Andersons pediatric cardiology 1817

Morphogenesis
Thefindingsregardingcardiacdevelopmentnowhelpinunderstandingthe
developmentoftheheartshavinguniventricularatrioventricularconnections.As
describedinChapter3,duringearlydevelopment,theorificeofthe
atrioventricularcanalissupportedalmostexclusivelyabovetheinletpartofthe
primaryhearttube,withthepresumptivearterialpediclessupportedbytheoutlet
component.Duringthisearlystage,furthermore,theapicalcomponentofthe
developingleftventricleballoonsfromtheinletcomponentoftheventricular
loop,whereasthatofthedevelopingrightventricleballoonsfromtheoutletpart
(Fig.69.23).Theballooningofthetwoapicalventricularcomponentsoccurs
concomitantwithdevelopmentoftheapicalmuscularseptum.Ifthetwo
pouchesdidnotgrowseparatelybutrathertherewasformationofageneral
apicalcomponentfromtheprimaryhearttube,theendresultwouldbeto
produceasolitaryventricleofindeterminatemorphology.Whetherthe
atrioventricularjunctionsareguardedbytwovalvesoracommon
atrioventricularvalvedependsonthepartitioningoftheatrioventricularjunction.
Shouldthepouchesforminnormalfashionbuttheatrioventricularjunction
remainsconnectedonlytotheinletpartoftheventricularloop,theendresult
wouldbedouble-inletleftventricle.Theapicalcomponentderivedfromthe
outletpartoftheloopformsthebasisoftheincompleterightventricle(Fig.
69.24).Again,thearrangementoftheatrioventricularvalveswoulddependon
themodeofdevelopmentoftheatrioventricularjunction.Theventriculoarterial
connectionspresentwoulddependonthedevelopmentoftheoutletportions.
Thepositionoftheincompleterightventriclewouldprobablybedeterminedby
theinitialloopingoftheprimarytube,butequallyitcouldbeinfluencedby
rotationoftheentireheart.


FIG.69.23 Thedevelopingheart(middle)candevelopabnormallysoas
toproduceasolitaryandindeterminateventricle(left)orahearthaving
dominantrightorleftventricle(RV/LV)(right).

FIG.69.24 Imagepreparedfromanepiscopicdatasetmadefroma
humanembryoatCarnegiestage14.Theapicalcomponentoftheright


ventricleisbeginningtoballoonfromtheoutletcomponentofthe
ventricularloop,butatthisearlystage,theatrioventricularcanalopens
exclusivelytothedevelopingrightventricle.Theonlyentrancetothe
developingrightventricleistheembryonicinterventricularcommunication.
Thecomparisonbetweenthedevelopingrightventricleandtheincomplete
rightventriclefoundintricuspidatresiaordouble-inletleftventricle(see
Fig.69.2)isstriking.

Double-inletrightventricleresultsfromtransferoftheentireinletpartofthe
primaryhearttubetotheapicalcomponentderivedfromtheoutlet,thisprocess
occurringsubsequenttoformationoftheapicalcomponentoftheleftventricle
(seeFig.69.23).Thisapicalcomponentwillformthebasisofthe
posteroinferiorlylocatedincompleteleftventricle.Valvarmorphologyand
ventriculoarterialconnectionswillagaindependonthedevelopmentoftheother
partsofthehearttube.Thepositionoftheincompleteventriclewilldependon
theinitialdirectionofventricularlooping.Rightwardloopingwillgivealeftsidedrudimentaryleftventricle,whereasleftwardloopingwillresultinarightsidedleftventricle.
Absenceofanatrioventricularconnectionismuchhardertoexplain.Any
offeredhypothesismustaccountfortheknownanatomicfacts,namelythatin
thesettingofabsenceoftherightconnection,theleftatriumcanbeconnectedto
amorphologicallyleft,right,orindeterminateventricle.Ineachofthese
circumstances,therightatrialanatomyisindistinguishable,asistheleftatrial
anatomyinthesettingofabsenceoftheleftconnectionirrespectiveofwhether
therightatriumisconnectedtoamorphologicallyright,left,orindeterminate
ventricle.Themostlikelyexplanation,therefore,isthattheatrioventricular
junctionsdevelopedasthoughtoproduceadouble-inletconnection,butfor

somereasontheright,orleft,atrioventricularorificefailedtodevelop.Indeed,
examinationofthedevelopinghumanhearthasshownthatastageexistsearlyin
itsformationthatisremarkablysimilartoabsenceoftherightatrioventricular
connection,albeitpriortoformationoftheatrialseptum(seeFig.69.24).
Thefindingsintheheartswithuniventricularatrioventricularconnectionsall
pointtothedevelopmentalinsultashavingoccurredatanearlystageof
formationoftheheart,priortothecompletionofcardiacseptation.Inthe
human,thismeanspriortotheendoftheeighthweekofdevelopment.Inthe
heartswithbiventricularatrioventricularconnectionsthatarefunctionally
univentricular,however,theabnormaldevelopmentmusthavetakenplacemuch
later,sinceinthesettingofhypoplasticleftheartsyndromeorpulmonaryatresia
withintactventricularseptum,theintegrityoftheventricularseptumindicates