Andersons pediatric cardiology 839
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (212.69 KB, 3 trang )
Morphogenesis
Almostthree-quartersofacenturyago,MaudeAbbottcommentedthat
understandingofcardiacdevelopmentshouldprovidethebasisforappreciation
ofthedifferenttypesofcongenitallymalformedhearts.24Thankstothe
developmentofepiscopicmicroscopy,25wenowhavetheevidenceinhandto
substantiateherprediction.Thefindingsusingthistechniquewithregardto
normaldevelopmentoftheventricularseptumprovideevidencepertinenttothe
conceptsusedpreviouslytodifferentiateanddescribethephenotypes.Evidence
fromgeneticallyperturbedmiceprovidesadditionalevidenceinthisregardto
supportthenotionthatalldefectscanbecategorizedintermsofbeingmuscular,
perimembranous,ordoublycommittedandjuxtaarterial.Asalreadydiscussedin
Chapter3,ballooningoftheapicalcomponentsofthedevelopingventricles,
fromtheinletandoutletcomponentsoftheventricularloop,producesthefirst
evidenceofdiscretemorphologicallyleftandrightventricularchambers.Asthe
apicalcomponentsballoon,theprimordiumofthemuscularventricularseptum
developsbetweenthem(Fig.32.15A).
FIG.32.15 Imagestakenfromathree-dimensionaldatasetprepared
usingepiscopicmicroscopyfromadevelopingmouseatembryonicday(E)
10.5.(A)Frontalsectionillustratinghow,atthisearlystage,the
atrioventricular(AV)canalopenstothecavityofthedevelopingleft
ventricle(single-headedarrow).Theinterventricularcommunication
(double-headedarrow)providesaccesstothedevelopingrightventricle.
Notethattheapicalmuscularventricularseptum(star)appearsbetween
theballooningapicalventricularcomponents.(B)Section,alsointhe
frontalplane,fromadevelopingmousesacrificedatE11.5.Expansionof
theatrioventricularcanalhaspermittedthecavityoftherightatriumto
opendirectlytothecavityofthedevelopingrightventricle(single-headed
arrow).Bythisstage,themusculatureoftheatrioventricularcanalis
becomingsequestratedtoformtheatrialvestibule;theshortarrowsmark
thedevelopingplaneofatrioventricularinsulation.Notethatthedeveloping
atrioventricularbundlecanberecognizedonthecrestofthemuscular
septum.
Previousinvestigationshadshownthattheatrioventricularbundleisformed
onthecrestofthedevelopingmuscularseptum.26Thesestudies,bychartingthe
locationofthebundleandfollowingtherelationshipbetweenthecrestofthe
septumandtheinsulatingplanebetweentheatrialandventricularsepta,showed
thatpostnatallytheinitialmuscularseptumgivesrisetotheentiretyofthe
muscularventricularseptum.Ithadbeensuggestedthatan“atrioventricular
canalseptum,”alongwitha“conalseptum,”werepartofthedefinitivemuscular
ventricularseptum.21Absenceofthealleged“atrioventricularcanal”component
wasusedtojustifydescriptionoftheperimembranousinletdefectsassociated
witheitheratrioventricularseptalalignmentormalalignmentasan
“atrioventricularcanal”ortypeIIIdefect.27,28AsshowninChapter3andas
discussedinChapter31,aseptumisindeedformedtodividetheatrioventricular
canal.Itisformedsubsequenttomuscularizationofthemesenchymalcapofthe
primaryseptumandthevestibularspine.29Thiscomponent,however,
subsequenttothecompletionofatrialseptation,issequestratedontheatrial
aspectoftheinsulatingplane.Itdoesnotformpartofthemuscularventricular
septum.(seeFig.32.15B).Thesequestrationoftheatrioventricularcanal
myocardiumandtheseparationoftheatrioventricularjunctionsbygrowthofthe
vestibularspineaccompanytheprocessofexpansionofthecanal.This,inturn,
involvesremodelingoftheinitialembryonicinterventricularcommunication
suchthattherightatrialcavityisplacedindirectcommunicationwiththe
developingrightventricle.Subsequenttothisstageofdevelopment,however,
theoutflowtractremainssupportedonlybythedevelopingrightventricle(see
Fig.32.16,left),eventhoughitsproximalpartisalreadybeingdividedinto
potentialaorticandpulmonarycomponentsbyfusionoftheoutflowcushions
(seeFig.32.16,right).
FIG.32.16 Sectionstakenfromthesameepiscopicdatasetasshownin
Fig.32.15B.Asseenatleft,althoughtherightatriumnowconnectsdirectly
withtherightventricle(single-headedarrow),theoutflowtractremains
exclusivelysupportedabovetherightventricularcavity.Thebloodentering
itsdorsalcomponent,whichwillbecometheaorticroot,mustpassatthis
stagethroughtheembryonicinterventricularcommunication(doubleheadedarrow).Thisarea,withthecrestofthemuscularventricularseptum
asitsfloor(star),isroofedbytheinnerheartcurvature.Thepanelatright
showshowtheendocardialcushionswithintheoutflowtractarefusingto
separatetheputativeaorticandpulmonaryoutlets.
Itisonlywithongoingdevelopmentthattheaorticcomponentoftheoutflow
tractismoveddorsallyandleftward.Thisprocessalsobringsthefusedproximal
outflowcushionsinlinewiththecrestofthemuscularventricularseptum,thus
wallingtheaortaintotheleftventricle.Andonlywhentheaorticrootis
committedtotheleftventricleisitpossibleforthedevelopingembryotoclose
thepersistingembryonicinterventricularcommunication.Thisisachievedbythe
growthofso-calledtuberclesfromtheventricularsurfacesoftheatrioventricular
cushions,withthetuberclessubsequentlybecomingthemembranouspartofthe
ventricularseptum(Fig.32.17A).Afterthecompletionofseptation,thecoreof
thefusedoutflowcushionmassisconvertedintoanareaoffibroadiposetissue.
Thisareaseparatesthemuscularizedsurfaceofthecushionsfromtheaorticroot
(seeFig.32.17B).Themuscularizedsurfacebecomesthefreestandingmuscular
infundibularsleeve,alsoknownasthepulmonaryconus.Becauseofthe
attenuationofthecoreoftheinitialembryoniccushionmass,thereisno“conal”
or“outlet”ventricularseptumtobefoundinthenormalpostnatalheart.The
postnatalventricularseptum,therefore,hasonlymuscularandmembranous
