Andersons pediatric cardiology 800
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leafletsofthetricuspidvalve.ThesearetheindirectGerbodedefects.29The
presenceofamitralvalveguardingadiscreteleftatrioventricularjunction
distinguishesthemfromatrioventricularseptaldefectswithacommon
atrioventricularjunction.Thisdistinctionholdsequallygoodforheartshaving
perimembranousinletdefectsalongwithacleftintheaorticleafletofan
otherwisenormallystructuredmitralvalve.Thislesion,the“isolated”cleft,
whichcanalsobefoundinthesettingofintactseptalstructures,isoften
consideredtobedirectlyrelatedtoatrioventricularseptaldefects.Ithasobvious
developmentalaffinities.30Theanatomicbasisofthetwolesions,however,is
quitedifferent.Italsousedtobeconsideredthatheartswithstraddlingofthe
tricuspidvalvehadanatrioventricularcanaltypeofventricularseptaldefect.31
Althoughtheselesionssharewithatrioventricularseptaldefectthepresenceof
valvarleafletsthatoverridetheseptum,theyhaveseparaterightandleft
atrioventricularjunctionsasopposedtothecommonatrioventricularjunction
(seeChapter32).Insummary,therefore,thepresenceofthecommon
atrioventricularjunctionistheanatomichallmarkofalltheheartswithdeficient
atrioventricularseptationthatarediscussedfurtherinthischapter.
Morphogenesis
Inthepastthestartingpointinconsideringthemorphogenesisofatrioventricular
septaldefectswasusuallytheassumptionthattheatrioventricularendocardial
cushionscontributedmarkedlytotheformationoftheseptalstructuresthatare
lackinginthemalformedheartsandalsototheatrioventricularvalvarleaflets.32
Itwasbecauseoftheseassumptionsthat“endocardialcushiondefect”cameto
enjoysomepopularityasadescriptiveterm.Theconceptsunderscoringthis
approachdonotbearrigorousexamination,neitherfromthestanceof
embryologicevidencenoronthebasisoftheanatomyofthedefinitivelesions.It
isveryattractivetoarguethatcompletefailureoffusionoftheendocardial
cushionswillresultinthe“complete”defect.Partialfailureoffusioncould
arguablyresultintheostiumprimumorpartial,defectand,insimilarfashion,
accountforthecleftintheaorticleafletofthemitralvalve.Butsomethingmuch
morefundamentalmusthappentoproducethegroupofheartsunderdiscussion.
Wehavealreadyseenhowtheiranatomydiffersfromthenormalstateinfar
morerespectsthanthepresenceofasimpleholeintheseptumcoupledwitha
cleftintheaorticleafletofthemitralvalve.Indeed,thefactthatisolatedcleftsin
theaorticleafletofthemitralvalvedoexistbuthavemarkedlydifferent
anatomyfromthegroupunderdiscussionisadditionalstrongevidencethatthe
abnormaldevelopmentismorethansimplefailureoffusionoftheembryonic
endocardialcushions.
Itcouldwellbethataninitialfailureoffusionofthecushionsisthefirststep
intheproductionofanatrioventricularseptaldefect.Butthecushionsdonotgo
ontoproduceallthevalvarleaflets,nordotheyformtheentiretyofthe
separatingatrioventricularjunctionalstructures.Formationofthevalvarleaflets
isaverylatedevelopmentalevent.Itoccurspredominantlybyunderminingof
ventricularmyocardium,afactwellknowntoGermanembryologistsand
anatomistsinthe19thcentury.33,34Thustheabnormalatrioventricularvalvar
leafletsfoundinatrioventricularseptaldefectswithacommonatrioventricular
junctionwillbeproducedandsculptedinpartfromthegrosslyabnormal
ventricularmass.Itistheabnormalarchitectureoftheventricles,specifically
theirjunctionwiththeatriums,thatunderscoresthecharacteristicphenotypic
morphology.Whensearchingforthemechanismthatresultsinthedevelopment
ofacommonatrioventricularjunction,therefore,itisnecessarytodirect
attentiontotheformationofthejunctionalregions.Thisiswhereitisimportant
tounderstandtheinitialfunctionoftheatrioventricularendocardialcushions.
Theyactastheforerunneroftheatrioventricularvalvesinearlydevelopment,
servingalsotogluetogetherthecentralpointofthedevelopingatrioventricular
junctions.Onlyafterthesuperiorandinferioraspectsofthejunctionhavebeen
stucktogethercantheaorticoutflowtractbeincorporatedintothedeveloping
leftventricle(seeChapter3).Theleafletsofthemitralandtricuspidvalvesare
subsequentlydelaminatedfromtheventricularwallsorelsesculptedinpartfrom
thefusedcushions,butinthesettingofanormallywedgedsubaorticoutflow
tract.Iftheendocardialcushionsdonotfusetogetherorifthereisanintrinsic
abnormalityintheformationoftheatrioventricularjunctions,thereislossofthis
focalpointforthesubsequentdevelopmentoftheventricles.Theatrioventricular
junctionswillnotgrowasseparateleftandrightcomponentswithaseptumand
thesubaorticoutflowtractinterposedbetweenthem.Instead,thesuperiorand
inferiormarginsofthejunctionswillretaintheirinitialcommonappearance.
Indeed,thereisremarkableaffinitybetweenthearrangementofthedeveloping
atrioventricularjunctionwiththeanatomyaspreviouslydescribedpriortothe
incorporation,duringdevelopment,ofthesubaorticoutflowtractintothe
developingleftventricle(Fig.31.13A).Itisthepersistenceoftheinitial
commonarrangementoftheatrioventricularjunction,therefore,thatwill
producetheprototypicventricularmassofanatrioventricularseptaldefectwith
acommonatrioventricularjunction.Themaldevelopmentwillalsoexplainthe
presenceofthebasicseptaldefectitself.Thisisbecause,inthenormalheart,the
fusingcushionsalsoprovidethefocalpointforappropriatedevelopmentand
alignmentoftheatrialandventricularseptalstructures.Intheabsenceofsucha
keystone,theprecisetypeofatrioventricularseptaldefectformedwilldependon
thewayinwhichtheatrioventricularvalvarleafletsaredelaminatedand
developedfromtheabnormalventricularmass.Inthisrespect,itshouldbe
rememberedthatatongueofvalvartissuejoinstogetherthebridgingleafletsin
theostiumprimumdefect.Paradoxically,therefore,alesionoftenpresumedto
developbecauseoffailureoffusionofendocardialcushionshascontinuityof
valvartissuethroughthecenterofthecommonjunction.
FIG.31.13 Sectionstakenfromepiscopicdatasetspreparedfrom
developingmouseembryos,sacrificedatembryonicdays11.5(A)and13.5
(B).(A)Frontalsectionshowingthemesenchymalcomponentsthat
surroundtheprimaryatrialforamen.(B)Short-axissectionviewedfromthe
ventricularapexdemonstratingthecomparabilityofthedeveloping
cushionsintheatrioventricular(AV)canalandthearrangementofthe
leafletsofthecommonatrioventricularvalve(seeFig.31.8).
Asyetthetissuesderivedfromthecushionshavestilltobedeterminedwith
certainty.Theabnormality,furthermore,involvesmorethantheatrioventricular
endocardialcushionsthemselves.Additionalmesenchymaltissuessurrounding
theembryonicprimaryatrialforamenareinvolvedinitsclosure.Theseinclude
themesenchymalcapcarriedontheleadingedgeoftheprimaryatrialseptum
