Andersons pediatric cardiology 85
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EmbryologyoftheHeart
RobertH.Anderson,NigelA.Brown,TimothyJ.Mohun
Abstract
Wehaveacquiredconsiderablenewinformationoverthepastdecadeswith
regardtodevelopmentoftheheart.Themostimportantnewfindinghas
beentheappreciationthatnotallofthecomponentsfoundpostnatallyare
representedintheinitiallinearhearttube.Instead,wenowknowthatnew
materialisaddedfromtheso-calledsecondheartfieldtotheinitial
componentofthetubeatitsarterialandvenouspoles.Theinitialpart,
derivedfromthefirstheartfield,eventuallyformslittlemorethanthe
definitiveleftventricle.Thischapterillustratesthetemporalchangesin
anatomythattakeplaceduringtransformationoftheinitialtube,witha
solitarylumen,intothefour-chamberedpostnatalorgan.Ouraccountis
basedlargelyonanalysisoflargenumbersofdevelopingmouseembryos
preparedusingthetechniqueofepiscopicmicroscopy.Thispermits
sectioningofthedatasetsinanyrequiredplane,replicatingtheapproaches
nowusedclinicallyintheevaluationofimagespreparedusingcomputed
tomographyormagneticresonanceimaging.Wesupplementourfindings
fromthedevelopingmousewithmaterialpreparedfromhumanembryos,
withthelatterfindingsshowingthatthechangesobservedinthemouseare
broadlyrepresentativeofhumandevelopment.
Weshowhow,subsequenttoloopingofthedevelopingventricular
componentofthedevelopingheart,pouchesareproducedinparallel
fashionfromtheatrialcomponentofthetubebytheprocessnowknownas
ballooning.Thesepouchesbecometheatrialappendages.Subsequent
ballooninginseriesfromtheventricularcomponentthenformstheapical
partsofthedefinitiveventricles.Wediscussthestepsinvolvedin
incorporationofthesystemicvenoussinusintothedevelopingrightatrium,
andtheformationofthepulmonaryvein.Thislatterstructureisinitiallya
solitarychannelformedwithinthepharyngealmesenchyme,whichusesthe
persistingdorsalmesocardiumtogainitsentrancetothedevelopingleft
atrium.
Wedescribeexpansionoftheatrioventricularcanal,thusprovidingtheinlet
ofthedevelopingrightventricle,andtransferoftheproximalpartofthe
outletpartofthehearttubetotheleftventricleastheaorticroot.Wedraw
parallelsbetweenthistransferandthesituationseenpostnatallyinthe
settingoftetralogyofFallot.
