Andersons pediatric cardiology 95
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FIG.3.20 Imagefromanepiscopicdatasetpreparedfromadeveloping
mouseembryoearlyduringembryonicday12.5.Theatrioventricularcanal
hasexpandedsuchthattherightventriclehasachieveditsowninlet
throughthedevelopingtricuspidvalve.Theoutflowtract,however,remains
supportedexclusivelybythedevelopingrightventricle,althoughthe
cushionsformedwithinthetractarefusingproximally(arrow)todividethe
intermediatecomponentintotheaorticandpulmonaryroots.Atthisstage,
thebloodenteringtheaorticroothastotraversetheembryonic
interventricularcommunication.
FIG.3.21 Sectionthroughahumanembryointhefrontalplane,
replicatingtheobliquesubcostalechocardiographiccut,showinghow
fusionofthemuscularizingproximalcushionsoftheoutflowtractwiththe
ventricularseptumwallstheaortaintotheleftventricle.Thestarshowsthe
coronarysinus.Notethepulmonaryvenousorificestillremainsadjacentto
thesinuscontainedintheleftatrioventricularjunctionatthisstageof
development.
FIG.3.22 Image,takeninthefrontalplane,fromanepiscopicdataset
preparedfromadevelopingmouseatembryonicday13.5.Itshowshow
tissuederivedfromtheventricularsurfacesoftheatrioventricularcushions,
knownastubercles,havefusedwitheachother,andwiththecrestofthe
muscularventricularseptum,towallthesubaorticoutlet,derivedfromthe
proximaloutflowtract,intothecavityoftheleftventricle.Thetubercleswill
formthemembranouspartoftheventricularseptum.
Beforemovingontoconsidertheformationofthesevalves,itisconvenient
firsttocompletethediscussionofthechangesthattakeplacewiththeoutlet
segmentofthehearttube,alongwiththearteriestowhichitgivesrisewithinthe
developingpharyngealmesenchyme.
