Andersons pediatric cardiology 87
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OriginoftheHeartTube
Recentstudieshavenowvalidatedthenotionthat,afterformationofthelinear
hearttube(seeFig.3.1),cellsarecontinuouslyaddedatbothitsvenousand
arterialpoles.1–3Thesourceofthisnewmaterialisthesecondheartfield,with
thelineartubeitselfderivedfromthefirstheartfield.Thecellsformingthese
fieldsarederivedfromtheheart-formingareas.Thesearelocatedwithinthe
originalembryonicdisc,whichisboundedbythejunctionoftheembryowith
theextraembryonictissuesformedbytheamnionandyolksac.Therearethree
germlayerswithinthedisc:theectoderm,whichfacestheamnioticcavity;the
endoderm,whichfacestheyolksac;andtheintermediatemesodermallayer
(Fig.3.2).Foldingofthisdisc,concomitantwithextensivegrowth,givesthe
embryoitscharacteristicshape.Thecellsproducingtheheart-formingareas,
initiallyfoundtoeithersideofthemidline,havemigratedfromthecranialpart
oftheprimitivestreakduringtheprocessofgastrulation.Withcontinuing
development,theyjoinacrossthemidlinetoformthecardiaccrescent(Fig.3.3).
Itremainstobedeterminedwhetherthefirstandsecondfieldsarediscrete
entities.Boundariesbetweenmorphologicregionsofthedevelopingembryoare
notnecessarilyformedatsubsequentstagesbythesamecellsaswerepresent
initially.Ifweacceptthatthematerialfromwhichtheheartisformedisderived
fromthesamebasicheart-formingareas,nonethelessthereisanobvious
temporalorderinthedifferentiationofitsfirstandsecondcardiaclineages.4
Thisordermayreflecttheevolutionarydevelopmentofthecardiovascular
system.Wheninitiallydevelopedduringevolutionoftheanimalkingdom,the
heartcontainednomorethanthecomponentsofthesystemiccirculation—
namely,anatrium,aventricle,andamyocardialoutflowtract.Thepulmonary
circulation,representedbytherightventricle,andmost,butnotall,oftheleft
atrium,includingtheatrialseptum,appearsappreciablylaterinevolutionary
development.Thereforeitisunlikelytobecoincidentalthattheatrialseptumin
mammals,alongwiththedorsalatrialwall,isformedfrommaterialthatisadded
totheheartrelativelylateinitsdevelopment.Theevolutionaryconsiderations
suggeststronglythatnovelpatterning,withdifferenttemporalsequences,but
withinthesameheart-formingarea,issufficienttoprovideallthematerial
neededtoconstructthefour-chamberedheartsofbirdsandmammals,albeitthat
notallprecursorsarepresentinthelinearhearttubewhenitisfirstseen(see
Fig.3.1).
FIG.3.2 Theembryonicdiscisformedasatrilaminarstructure,withthe
mesodermalstructuressandwichedbetweentheendodermaland
ectodermallayers.Thecardiaccrescentisderivedfromtheprimitive
streak,asshowninFig.3.3.
FIG.3.3 Cellsmigratebilaterallyfromtheprimitivestreakintothe
mesodermallayeroftheembryonicdisc,initiallygivingrisetotheheartformingareas,andthenthecardiaccrescent.Temporalmigrationsofcells
fromthecardiaccrescentthenproducethehearttube,withtwoofthese
migrationscurrentlyidentifiedasthefirstandsecondlineages,orheart
fields.
