Andersons pediatric cardiology 89
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FIG.3.6 FrontalsectionthroughahumanembryoatCarnegiestage12,
equivalentataroundthebeginningof6weeksofgestation.Ithasbeen
preparedusingthetechniqueofepiscopicmicroscopy.Theatrial
appendagesareexpanding,orballooning,insymmetricalfashionfromthe
atrialcomponentoftheprimarytube(whitearrows).Theventricularapical
components,incontrast,areballooninginseriesfromtheinletandoutlet
componentsoftheventricularloop,thusgivingrise,respectively,tothe
developingleftandrightventricles.Theatrioventricularcanalisinitially
drainingexclusivelytotheinletoftheloop,butitsparietalwallalready
providescontinuitybetweenthewallsofthecomponentsthatwillbecome
therightatriumandrightventricle(redarrow).Notethattheprimordiumof
themuscularseptum,formingthecaudalmarginoftheinterventricular
communication,isproducedconcomitantwiththeballooningoftheapical
components.
FIG.3.7 Subsequenttoballooningoftheatrialandventricularcavities,
separatesystemicandpulmonarystreamsalreadyexistthroughtheheart,
despitethefactthattheatrioventricular(AV)canaldrainstothedeveloping
rightventricle,andtheoutflowtractissupportedexclusivelybythe
developingrightventricle.Boththestreamspassthroughtheembryonic
interventricularcommunication,shownbythegrayring.Thechamber
myocardiumisshowninbeige,whiletheprimarymyocardiumoftheinitial
hearttubeisshowningray.Itisremodelingoftheprimarymyocardiumthat
willeventuallyresultwitheachventricleachievingitsseparateinletand
outletcomponents.Notethepresenceofthemediastinalmyocardium,
showninblue,fromwhichisderivedtheprimaryatrialseptum,andwhich
surroundstheorificeofthedevelopingpulmonaryvein.
DevelopmentoftheVenousComponents
Theongoingadditionoftissuesfromtheheart-formingareasgivesrisenotonly
totheprimaryatrialcomponentofthehearttube,butalsotothesystemicvenous
tributariesatitsvenouspole.Subsequenttotheprocessoflooping,thereis
symmetrybetweenthevenouschannelsformedinbothsidesofthedeveloping
embryo.Thesechannels,whichprovidereturnofthebloodstreamsfromthe
yolksac,theplacenta,andtheembryoitself,cometogethertodrainintothe
atrialcomponentofthehearttubethroughvenousconfluences,whichareknown
asthehornsofthesystemicvenoussinus(Fig.3.8).Thesystemicvenoussinus,
orsinusvenosus,isanatomicallydiscreteinloweranimals,suchasfish.Nosuch
anatomicallydiscretestructure,however,istobefoundintheearlystagesof
developmentofthemammalianheart.Instead,thevenoustributariesonboth
sidesoftheembryosimplyemptyintotheatrialcomponentthroughthesinus
horns(seeFig.3.8).Onlyafterthesystemicvenoustributarieshavebeen
remodeledtodraintotherightsideoftheinitialatrialcomponentoftheheart
tubedoesitbecomepossibletorecognizestructuresdemarcatingtheirborders.
Thesestructuresarethevalvesofthesystemicvenoussinus.Akeypartof
normaldevelopment,therefore,isremoldingofthesystemicvenoustributaries.
Thisprocessinvolvestheformationofanastomosesbetweenthevariousvenous
systemssuchthatleft-sidedvenousreturnisshuntedtotherightsideofthe
embryo.Amajoranastomosis,thevenousduct,orductusvenosus,divertsthe
umbilicalvenousreturnfromtheplacentatothecaudalpartofthecardinal
venoussystem.Thevitellineveins,drainingtheyolksac,largelydisappear,
althoughsomeofthesestructuresareincorporatedintothevenoussystemofthe
liver.Asecondimportantanastomosisdevelopsinthecranialpartoftheembryo,
divertingthevenousreturnfromtheleft-sidedtotheright-sidedcardinalvein.
Withthisshiftofthecranialvenousreturntotheright-sidedcardinalchannel,
andwiththedisappearanceoftheleft-sidedvitellineandumbilicalveins,thereis
gradualdiminutioninsizeoftheleftsinushorn.Asitdiminishesinsize,soits
walls,whichremainsseparatedfromthoseoftheleftatrium,become
incorporatedintothelefthalfofthedevelopingatrioventricularjunction,
eventuallybecomingthecoronarysinus(Fig.3.9).
