Deconstructing
the
molecular
mechanisms
shaping
the
vertebrate
body
plan
Rita
Aires
1,
Andre´ Dias
1and
Moise´s
Mallo
Thelargedisplayofbodyshapesandsizesobservedamong
vertebratesultimatelyrepresentvariationsofacommonbasic
bodyplan.Thislikelyresultsfromtheuseofhomologous
developmentalschemes,justdifferentiallytinkeredbothin
amplitudeandtimingbynaturalselection.Inthisreview,wewill
revisit,discussandcombineoldideaswithnewconceptsto
updateourviewonhowthevertebratebodyisbuilt.Recent
advances,particularlyatthemolecularlevel,willguideour
deconstructionoftheindividualdevelopmentalmodulesthat
sequentiallyproducehead,neck,trunkandtailstructures,and
thetransitionsbetweenthem.
Address
InstitutoGulbenkiandeCieˆncia,RuadaQuintaGrande6,2780-156 Oeiras,Portugal
Correspondingauthor:Mallo,Moise´s(mallo@igc.gulbenkian.pt) 1
Equalcontribution.
CurrentOpinioninCellBiology2018,55:81–86
ThisreviewcomesfromathemedissueonDifferentiationand disease
EditedbyKatjaRo¨perandXose´ RBustelo
ForacompleteoverviewseetheIssueandtheEditorial
Availableonline14thJuly2018
https://doi.org/10.1016/j.ceb.2018.05.009
0955-0674/ã2018ElsevierLtd.Allrightsreserved.
Introduction
Formationofthevertebratebodyisadynamicandcomplex
processthatstartsattheendofgastrulation.Progressively,
from head to tail, the body axis is laid down by the
sequential addition of primodiae for the different body
structuresattheposteriorembryonicend[1].Thisprocess canbedividedintofoursequentialstages.Thefirststage
comprises production of head structures, essentially
formed bythe brainandmost sensoryorgansencasedin
arigid skull.Aftercompletionof head structures,a
pro-foundswitchin developmentalmechanismsleadstothe
formation ofthetrunk,whichcontains the largest partofthe
respiratory, digestive,reproductiveandexcretory organs.
Theseareheldandprotectedbyaribcageandamobile
axialskeletoncomposedofvertebraethatalsoenclosethe
spinal cord. The neck emerges as a transitory region
betweentheheadandthetrunk,alreadycontainingspinal
cord and vertebral column but still not associated with
major organicsystems.Thelast stageof vertebratebody
formationistaildevelopment,whichyieldsa
muscle-skel-etalstructureinvolvedinlocomotion,balance,defenceand
intraspecificcommunication.Insomespecies,suchasin
humans,taildevelopmenthasbeenmodifiedinsuchaway
thatonlyavestigialelementisleft[2,3].
Thehead,neckandtrunkallrelyontheepiblasttoproduce theirneuraltubeandrequiretheactivityofthePrimitive
Streak (PS) to generate mesodermal components. This
contrastswithtaildevelopment,whichdependsonthetail
budtobuildbothitsmesodermandneuraltube[4].These
differences were initially observedby Holmdahlalmost
100 years ago, coiningthe terms primary andsecondary
body for PS-dependentand tailbud-dependent regions,
respectively [5]. Today, recent advances enable us to
addresstheformationofthevertebratebodyatamolecular
level,thusexpandingHolmdahl’sclassicalvision.
New
layers
in
the
regulation
of
PS
formation
FormationofthePSisakeystepinvertebrate
develop-ment. This is a complex process involving a variety of
cellularandmolecularinteractionslimitingaratherbroad
cellularcompetencetoaspecificareaofamorphogenetic
field[6].Recentfindingshavediscoveredextraregulatory
layers to this process. Classic genetic experiments had
shownthattheTransforming GrowthFactor-b(TGF-b)
familymemberNodal(SeeBox1)isacentralcomponentof
the molecularnetworkinducingPS formation[6].
