Andersons pediatric cardiology 92
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FIG.3.14 Imageinfour-chamberplanefromahumanembryoat
Carnegiestage16.Thesectionshowshowtheprimaryatrialseptum,with
itsmesenchymalcap,isgrowingtowardthesuperioratrioventricular(AV)
cushion.Thecranialoriginoftheseptumhasbrokendowntoformthe
secondaryforamen.Theprimaryatrialforamen(bracket)isthespace
betweenthemesenchymalcapandtheatrioventricularcushion.Notethe
venousvalvesmarkingtheboundarybetweentherightatrialcavityandthe
systemicvenoussinus.
FIG.3.15 DrawingbasedontheillustrationofWilhelmHistheElder
showinghowthevestibularspinegrowsintothespacebetweentheleading
edgeoftheprimaryseptumandtheatrioventricularcanal.
Themesenchymaltissueofthespine,togetherwiththemesenchymalcapon
theprimaryseptum,thenmuscularizetoformthebuttressatthebaseoftheatrial
septum,thusanchoringtheseptumfirmlyagainstthefusedatrioventricular
cushions(Fig.3.16).Subsequenttothesechanges,withtheatrialseptum
separatingthenewlyformedrightandleftatriums,asecondaryforamenremains
presentatthesiteofitsbreakdownattheatrialroof.Thisholeisanessentialpart
ofthefetalcirculation,permittingtherichlyoxygenatedplacentalreturntoreach
theleftsideofthedevelopingheart,andthenthedevelopingbrainthroughthe
aorta.Asthepulmonaryveinsareincorporatedintotheatrialroof,theensuing
foldingbetweenthepulmonaryandsystemicvenousstructurescreatesacranial
buttressagainstwhichtheflapoftheprimaryseptumisabletoclose(seeFig.
3.16).Thespacebetweentheflapvalveandthefoldisthedefinitiveoval
foramen.Theforamenremainspatentduringfetallife,closingpostnatallyin
mostbutnotallindividuals.9
FIG.3.16 Imagepreparedfromanepiscopicdatasetfromamouseat
embryonicday17.5,whichisjustpriortobirth.Thedouble-headedarrow
showstheovalfossa,withtheprimaryatrialseptumformingitsfloor.The
cranialmarginofthefossaistheinfoldingbetweentheattachmentsofthe
systemicveinstotherightatrium,andthepulmonaryveintotheleftatrium.
Theinferiormarginofthefossaisformedbythebuttressderivedby
muscularizationofthemesenchymalcapandvestibularspine.More
craniallystill,theflapvalveformedfromtheprimaryseptumhasyettofuse
withthesuperiorfold.Thisdoesnothappenuntilafterbirth.Thespace
betweentheleftvenousvalveandtheseptumisthesmallpartofthebody
ofthedevelopingatriumremainingontherightside.SCV,Superiorcaval
vein.
Theprimaryseptumitselfinitiallygrowsbetweenthesiteoftheleftvenous
valveandtheorificeofthenewlyformedpulmonaryvein.Allthetissuetothe
leftofthevenousvalveismediastinalmyocardium.Thistissueformsthebody
ofthedevelopingatrium.Thelargerpartofthisbodyiscommittedtothe
definitiveleftatrium,withonlyasmallpartremainingintherightatrium
betweentheleftvenousvalveandtheseptum(seeFig.3.16).Eachatrium
eventuallypossessesapartofthebodyderivedfrommediastinalmyocardium,
withthelargerpartcommittedtothemorphologicallyleftatrium.Eachatrium
alsopossessesanappendage,formedbybilateralballooningfromtheinitial
lumenoftheprimaryhearttube.Thechambersalsopossessavestibule,derived
oneachsidefromtheinitialprimarymyocardiumoftheatrioventricularcanal.
