Andersons pediatric cardiology 96
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (183.92 KB, 3 trang )
OutletSegment
Wheninitiallyformed,theentiretyoftheoutflowtract,supportedbytheoutlet
oftheventricularloop,hasmyocardialwalls.Atthemarginsofthepericardial
cavity,thesolitarycavityofthismyocardialtubebecomescontinuouswiththe
so-calledaorticsac.Thesacitselfisamanifoldthatgivesrisetothearteriesthat
extendthroughthearchesofthepharyngealmesenchyme.Itisgrowthofa
protrusionfromthemidlineofthedorsalwallofthesac,originatingbetweenthe
originsofthearteriesextendingthroughthefourthandsixtharches(Fig.3.23),
thatwilleventuallyfusewiththedistalendsoftheoutflowcushionssoasto
separatethesolitarycavityoftheoutflowtractintothepulmonaryandaortic
channels.Attheearlystageofitsdevelopment,whenthetractstillhas
exclusivelymuscularwalls,acircumferentiallayerofcardiacjellyliesbetween
endocardiumandmyocardiumthroughtheentiretyofthetube.Atthisstage,the
tracthasanobviousbend,permittingthedistinctionofproximal,intermediate,
anddistalparts(Fig.3.24).15
FIG.3.23 ScanningelectronmicrographofahumanembryoatCarnegie
stage14showingthejunctionofthedistaloutflowtractwiththeaorticsac.
Thesacisnomorethanamanifoldwithinthepharyngealmesenchyme
thatgivesrisetothearteriesrunningthroughthepharyngealarches.Atthis
stageofdevelopment,itispossibletorecognizeonlythethird,fourth,and
sixtharches.Thestarshowstheareaofthedorsalwallofthesac,between
theoriginsofthearteriestothefourthandsixtharches,whichwillprotrude,
inanobliquefashion,intothedistaloutflowtractandbecomethe
embryonicaorto-pulmonaryseptum(seeFig.3.27).
FIG.3.24 Scanningelectronmicrographshowingtheexternalaspectofa
humanembryoatCarnegiestage14,whentheoutflowtractisamuscular
tube.Thepresenceofanobviousbendpermitsrecognitionofproximal,
intermediate,anddistalcomponents.
Thearteriestakingoriginfromtheaorticsacinitiallyruninsymmetrical
fashionthroughthedevelopingpharyngealarches,encirclingthegutandthe
developingtracheobronchialgroove,andunitingdorsallytoformthedescending
aorta.Itisfrequentforcartoonsusedintextbookstoshowsixpairsofarch
arteries.Inreality,therearenevermorethantwoorthreepairsofarches,along
withtheirarteries,tobeseenatanyonetime.Attheinitialstages,theaorticsac
hasrightandlefthorns,witheachhorngivingrisetothearteriesofthefirstto
thirdarches.Thearteriesofthefirstandsecondarchesrapidlybecome
assimilatedintothearterialsystemoftheheadandface.Bythetimeitbecomes
possibletorecognizethearteriesofthefourthandsixtharches,thearteries
withinthefourtharcharefeedingthearteriesofthethirdarches(seeFig.3.23).
Itisthennolongerpossibletorecognizethearteriesoftheinitialtwoarchesas
encirclingstructures(Fig.3.25).
FIG.3.25 Leftaspectofthearchescoursingthroughthepharyngeal
mesenchymaofadevelopingmouseatembryonicday11.5,acomparable
stagetothehumanheartshowninFig.3.23.Thisimageshowsa
reconstructionofanepiscopicdataset.Theaorticsac(shortarrows)gives
risebilaterallytothearteriespassingthroughthethird,fourth,andsixth
pharyngealarches.Thearteriesthenjoinbilateralaortas(longarrows)that
fusecaudally(star).Thecavityofthedistaloutflowtract(OFT)atthisstage
remainsasolitarystructure,joiningthecavityoftheaorticsacatthe
marginsofthepericardialcavity,whichisreconstructedingreen.(Courtesy
Dr.SimonBamforth,NewcastleUniversity.)
Asdevelopmentproceeds,newnonmyocardialmaterialisaddedtothetubeat
themarginsofthepericardialcavity.Thisresultsineffectiveregressionofthe
distalborderofthemyocardialwallsawayfromthemarginsofthepericardial
cavity.Thenonmyocardialtissueinitiallygrowsintothepericardialcavityon
eithersideinhorseshoefashion,formingtheparietalwallsoftheaortaandthe
pulmonarytrunk.Concomitantwiththeformationofthenonmyocardialparietal
walls,thecentralareabetweentheoriginsofthefourthandsixtharcharteries