How-ever,thepre-gastrulatingamnioteembryocontainsNodal
activity scatteredthroughoutthe epiblast.Nevertheless,
thisactivityisunabletotriggergastrulationbecauseitis keptbelowacriticalthresholdlevelbyspecificinhibitors, includingthe Cerl1ortheLeftyproteins[7].Yet,itstill
allowsforsporadiccelldelaminationthroughoutthe
epi-blast[8].Ithasnowbeenshownthatthisscatteredactivity
converges in a restricted domain as a consequence of
stereotyped cell movements within thisepithelial layer.
This convergencetriggers apositive feedbackloop that
promotes a local increase of Nodal signalling and of
Crumbs2-dependentstimulationofEpithelial to
Mesen-chymalTransition(EMT)-associatedprocesses,confining
andamplifyingthecelldelaminatingactivityinthatareaof theepiblasttogeneratethePS[8,9].Rathersurprisingly,it
was recently found that regulation of Lefty expression
involvesanepigeneticmechanism,requiringtheactivity
oftheTetgenestokeeptheirpromotersnon-methylated.
Indeed, in Tet mutant embryos Nodalactivity becomes
excessively highthroughout theepiblast,leading to the
The
neuro-mesodermal
progenitors
as
a
common
theme
in
post-cranial
body
formation
Head,neckandtrunkhavebeenclassifiedasprimarybody
onthebasis ofcommongeneraldevelopmentalfeatures.
However,therearefundamentalmechanisticdifferences
betweentheproductionofheadandpost-cranialvertebrate
structures(i.e.neck,trunkandtail)[4].Twoparticularly
relevant indicators of this change are thecontrol of PS
activity, that becomes independent of Nodal signalling
[11],andthedrasticmodificationinthebuildingofbody structures.Inparticular,whiletheprimordiaeforthe
head-associated neural and mesodermal structures are mostly
shapedconcomitantlywiththeappearanceandearly
orga-nizingactivityofthePS,thepostcranialbodyis progres-sivelylaiddownastheembryoextendsatitscaudalend.In
theneuraltubeandsomiticmesoderm,thisdevelopmental
switchmarkstheoutsetofthespinalcord andvertebral
columnandisfunctionallylinkedtotheappearanceofthe
Neuro-MesodermalProgenitor(NMP)[12].Thiscell
pop-ulationwasidentifiedingraftingexperimentsbyitsability
to generate neural and paraxial mesodermal tissues
throughoutdevelopment[13,14],andfurthercharacterized byretrospectiveclonalanalysesinthemouseembryo[15].
Despite some controversy, it is generally accepted that
NMPsarethe origin of the spinal cordand paraxial (somitic) mesodermfoundintheneck,trunkandtailstructures,thus
contributingtobothprimaryandsecondarybodystructures
[12,15].Inamnioteembryos,NMPsinvolvedinneckand
trunkdevelopmentarelocatedintwospecificareasofthe
epiblast,thenode-streakborderandtheanteriorpartofthe caudallateralepiblast[14,16].Thesecellsarethen
reallo-catedtotheChordo-NeuralHinge(CNH),withinthetail
bud,whentheembryoengagesinsecondarybody
forma-tion[14].TheobservationthattheCNHcontainsremnants
of the blastopore lip in amphibians and the node in
amniotes [17,18] suggests a mechanistic continuity
between primary and secondary body formation, which
was further supported by retrospective clonal analyses
revealingthattailbudNMPsaredirectderivativesoftheir trunkcounterparts[15].Basedinthepreeminentrolethat
NMPshaveduringvertebratedevelopmentwe,therefore,
proposethat,inadditiontoprimaryandsecondarybody,
vertebratedevelopmentcouldbefurtherdividedin
NMP-independentandNMP-dependentstages.
