Andersons pediatric cardiology 93
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Thevenouscomponentshavedisparateorigins.Thesystemicvenouscomponent
isinitiallyformedfromprimarymyocardium.Thepulmonaryvenous
component,alongwiththeatrialseptum,incontrast,isderivedfromthe
mediastinalmyocardium.
AtrioventricularCanal
Inearlystages,thejunctionbetweenthedevelopingatrialcomponentandthe
inletoftheventricularloopisacanalwithfinitelength(seeFig.3.13),whichis
thenseptatedbyfusionofthesuperiorandinferioratrioventricularendocardial
cushions(Fig.3.17).Thesedevelopwithinthecanalbytheprocessofepithelialto-mesenchymaltransformation.Thecushionswilleventuallyprovidethe
foundationsforformationoftheaorticleafletofthemitralvalve,andtheseptal
leafletofthetricuspidvalve.Theyalsocontributetoclosureofthe
interventricularforamen,formingintheprocessthemembranouspartofthe
septum.Onlysubsequenttodelaminationoftheseptalleafletofthetricuspid
valve,arelativelylateevent,doesthisfibrouspartbecomeseparatedintoits
atrioventricularandinterventricularportions.10Muchhasbeenlearnedinrecent
yearsconcerningtheepithelial-to-mesenchymaltransformationsthattakeplace
duringformationofboththeatrioventricularandtheoutflowcushions,11butas
indicatedweareconfiningourattentiontothemorphologicchanges.Inthis
regard,ithadbeenthoughtthatfailureoffusionoftheatrioventricularcushions
underscoredthedevelopmentofheartswithatrioventricularseptaldefectand
commonatrioventricularjunction.Heartscandevelopwithallthefeaturesof
atrioventricularseptaldefect,however,whenthecushionsthemselveshave
fused.Theproblemunderscoringtheabnormality,therefore,isonethatpermits
theretentionofthecommonjunction,usuallyinvolvingfailureofgrowthofthe
vestibularspine.12Normally,nonetheless,theatrioventricularcushionsdofuse
soastodividetheatrioventricularcanalintoright-sidedandleft-sidedchannels
(seeFig.3.17).Withongoingdevelopment,theinitialmusculatureofthe
atrioventricularcanalbecomessequestratedontheatrialaspectoftheplaneof
atrioventricularinsulationtoformtherightandleftatrialvestibules(Fig.
3.18).13
FIG.3.17 Scanningelectronmicrographshowingtheatrioventricular
junctionsofthedevelopinghumanheartatCarnegiestage16.Theimage
showstheventricularaspectoftheatrioventricularcanaljustpriortofusion
oftheatrioventricular(AV)endocardialcushions.
