe
distalmarginoftheoutflowcushionsistheembryonicaortopulmonary
foramen(double-headedarrow).
FIG.3.27 Left-sidedviewofareconstructionofthearteriescoursing
throughthepharyngealarchesofadevelopingmouseatembryonicday
12.5.Thedistalpartoftheoutflowtracthasnowseparatedintothe
intrapericardialarterialtrunks,withthepulmonarytrunkarisingcranially
andtheaorticrootpositionedcaudally.Thethirdandfourtharcharteries
havenowbecomethebrachiocephalicarteryontherightside,withtheleft
fourtharcharteryformingthetransversecomponentoftheaorticarch.The
arteryoftheleftsixtharchisnowrecognizableasthearterialduct.The
pulmonaryarteriesarenowseenarisingfromtheflooroftheoriginalaortic
sac.Notealsothepresenceofacollateralchannelextendingbetweenthe
terminalcomponentsofthearteriesofthefourthandsixtharches.These
collateralchannelsarefoundinhalfofalldevelopingmouseembryos.
Theyareoftenconfusedwitharteriesofthefiftharch,whichdoesnotexist
inthemouse.(CourtesyDr.SimonBamforth,NewcastleUniversity.)
Bythisstage,thefourthandsixtharcharteries,arisingfromthecranial
componentoftheaorticsac,haveremodeledtoformthebrachiocephalicartery
ontherightside,withtheleftfourtharcharterypersistingasthetransversepart
oftheextrapericardialaorta(seeFig.3.27).Duringthesameperiod,the
protrusiontakingoriginfromthedorsalwalloftheaorticsachasgrowntodirect
theflowfromtherightsideofthedistaloutflowtracttothearteriesofthethird
andfourtharches,withtheflowfromtheleftsidedirectedintotheleftsixtharch
artery.Inthisway,theaorticandpulmonarycomponentsoftheremodeling
distaloutflowtractaredirectedintotheappropriateextrapericardialchannels
(seeFig.3.27).Withcontinuinggrowth,theprotrusionthenfuseswiththedistal
endsofthecushions,whichthemselveshavedevelopedoverthesameperiod
withinthecomponentsoftheoutflowtract,whichhaveretainedtheirmyocardial
walls(Fig.3.28).Duringthisperiod,withintheintermediateandproximalparts
oftheoutflowtract,theoriginalcardiacjellyhasproliferatedbytheprocessof
epithelial-to-mesenchymaltransformationtoformtheoutflowcushions.These
opposingcushionsspiralwhentraceddistallytoproximally(Fig.3.29).
FIG.3.28 Image,incomparableorientationtothatshowninFig.3.26,
fromamouseataslightlylaterstageofdevelopment.Bythisstage,the
aortopulmonaryseptumformedbytheprotrusionfromthewalloftheaortic
sachasfusedwiththedistaloutflowcushions(dashedline),withthedistal
outflowcushionsthemselvesalsofused.Thefusionhasobliteratedthe
aortopulmonaryforamen,convertingthedistaloutflowtractintothe
intrapericardialarterialtrunks.Afurtherimportantchangeisthe
developmentoftheintercalatedcushions(stars)intheintermediatepartof
theoutflowtract.Thecushionsdevelopingwithintheproximalpartofthe
outflowtract,however,areasyetunfused.