NMPsarecurrentlydefinedbytheirco-expressionof
Bra-chyury (T) and Sox2, mesodermal and neural markers,
respectively[14,16]. Thesecells are thus thought to be
inatransientstatebetweenthesetwolineages,andtheir
ultimate choice resulting from the balance between the
networkscontrollingeachfate.Avarietyofinvitroandin vivoexperimentsareconsistentwiththisideaandhavealso identifiedWnt3aasakeyregulatoroflineagechoiceinthese
cells. Collectively, these studies show that NMPs take
neural routes when exposed to low or negative Wnt3a
activityandmesodermalfatesuponincreasedandsustained
Box1 . Cdxgenes
CdxhomeoboxgenesbelongtotheParaHoxgeneclusterandencode severalimportanttranscriptionfactorsthatcontrolaxialpatterning.
Cerl1
Thisgeneencodesacytokineandfunctionsasanantagonistofthe TGF-bfamily.Itisinvolvedinvertebrateheadandheartinduction andintheformationandpatterningofthePS.
Crumbs2
ThisgeneencodesforamemberoftheCrumbscellpolaritycomplex familyofproteins.Recentdatasuggeststhat,inmammals,Crumbs2 isnecessarytoremove,ratherthanmaintain,apicaljunctions.It playsakeyroleinmesodermproductionbycompletingtheEMTof epiblastcellsinthePS.
Gdf11
Gdf11belongstotheTGF-bsuperfamilyofproteins.Itbindstothe growthfactor-breceptorAlk5,regulatingtheexpressionofkeygenes (e.g.Hoxgenes)duringembryonicdevelopment.Itisinvolvedinthe Anterior-Posterior(AP)patterning,particularlyduringtailbudandtail formation.
Leftygenes
LeftygenesencodeantagonistsofNodalactivity,bypreventingthe interactionbetweenNodalanditsreceptors.Lefty-1iscrucialduring gastrulation,confiningNodalactivitytothefuturePSregion.Itisalso involvedtheestablishmentofleft-rightasymmetry.
Nodal
NodalisasignalingmoleculeoftheTGF-bsuperfamily.Bindingof Nodaltospecificsurfacereceptorstriggersasignalingpathway involvingrecruitmentandactivationoftheSMADtranscription fac-torsfamily.ItplayskeyrolesinPSformationandintheestablishment ofleft-rightsymmetry.
Oct4(Pou5f1)
Embryonicstemcellsaregovernedbyacoreofkeytranscription factorsincludingOct4.Thisgeneencodesaproteinessentialfor pluripotencyandself-renewalpropertiesduringembryonic devel-opment.Additionally,ithasrecentlybeenshowntoplayakeyrole controllingvertebratetrunklength.
Sox2
Sox2isamemberoftheSRY-relatedHMG-box(SOX)familyof tran-scriptionfactorsthatplayscriticalrolesinavarietyofdevelopmental processes.Itisinvolvedinmaintainingtheself-renewalofneural progenitorcells,thusplayingakeyroleinneuraltubeformation.
T
TheT-boxtranscriptionfactorBrachyury(T)playsseveralkeyroles duringembryonicdevelopment,particularlyinmesodermformation anddifferentiationbytranscriptionalregulationofimportant meso-derm-associatedgenes.Itisalsocrucialfornotochorddevelopment.
Tetgenes
TetgenesencodeforproteinsoftheTET(ten-eleventranslocation) familythatcatalyseDNAdemethylationandcontrolgeneexpression throughepigeneticmechanisms.
Wnt3/Wnt3a
Wntproteinsareinvolvedinkeyintercellularsignallingwithmultiplecrucial rolesduringembryonicdevelopment.Wnt3regulatesAPpatterninginthe earlyembryoandPSformation.Wnt3aisnecessaryformesoderm production,asitcontrolsprogenitorcellfateduringaxialelongation.
Wnt3aactivity,whichisfurtherpromotedandstabilizedby Tbx6andMsgn1[19,20,21,22,23–25,26,27–29].
