Andersons pediatric cardiology 98
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FIG.3.29 ImageusingthesameepiscopicdatasetasFig.3.25,but
reconstructedtoshowthespiralingcourseofthecushionsextending
throughtheproximalandintermediatepartsoftheoutflowtract.The
reconstructionalsoshowsthesolitarylumenofthedistaloutflowtract,and
thearteriesarisingfromtheaorticsactopassthroughthethird,fourth,and
sixthpharyngealarches.
Thecushionthatislocatedcaudallyattheintermediatepartoftheoutflow
tractturnsbeneaththeothercushionatthebendandachievesaseptallocation
withintheoutletoftherightventricle.Thecushionthatispositionedcraniallyat
itsdistalmarginspiralstoattachparietallytothewalloftheoutletoftheright
ventricle.Thismeansthat,asthecushionsapproximateoneanother,fusingalong
theirfacingsurfaces,theproximaloutflowtractwilleventuallybeseparatedinto
ventralanddorsalchannels,withright-sidedandleft-sidedchannelsformed
distally.Fusionofthecushionsalongtheirlength,however,doesnotoccuratthe
sametime.Itcommencesdistally,withtheactofclosuremovinginproximal
direction.Theoutflowcushions,likethecushionswithintheatrioventricular
canal,areinitiallyformedbytheprocessofepithelial-to-mesenchymal
transformation.Anadditionalmigrationofcellstakesplaceintheoutflowtract,
thistimefromtheneuralcrest.Thecellsderivedfromtheneuralcrest,
furthermore,notonlypopulatethecushions,butalsomigratetooccupythe
surfaceoftheprotrusionfromthedorsalwalloftheaorticsac.Whenthetwo
componentseventuallyfusetoobliteratetheembryonicaortopulmonary
foramen,therefore,theactoffusiondependsontheappropriatemigrationfrom
theneuralcrest.Thisfusionofthedistalendsoftheoutflowcushionswitheach
other,andwiththeaortopulmonaryseptum(seeFig.3.28),thenconnectsthe
right-sidedandcaudalchannelofthedistaloutflowtracttotheextrapericardial
systemicarteries.Thisleavestheleft-sidedandcranialchanneloftheoutflow
tractincommunicationwiththecaudallylocatedrightandleftpulmonary
arteriesandthearteryoftheleftsixtharch.Thechangesinthedistalpartofthe
outflowtract,therefore,haveproducedtheintrapericardialpartsoftheaortaand
pulmonarytrunk,withthemajoroutflowcushionshavingalsofusedinthe
intermediatepartoftheoutflowtract.Additionalcushionshavealsodeveloped
duringthisperiodwithintheintermediatepartoftheoutflowtract,withone
cushionbeingplacedmuchmorecraniallycomparedwiththeother(seeFig.
3.28).Thesearetheintercalatedcushions.Cavitationwithinthedistalendsof
thesecushions,andalsowithintheoppositeendsofthefusedmajorcushions,
thenproducestheprimordiumsoftheaorticandpulmonaryvalves(Fig.3.30).
Thearterialvalves,therefore,areformedwithintheintermediatecomponentof
theoutflowtract(seeFig.3.28),whichinitiallyretainsitsmyocardialwalls.16
Asthecushionscavitate,sothereisongoinggrowthofthenonmyocardial
tissuesfromthesecondheartfield.Thisformsthewallsofthesinusesofthe
arterialroots,withthecavitatingcushionsthemselvesremodelingtobecomethe
valvarleafletsandtheirsemilunarhinges.Althoughthemiddlepartsofthe
majorcushionshavefusedtoseparatetheaorticandpulmonaryroots,the
opposingedgesofthecushionsthemselvesdonotfuse,thusproducingthe
trifoliatearrangementoftheaorticandpulmonaryvalves(seeFig.3.30).With
ongoingdevelopment,thecentralcomponentofthefusedcushionsbreaksdown
alongalinethatisnormaltothelineoffusion.Inthisway,thenewlyformed
aorticrootisseparatedfromthepulmonaryroot.Thiscentralpartofthefused
cushionswasinitiallyoccupiedbythecellsderivedfromtheneuralcrest,which
dieduringthisprocessbyapoptosis.17
FIG.3.30 Short-axissectionthroughtheintermediatepartoftheoutflow
tractofahumanembryoatCarnegiestage18showingthedeveloping
primordiumsofthearterialvalves.Notethatbothvalvesarestill
surroundedbyamyocardialcuff.Thevalvarleafletsareformingbya
processofcavitationofthecushions.Notealsothesiteoftheinitialzoneof
fusionbetweenthecushions.Thearterialrootswilleventuallyseparateat
rightanglestothisplane.
Attheinitialstageofformationofthearterialvalvesandtheirsupporting
sinuseswithintheintermediatepartoftheoutflowtract,thecomponentsofthe
majorcushionsoccupyingtheproximalpartoftheoutflowtractremainunfused
(seeFig.3.20).Withfurtherdevelopment,thesecushionsnotonlyfusewitheach
other,butalsofusewiththecrestoftheventricularseptum,thuswallingthe
aorticrootintotheleftventricle(seeFig.3.21).Aswiththecushionsinthe
intermediatepartoftheoutflowtract,thecentralcoreoftheproximalcushionsis
alsooccupiedbythecellsthatmigratedfromtheneuralcrest.And,asoccurred
withinthedevelopingarterialroots,thesecellsagaindisappearbytheprocessof
apoptosis.Thesurfaceofthefusedproximalcushions,incontrast,becomes
muscularized,thusformingthesubpulmonaryinfundibulum(Fig.3.31).18
Subsequenttoclosureofthepersistingembryonicinterventricularforamenby
thetuberclesoftheatrioventricularcushions(seeFig.3.31),themusculatureof
theinnerheartcurvatureinitiallycontinuestoseparatethedevelopingleafletsof
theaorticandmitralvalves.Itisonlyatamuchlaterstagethatthismyocardium
disappears,producingtheaortic-to-mitralvalvarcontinuitythatisafeatureof
thepostnatalheart.Similarly,atmuchlaterstages,oncemorebyaprocessof
apoptosis,theentiretyofthemyocardialwallssurroundingthedeveloping
