Andersons pediatric cardiology 90
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FIG.3.8 Scanningelectronmicrograph,preparedfromadeveloping
mouseatembryonicday9.5,takenbymakingasectionthroughtheatrial
componentofthetubeatthesiteofitsconnectiontothepharyngeal
mesenchymethroughthedorsalmesocardium.Theopeningsofthe
systemicvenoustributariesareseenthroughvenousconfluences,which
areknownasthehornsofthesystemicvenoussinus.Notethat,atthis
stageofdevelopment,therearenoboundariesbetweentheatrial
componentandthesinushorns.
FIG.3.9 Scanningelectronmicrograph,preparedfromadeveloping
mouseataslightlylaterstageofembryonicday10.5,showinghowthe
systemicvenoustributarieshavebecomeconnectedtotherightsideofthe
atrium,withtheirjunctionswiththeatriumbecomingdistinctasthevalves
ofthesystemicvenoussinus.Notethepulmonaryridgesmarkingthesite
ofthedorsalmesocardium(seeFig.3.11).Thewallsoftheleftsinushorn
havebeenincorporatedintothedevelopingleftatrioventriculargroove.
Theremodelingofthesystemicvenoussinussetsthescenefordevelopment
ofthepulmonaryvenoussystem.Thepulmonaryveins,ofcourse,cannotbe
formedwithoutdevelopmentofthelungsthemselves.Theseappearasbuds
withinthemediastinalmesenchymeontheendsofthebifurcating
tracheobronchialtube.Venousstructuresthendevelopwithinthelungbuds,
concomitantwithcanalizationofavenouschannelfromamidlinestrand
initiallyformedwithinthemediastinaltissues.Whencanalized,themidline
channel,orprimarypulmonaryvein,drainsthedevelopingintrapulmonary
venousplexusesfrombothlungs.Itjoinstheheartatthesiteofthepersisting
dorsalmesocardium.Mostofthemesocardiumthatinitiallyconnectedthelength
ofthehearttubewiththemediastinumbreaksdownduringventricularlooping.
Whenviewedinternally,theedgesofthepersistingmesocardialconnectionare
seenastworidges,thepulmonaryridges,whichbulgeintothelumenofthe
atrialcavity(seeFigs.3.9and3.10).Thecanalizingprimarypulmonaryvein
openstotheatrialcavitybetweentheseridges,itsopeningbeingadjacenttothe
developingatrioventricularjunction(Fig.3.11).
FIG.3.10 Theimage,preparedfromanepiscopicdatasetfromamouse
embryoatembryonicday10.5,showshowthepulmonaryridges(stars)
markthesiteofthepersistingdorsalmesocardium.Notetheformationof
thelungbudsinthemediastinalmesenchyme,withthemid-pharyngeal
strandpointingtothepitbetweentheridges.Thestrandwillcanalizeto
becomethepulmonaryvein.Notealsothevenousvalvesnowmarkingthe
junctionoftherightatriumwiththesystemicvenoussinus.
FIG.3.11 SectionfromahumanembryoatCarnegiestage14cutinthe
long-axisplane.Itshowsthelocationofthesolitarypulmonaryvenous
orificeimmediatelycranialtotheleft-sidedsinushorn,nowincorporated
intotheleftatrioventricularjunctionasthecoronarysinus.Notethatthat
coronarysinuspossessesitsownwalls,whicharediscretefromthewalls
oftheleftatrium.
Controversyhaslongragedastotherelationshipbetweenthenewlyformed
pulmonaryvenousconfluenceandthetributariesofthesystemicvenoussinus.
Duringnormaldevelopment,theprimarypulmonaryveinhasneverhadany
connectionwiththesystemicvenoustributaries.Itformsasanewstructure
withinthemediastinum,openingwithinthemediastinalmyocardiumtothe
cavityoftheleftatrium.Assuch,itispositionedtotheleftofthesiteof
appearanceoftheprimaryatrialseptum,whichisalsoderivedfrommediastinal
myocardium(seeFig.3.5).Thepulmonaryvenousstructuresaresimilarly
recognizablefromtheoutsetoftheirdevelopmentasbeingderivedfrom
mediastinalmyocardium,whereasthesystemicvenoustributariesinitially
possessaprimarymyocardiallineage,andcanbeidentifiedinmolecularterms
