Andersons pediatric cardiology 1175
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FIG.44.12 Imagespreparedfromepiscopicdatasetsobtainedfrom
developingmouseembryossacrificedduringembryonicday12.5.(A)
Frontalsectionthroughtheoutflowtract,whicharisesatthisstageabove
thecavityoftherightventricle.Arrowsshowthejunctionbetweenthe
outflowtractandthepharyngealmesenchyme.Theoutflowtract
possessesdistal,intermediate,andproximalcomponents.Thecushions
separatingtheoutflowtractintoaorticandpulmonarychannelsoccupyonly
theintermediateandproximalparts,whichhavemyocardialwalls.The
distalpart,atthisstage,hasbeenseparatedbyaprotrusionthathas
grownfromthedorsalwalloftheaorticsac.Thedottedlineshowstheline
offusionoftheprotrusionwiththedistalendofthemajoroutflowcushions,
whichbythisstagehavefusedwitheachother.However,theproximal
outflowcushionsremainunfused.Thestarsshowtheintercalated
cushions,whichareformedintheintermediatepartoftheoutflowtract.(B)
Viewoftheshortaxisthroughtheintermediatepartoftheoutflowtractas
viewedfromaboveandtheright.Theinterrelationshipsbetweenthe
intercalatedandmajorcushionsareformingtheprimordiumsoftheaortic
andpulmonaryroots.Notethatthatparietalpartsofthemajoroutflow
cushionsremainunfusedinbothroots,althoughfusedcentrally.Theywill
separateatrightanglestotheplaneoffusionastherootsseparateone
fromtheother.
Atthestageatwhichtherootsbegintoform,theoutflowtractitselfremains
supportedabovethecavityoftherightventricle.Itisnotuntilembryonicday
13.5inthedevelopingmousethattheaorticrootbeginsitstransfertothe
developingleftventricle.Concomitantwiththetransferoftheroot,theinitial
embryonicinterventricularcommunicationbecomesreorientedtoformthe
entrancetothedevelopingleftventricularoutflowtract(Fig.44.13A).Asthe
aorticrootachievesitstransfer,acomponentoftheproximaloutflowtract
myocardiumissimilarlytransferredintotheleftventricle,whereitformsthe
summitoftheparietalleftventricularwall.Atthesametime,theproximal
outflowcushionsthemselvesfusewitheachotherandmuscularize,fusingatthe
sametimewiththecrestofthemuscularventricularseptum.Thispermitsthe
tuberclesfromtheventricularaspectsoftheatrioventricularcushionstoclose
thepersistingembryonicinterventricularcommunication,thuscompleting
ventricularseptation(seeFig.44.13B).
FIG.44.13 Imagesfromepiscopicdatasetspreparedfromdeveloping
mouseembryossacrificedatembryonicday13.5(A)and14.5(B).Both
aresectionedinthefrontalplane.Theimagesshowhowtheaorticrootis
transferredtotheleftventricle,withreorientationoftheinitial
interventricularcommunication,takingwithittheproximaloutflowtract
myocardium,whichsupportsthecushionsthanarebeginningtocavitateto
formtheleafletsoftheaorticvalve.Notethestemoftherightcoronary
artery,whichisgrowingoutfromtheaorticwall.
Theparietalunfusedmarginsofthedistaloutflowcushionshavealready
beguntocavitateandseparateawayfromthewallsoftherootwhiletheroot
itselfretainsalocationabovethecavityoftherightventricle(seeFig.44.13A).
Asthecushionscavitatetoformtheleaflets,thereisadditionalgrowthof
nonmyocardialtissuefromthesecondheartfieldtoformthewallsofthevalvar
sinuses.Thisgrowthofthenonmyocardialtissuescontinuesuptoterminthe
mouse,whichtakesplaceattheendofembryonicday18.5.Itisthe
nonmyocardialtissuesthatformthevalvarsinuses,withthecushionscavitating
soastoformthesemilunarleaflets(Fig.44.14).
FIG.44.14 Imagestakenfromepiscopicdatasetspreparedfrom
developingmouseembryossacrificedatembryonicday17.5(A)and18.5
(B).Theyshowhowadditionalgrowthofnonmyocardialtissuesfromthe
secondheartfieldhasformedthevalvarsinuses,withthedistalpartsofthe
outflowcushionscavitatingtoformthesemilunarvalvarleaflets.The
imagesshowcutsacrossthetwocoronaryaorticsinuses,whichretainthe
proximaloutflowmyocardiumattheirbases.
Wheninitiallytransferredtotheleftventricle,allthreedevelopingvalvar
leafletsaresupportedbyproximaloutflowtractmyocardium.Itisonlyatthe
endofembryonicday15.5inthemousethattheareabetweenthenonadjacent,
ornoncoronary,leafletbecomestransformedintofibroustissue.Ithasnowbeen
establishedthatexcessivefusionoftheparietalmarginsofthemajoroutflow
cushionsisthemechanismproducingthebifoliateaorticvalvewithfusionofthe
twoleafletsarisingfromthecoronaryarterialsinuses.Thisprocesshasbeen
demonstratedinacolonyofSyrianhamsters.18Ithasalsobeenshownthatmice
withknockoutoftheeNOSgenedevelopbifoliatevalveswithfusionofthe
leafletsdevelopedfromtherightwardofthemajorcushionsandtheintercalated
aorticcushion.19Thereforeitcanreasonablybepresumedthatexcessivefusion
ofthemajorcushions,combinedwithfusionwiththeintercalatedcushion,will
producetheunifoliateandunicommissuralvariant.Itisalsocurrentlyknown
thatcellsderivedfromtheneuralcrestpopulatethemajorcushionstoformthe
valvarleaflets,withwhichthevalvarsinusesareformedbytissuesderivedfrom
thesecondheartfield.However,thecellsfromtheneuralcrestarelessabundant
intheaorticintercalatedcushion.Evidencefromotherknockoutmicesuggests
thatfailureofformationoftheintercalatedcushionproducesabifoliatevalve
