Andersons pediatric cardiology 91

bytheirexpressionofthetranscriptionfactorTbx18.6,7Whenfirstseen,the
pulmonaryveininboththemurineandhumanheartdrainstotheheartdirectly
adjacenttotheatrioventricularjunction(seeFig.3.11).Thereisthensignificant
remodelingsothat,atfirst,separateorificesdrainthebloodfromtherightand
leftlungs.Eventually,inthehumanheart,fourorificesareformedatthecorners
oftheatrialroot(Fig.3.12).Onlyatthelatestagesisitpossibletoseeformation
oftheso-calledsecondaryatrialseptum,whichisnomorethanthefoldbetween
theright-sidedpulmonaryveinsandthesystemicvenoustributaries(seeFig.
3.12).

FIG.3.12 Sectionfromahumanembryoafterthecompletionofseptation
at8weeks’gestation.Thepulmonaryvenouscomponentnowformsthe
leftatrialroof.Notethediminutioninsizeoftheleftsuperiorcavalvein,and
appearanceofthesuperiorinteratrialfold(arrow).Itisthefoldthatprovides
asuperiorbuttressfortheflapvalveoftheovalforamen.Thevestibular
spineandmesenchymalcaphavemuscularizedtoformtheinferior
buttressoftheatrialseptum.


SeptationoftheAtrialChambers
Theshiftrightwardofthetributariesofthesystemicvenoussinus,sothatthey
openexclusivelytothedevelopingrightatrium,permitsseptationoftheatrial
componentoftheheart.Bythisstage,theadditionofthenewmediastinal
myocardiumhasformedthelargerpartofthebodyofthedevelopingatrial
component(seeFig.3.7).Theatrioventricularcanal,ofcourse,waspresentfrom
theoutset,andisalsocomposedofprimarymyocardium.Themyocardiumof
theatrialcomponentitselfwasalsoinitiallycomposedofprimarymyocardium,
butaswehaveseen,thetwoappendagesballooninsymmetricalfashionfrom
thislumentract(seeFig.3.6).Itisatthisstagethattheprimaryatrialseptum,or
“septumprimum,”growsasashelffromtheatrialroof(Fig.3.13).Bythetime
theprimaryatrialseptumhasappeared,endocardialcushionshavealso

developedwithintheatrioventricularcanal.Thecushions,positionedsuperiorly
andinferiorlywithinthecanal,growtowardeachothertodivideitintorightsidedandleft-sidedchannels.Asthecushionsgrowtowardeachother,sothe
primaryseptumalsogrowstowardthecushions,carryingonitsleadingedgea
furthercollectionofendocardialtissue,themesenchymalcap.Bythetimethe
primaryseptumandmesenchymalcaphaveapproachedthecushions,thecranial
partoftheseptum,atitsoriginfromtheatrialroof,hasbrokendown,creating
thesecondaryinteratrialforamen.Theprimaryforamenisthediminishingspace
betweenthemesenchymalcapandthefusingatrioventricularendocardial
cushions(Fig.3.14).Fusionofthemesenchymalcapwiththeendocardial
cushionsobliteratestheprimaryatrialforamen.Sincethisprocessoccurstothe
rightsideofthepulmonaryridges,thesolitaryopeningofthenewlycanalized
pulmonaryveiniscommittedtotheleftsideofthedividingatrialcomponent.
Thebaseofthenewlyformedatrialseptum,formedbythemesenchymalcap,is
thenfurtherreinforcedbygrowthintotheheartofmesenchymaltissuesthrough
therightpulmonaryridge.InitiallyillustratedbyWilhelmHistheElder,this
protrusionseeninthecaudalwalloftheatriumwaslabeledthevestibularspine,
or“spinavestibuli”(Fig.3.15).8Itisnowfrequentlydescribedasthedorsal
mesenchymalprotrusion.


FIG.3.13 FrontalsectionfromadevelopinghumanembryoatCarnegie
stage14.Theprimaryatrialseptumcanbeseengrowingfromtheatrial
roof,carryingonitsleadingedgeacapofmesenchyme.Atthisearlystage,
theatrioventricularcanal,whichhassignificantlength,opensexclusively
intothedevelopingleftventricle(LV).RV,Rightventricle.