Andersons pediatric cardiology 94
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FIG.3.18 Locationoftheringofcells(arrows)demarcatedbythe
antibodytothenodoseganglionofthechick.Arrangementbefore(A)and
after(B)expansionoftheatrioventricularcanal.Thelocationofthe
antibodyintherightatrioventricularjunctionshowsthattheinitial
musculatureofthecanalbecomessequestratedastheatrialvestibules.
Thecomponentonthecrestofthemuscularventricularseptumbecomes
theatrioventricularbundle.
FurtherDevelopmentoftheVentricular
Loop
Functionalseparationoftheleft-sidedandright-sidedbloodstreamstakesplace
longbeforethecompletionofventricularseptation(seeFig.3.7).Development
oftheventriclesproceedsbypartitioningthesebloodstreamssuchthattheone
originatingfromtherightsideoftheatrioventricularcanalbecomeschanneledto
thepulmonarytrunk,whiletheonecommencingattheleftsideofthe
atrioventricularcanaliscommittedtotheaorta.Thisrequiressignificant
remodelingoftheinnercurvatureoftheventricularloop.Expansionoftheright
sideoftheatrioventricularcanalplacesthecavityoftherightatriumindirect
communicationwiththeapicalpartoftherightventricle,itselfballoonedfrom
theoutletpartoftheinitialventricularloop.Appreciationoftheprocessof
apicalballooning(Fig.3.19)isthekeytounderstandingtheformationof
incompleterightventricles,asseeninmalformedheartssuchasdouble-inletleft
ventricleortricuspidatresia.Theunifyingfeatureoftheselesionsisthatthe
atrialchambersretaintheircommunicationexclusivelywiththedominantleft
ventricle.
FIG.3.19 Imagetakenfromanepiscopicdatasetpreparedfromahuman
embryoatCarnegiestage14.Thisispriortoexpansionofthe
atrioventricularjunction.Thedevelopingrightventriclealreadypossesses
itsapicaltrabecularcomponent,whichisformedbyballooningfromthe
outletcomponentoftheventricularloop.Atthisearlystage,thedeveloping
rightventriclesupportstheentiretyoftheoutflowtract,whichitselfcontains
outflowcushions.Thebloodenteringtheoutflowtractatthisstagehasto
passthroughtheembryonicinterventricularcommunication.
Remodelingoftheinitialprimaryhearttubealsopermitsthedorsalhalfofthe
outflowtracttobecometheoutletfromtheleftventricle.Priortothis
remodeling,cushionsdevelopthroughouttheoutflowtract,fusingtoproduce
separatepulmonaryandsystemicventricularoutlets.Duringtheinitialstages,
however,bothdevelopingoutletsremainsupportedbytherightventricle,which
subsequenttotheexpansionoftheatrioventricularcanalhasacquireditsown
inlet(Fig.3.20).Remodelingoftheprimarytubethenpermitsthedorsalaortic
roottobetransferredtothedevelopingleftventricle.Atthesametime,this
transferbringstheleadingedgeofthefusedproximaloutflowcushionsinto
alignmentwiththecrestofthemuscularventricularseptum(Fig.3.21).The
proximalcushionsthemselvesthenundergomyocardializationtoformthe
freestandingmuscularsubpulmonaryinfundibulum.Afterthisremodeling,the
persistingembryonicinterventricularcommunicationcanthenbeclosedby
appositionoftissuesderivedfromventricularsurfacesoftheatrioventricular
cushions(Fig.3.22).14Thesetissuesderivedfromtheso-calledtuberclesofthe
atrioventricularcushionswilleventuallybecomethemembranouspartofthe
ventricularseptum.Delaminationoftherightmarginsofthefusedcushionsfrom
thesurfaceoftheseptumtoformtheseptalleafletofthetricuspidvalvewill
thenseparatethenewlyformedmembranousseptumintoitsinterventricularand
atrioventricularcomponents.10Thecomponentsofthecushionsremaininginthe
leftventriclewillremoldtoformtheaorticleafletofthemitralvalve.
