Andersons pediatric cardiology 262
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NormalPlacentalDevelopment
Theembryo'searlygrowthanddevelopmenttakeplaceinafairlyhypoxic
environmentandaresupportedbyuterineglandsecretions,whicharedirectly
absorbedthroughphagocytosis.1–3Itisnotuntiltheendofthefirsttrimesterthat
nutrientdeliveryshiftstoblood-basedexchangeviatheplacenta.Inpreparation,
however,theplacentabeginstoformsoonafterfertilizationanddevelopsin
tandemwithorgansintheembryo.
Thematureplacentaconsistsofthreelayers.Thechorionicplateisthelayer
closesttothefetus,wheretheumbilicalcordattaches.Fromthechorionicplate,
acomplexnetworkofnutrient-absorbingvilliextendsintotheintervillousspace
(Fig.11.1).Thevilliareorganizedintolobules,orvilloustrees,andcorrespond
withcotyledonsonthematernalsurfaceoftheplacenta.Maternalspiralarteries
provideflowintotheintervillousspaceofeachlobule,allowingfornutrientand
gasexchange.Thespacedrainsviaendometrialveinsbetweenarterial
pulsations.4,5Whilemanyvillifloatinthisnutrient-richbath,anchoringvilli
traversetheintervillousspacetoconnectthechorionicplatetothebasalplateon
thematernalside.Thebasalplateisacombinationofmaternalendometrium,
whichhasremodeledintothedecidua,andfetaltissues.Aspregnancy
progresses,fibrin,secretionsandapoptoticcellsaccumulateinvariouslayersin
theplacenta;thelayerthataccumulatesinthebasalplateiswheretheplacenta
willdetachatbirth.
FIG.11.1 Structureanddesignofthehumanplacenta,highlightingthe
fetalandmaternalvascularrelationships.(FromThibodeauGA,PattonKT.
AnatomyandPhysiology.7thed.St.Louis:Mosby;2010.)
Thiscomplexorganbeginstoformbetween6and7dayspostfertilization,
withimplantation.5,6Onday5,ablastocystiscreatedwhencellspolarizeintoan
innercellmassandafluidcavity,surroundedbyasinglelayeroftrophoblast
cells.Trophoblastcellsaretheprecursorstotheplacentaandfetalmembranes,
whereastheinnercellmassgivesrisetotheembryo,theumbilicalcord,andthe
placentalmesenchyme.Implantationbeginswiththeblastocystadheringtothe
uterineepithelium,andiscompletewhentheblastocystisfullyembeddedinthe
endometrium,12dayspostfertilization.5–7
Onlytrophoblastcellsthatlieovertheinnercellmassareabletofacilitate
successfulimplantation.Thustheblastocystmustbeorientedwiththeinnercell
massclosesttotheuterineepitheliumforoptimalplacentation.5Twotrophoblast
celltypesemerge:cytotrophoblasts,thestemcellsoftheplacenta,andthe
multinucleatedsyncytiotrophoblast,whichformsfromthefusionof
cytotrophoblasts.Thesyncytiotrophoblastwilleventuallybecomeasingle
multinucleatedbodythatcoverstheentiresurfaceoftheintervillousspace,thus
servingasthebarrierbetweenfetaltissuesandmaternalblood.5Oncethe
blastocysthasformedastableattachment,thesyncytiotrophoblastsecretes
digestiveenzymesthatallowforthefinal,invasivestageofimplantation.The
syncytiotrophoblastformsprojectionsthatadvanceintofluid-filledspaces,
calledlacunae,createdbytheenzymes.Astheseprojectionsortrabeculaegrow,
thelacunaecoalesceandexpand,erodingintomaternalcapillaries.Maternal
bloodcellsarenowabletoenterthefluid-filledspacesurroundingthe
trabeculae.However,exposuretoarterialmaternalbloodflowandthe
establishmentofplacentalcirculationdoesnotoccuruntiltheendofthefirst
trimester.
Between11and13daysafterfertilization,cytotrophoblastcellscreate
invaginationsinthetrabeculaeofsyncytiotrophoblastandbecomeprimaryvilli.
Thefluid-filledspacearoundthevillihasnowbecometheintervillousspace.
Aroundday16,mesenchymalcellsfollowtheinvaginationsmadebythe
cytotrophoblastcellsandtransformthevilliintosecondaryvilli.Finally,tertiary
villiform,whichcontainmesenchymalconnectivetissue,fetalvessels,and
macrophages,encasedinthesinglemultinucleatedsyncytiotrophoblast.
Aroundthistime,somecytotrophoblastcellsalsobeginmigratingbeyondthe
syncytiotrophoblastintomaternaltissues,becomingextravillousorinterstitial
trophoblasts.Trophoblastinvasionwillcontinuethroughthesecondtrimester,
eventuallyextendingintotheinnerthirdofthemyometrium.Thisinvasionis
facilitatedbymigration-inducingchemokinesinthedecidua,while
simultaneouslyinhibitedbycytokinespromotingattachmentoverfurther
invasion.2,7Extravilloustrophoblaststraveldownwithinthelumenofthe
endometrialspiralarteries,replacingendothelialcells,whileinterstitial
trophoblastsenterthevesselwallsdirectly.4,5Thespiralarteriesofthe
endometriumlosesmoothmuscleanddilateinresponsetohormonalsignaling
andthistrophoblastinvasion.2,4Initially,thedensityofextravilloustrophoblast
invasionissuchthatthespiralarteriesareeffectivelyplugged,preventing
arterialmaternalbloodflowintotheintravillousspace.By10to12weeksafter
implantation,theplugsloosenandtheunobstructedspiralarteries,now
continuouswiththeintervillousspace,bathethevilliwithmaternalblood.The
uteroplacentalcirculationisthusestablished,withspiralarteriesfeedingintoa
large,low-impedancelakesurroundingthevilli.Nutritionandoxygenexchange
occurasthebloodflowsoverthevilli.4,5From15weeksonward,asthe
uteroplacentalcirculationintroducesoxygentoapreviouslyhypoxemic
environment,furtherinvasionofthespiralarteriestakesplace.Remodelingof
