Andersons pediatric cardiology 86
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Withregardtotheoutletpartofthehearttube,weshowhowitinitially
possessesexclusivelymyocardialwalls,withadditionalgrowthatthe
arterialpoleproducingeffectiveproximalregressionofthemyocardial
borderconcomitantwithformation,first,oftheintrapericardialtrunks,and
thenthearterialroots.
Wecompleteouraccountwithdiscussionoftheformationofthecardiac
valves,theconductionsystem,thecoronaryarteries,andtheventricular
walls.
Keywords
Episcopicmicroscopy;hearttube;ballooning;septation;ventricularloop;
outflowtract
Introduction
Fromthefunctionalpointofview,theheartissimplyaspecializedpartofthe
vascularsystem.Nonetheless,developmentoftheheartasaspecializedpumpis
ofgreatsignificance.Wehavelearnedagreatdealoverrecentdecadesregarding
theoriginofthemuscularpartsofthispumpingorgan.Untilrecently,itwas
believedthattheinitiallineartube,whichgivesrisetotheheart,containedthe
precursorsofallthecomponentsasseeninthepostnatalorgan.Wenowknow
thatthisisnotthecase,andthattissueiscontinuallyaddedtothehearttubeasit
growsandloops.Theinitialstraightpartofthetube(Fig.3.1)eventuallyforms
littlemorethantheleftventricle.Thisknowledgenowpermitsbetter
interpretationofthemorphogenesisofmanycongenitalcardiacmalformations.
Theopeningsectionsofthischapterdiscussthenewevidencethathasemerged
concerningtheappearanceofthecardiaccomponents.Thereafter,werevertto
providinganaccountoftheso-calledcardiacsegments.Inthisrespect,wedo
notuse“segment”initsbiologicsense.Thuswedonotimplythateach
purportedsegmentisidenticaltotheothers,asisseenininvertebratessuchas
annelids.AsexplainedinChapter1,however,thesegmentalapproach,inits
sequentialmodification,isnowthepreferredmeansofdescribingthecascadeof
informationacquiredduringclinicaldiagnosis.Wewillcontinue,therefore,to
describethecardiaccomponentsassegments,inthiswayprovidingthe
necessarybackgroundtointerprettheanomalousdevelopmentthatleadstothe
congenitalmalformationsdescribedinthebodyofthisbook.Thereare
discrepanciesbetweenthetermsusedbybiologiststodescribedevelopingheart
andtheattitudinallyappropriatetermsusedbyclinicianswhendescribingthe
formedorgan.Biologistsandembryologistsusetheterm“anterior”todescribe
structuresthataretowardthehead,and“posterior”forthosetowardthefeet.We
circumventtheseproblemsbydescribingcranialandcaudalstructures.Toavoid
furtherconfusion,wealsoavoiduseoftheterms“anterior”and“posterior”
whendescribingstructureslocatedtowardthespineandsternum,asisthewont
ofclinicians.Forthispurposeinthischapter,weusetheadjectives“dorsal”and
“ventral.”Rightandleft,ofcourse,retaintheirtime-honoredusage.Throughout
thechapter,weconcentrateonillustratingthemorphologicchangesthattake
placeduringdevelopmentoftheheart.Hugeadvanceshavebeenmadeoverthe
pastquartercenturyinunderstandingthegeneticandmolecularchangesthat
underscorethemorphologicandtemporalremodeling.Spacedoesnotpermitus,
however,toassessthesefeaturesinthedepththeydeserve.Wethereforerestrict
ourselvestoconsiderationofthechangingmorphology.
FIG.3.1 Scanningelectronmicrographfromadevelopingmouseat
embryonicday9.5showingthelinearhearttuberevealedsubsequentto
dissectionoftheparietalpericardium.Ithadbeenbelievedthatallpartsof
theorganwererepresentedintheso-calledlineartube.Wenowknowthat
thispartgivesriseonlytothedefinitiveleftventricleandtheventricular
septum.Newmaterialisbeingaddedtothetubefromtheheart-forming
areasatboththecranialarterialandthecaudalvenouspoles.Notethat
thereisalreadyanasymmetricalarrangementofthedevelopingvenous
poleofthetube,asshownbythestar.
