Andersons pediatric cardiology 88
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FormationoftheCardiacLoop
Theinitialhearttubeismoreorlessstraight(seeFig.3.1).Withtheadditionof
thenewmaterialatitsarterialandvenouspoles,itrapidlybecomesS-shaped,in
thiswayachievingitsventricularloop(Fig.3.4).Experimentshaveshownthat
thetubewillcontinuetoloopevenwhendeprivedofitsnormalarterialand
venousattachments,andwillalsoloopwhennolongerbeating.Looping,
therefore,isanintrinsicfeatureoftheheartitself,althoughtheprecisecausehas
stilltobedetermined.Thetubeusuallycurvestotheright,withthedirectionof
turningbeingindependentoftheoverallleft-rightasymmetryofthedeveloping
embryo.Itisoftensaidthatrightwardloopingisthefirstsignofbreakingof
cardiacsymmetry.Thisisincorrect.Theregionofthetubethatwillbecomethe
atrioventricularcanalisasymmetricalevenbeforethebeginningoflooping(see
Fig.3.1).
FIG.3.4 Scanningelectronmicrographshowingthedevelopingmouse
heartduringtheprocessofventricularlooping.Theventricularpartofthe
tubehasinletandoutletcomponentsformedinseries.
CardiacSegments
Theprocessofloopingofthehearttubesetsthescenefortheappearanceofthe
buildingblocksoftheventricles,withadditionalingrowthoftissuesfromthe
heart-formingareasproducingtheprimordiumsofthearterialtrunksandthe
atrialchambersatthearterialandvenouspoles,respectively.Itusedtobe
thoughtthatfivesegmentscouldberecognizedintheinitiallineartube.Wenow
knowthatthisisnotthecase.Thedevelopmentofthecardiacchambersdepends
ontheexpansion,orballooning,oftheircavitiesfromthelumenofthe
componentsoftheprimarytube.Subsequenttolooping,thecellsthatmadeup
theinitialcomponentsofthelineartubearenegativeforbothconnexin40and
atrialnatriureticpeptide,characterizingthemasprimarymyocardium(Fig.3.5).
Asthecavityofthelineartubebeginstoballoonoutfrombothitsatrialand
ventricularcomponents,themyocardiumformingthewallsoftheballooning
componentschangesitsmolecularnature,beingpositiveforbothconnexin40
andatrialnatriureticpeptide.Thismyocardiumiscalledchamber,orsecondary,
myocardium.Thepartsballooningfromtheatrialcomponentoftheprimarytube
dosoinrelativelysymmetricfashion(Fig.3.6).Thepouchesthusformedwill
eventuallybecometheatrialappendages.Examinationoftheatrialcomponentof
theheartatthisearlystage,however,revealsthepresenceofathirdpopulation
ofcells(seeFig.3.5).Thesecellsarepositiveforconnexin40butnegativefor
atrialnatriureticpeptide.Theymakeupthepartofthetubethatretainsits
connectionwiththedevelopingmediastinumthroughthedorsalmesocardium.
Hencetheyaredescribedasrepresentingmediastinalmyocardium.5Thesecells
formthedorsalwalloftheleftatriumandasmallpartofthedorsalwallofthe
rightatrium.Theyprovidethesiteofformationoftheprimaryatrialseptumand
giverisetothepulmonaryvenousmyocardium.Ballooningalsotakesplace
fromtheventricularpartofthehearttube.Unlikethesituationintheatrial
component,wheretheappendagesofbothdefinitiveatriumsballooninparallel,
thepouchesthatformtheapexesoftheleftandrightventriclesballoonin
sequencefromtheinletandoutletcomponentsoftheventricularloop.The
ballooningoftheapicalcomponentsinseriesproducestheprimordiumofthe
muscularventricularseptum(seeFig.3.6).Theprocessofcardiacseptation
requiresappreciableremodelingoftheinitiallumenoftheprimaryhearttube.
Thisisbecause,subsequenttoloopingandaftertheinitialphasesofballooning,
thebloodpassingthroughtheatrioventricularcanaldrainstotheinletofthe
ventricularloop,albeitthatadirectconnectionalreadyexiststhroughits
rightwardmarginbetweenthedevelopingwallsoftherightatriumandright
ventricle(seeFig.3.6).Atthisinitialstage,furthermore,thedevelopingoutlet
segmentofthehearttubeissupportedexclusivelybytheoutletpartofthe
ventricularloop,fromwhichwilldeveloptherightventricle.Again,adirect
connectionalreadyexiststhroughthewallsofthetubebetweenthedeveloping
leftventricleandthearterialsegment(Fig.3.7).Remodelingofthelumenofthe
primarytube,alongwiththeconcomitantrearrangementsofthejunctionswith
thedevelopingatrialandarterialsegments,willunderscorethedefinitive
arrangement,whichthenpermitseventualclosureoftheplanebetweenthe
systemicandpulmonarybloodstreams.
FIG.3.5 Adjacentsectionsfromthehearttubeprocessedtoshow
expressionofeitherconnexin40(Cx40)oratrialnatriureticfactor(ANF).
Theimagesshowhowitispossibletodistinguishthreespecificmyocardial
phenotypes.Themediastinalmyocardiumisshownintheredoval,the
primarymyocardiumoftheatrioventricularcanalbythebrackets,andthe
chambermyocardiumbythearrows.