NMPs are involved in the formation of primary and
secondarybodystructures.Somefactors,likeT,Wnt3a,
Cdx,andFgfsignallingregulatetheiractivityatallaxial levels[19,30,31,32,33,34].However,recentstudies
indi-cate that genetic networks regulating early trunk and
earlytailprogenitorsinthemouseseemtobe
fundamen-tallydifferent,relyingondistinctivesubsetsofgenesfor trunkandtailformation[22].Amongthem,Oct4stands
out as a key regulator of trunk development, being
dispensable attail bud stages.Indeed, conditional Oct4
inactivation after completion of its role during early
pluripotentstagesproducedembryoslackingtrunk
struc-tures,but thatstillcontained recognizabletailbud
deri-vatives [35]. Conversely, sustained Oct4 activity in the
epiblast was sufficient to extend formation of trunk
structures,maintainingtypicalprimarybodygrowth
char-acteristics,aswellasdelayingorevencompletely
block-ing thetransition intosecondary bodyformation [36].
The importance of Oct4 for trunk development was
further supported by the observation that in snake
embryos,characterizedbyextremelylongtrunks,
recom-bination eventsmostlikelybroughtOct4underthe
con-trolofregulatoryelementsthatkeptitsexpressionactive for exceptionallylongdevelopmentaltimes[36].
Similar
structures,
different
mechanisms
Somites areanotherkeyelement ofvertebrate
develop-ment.Interestingly,althoughtheyareproduced
through-outmostofthemainbodyaxis,themechanismsregulating
theirformationvaryatdifferentaxiallevels.Thefirstfew somites,whichformtheoccipitalboneinsteadofvertebrae [37],arenotbuiltfollowingthegeneralmodelof somito-genesis,astheyareparticularlyresilienttogenetic
altera-tionsthatstronglycompromisethedevelopmentofmore
caudalsomites[38,39].Interestingly,recentobservations
indicate thatformation of NMP-derived somites is also
mechanistically different during primary and secondary
bodyformation.Inparticular,whilecompleteinactivation
of Lfnginterferesgloballywithpostcranialsomitogenesis
[40,41],oscillatory expression of thisgene isrequiredto buildtrunksomitesbutdispensableattaillevels[42].Tail
somiteswerealsoshowntobemoreresilienttoreductions
in Lfng dosagethan theirtrunk counterparts [43].
Con-versely,Hoxb6readilyinterfereswithsomiteformationat
taillevelsbuthavenonegativeeffectsonthesameprocess
in more anteriorregions,further supporting mechanistic
differencesinsomitogenesisduringprimaryandsecondary
bodyformation[44].
Regulating
transitions
between
compartments
Based ontheabovediscussion, vertebratedevelopment
seemstorequireatleasttwomajorfunctionaltransitions:
from NMP-independent to NMP-dependent and from
primarytosecondarybodyformation(seeFigure1).The
firsttransitionentailsaswitchincanonicalWntsignalling
fromrequiringWnt3atheadstagestorelyonWnt3awhen
entering the NMP-dependent phase [[32],19]. The
mechanismsunderlyingthischangearenotknown.
How-ever, in mouse embryos carryingastabilized versionof
theAxin2protein,Wntsignallingwasreducedduringthe
Wnt3phasebutstimulatedafterswitchingtothe
Wnt3a-drivenstage[45],indicatingthattheWnt3toWnt3aswap
isassociatedwithamechanisticchangeinthecanonical
Wntpathway.Duringthisfirsttransition,axialextension
also becomes dependent on T and the Cdx genes
[31,32,46]. Wnt3a, T and Cdx activities might indeed
cooperatetopromotetheemergenceofNMPsasrecent
genomic studiesrevealedtheopening ofkey enhancers
uponWnt-inducedNMPformation,followedby
cooper-ativebindingofTandCdxproteinstoregulatoryregions
of Wnt and Fgf signalling targets involved in NMP
activity[32].
Thetransitionfromheadtoneck/trunkdevelopmentalso
requireschangingthedevelopmentalpotentialofepiblast
cellsfatedtointermediateandlateralmesoderm.During
headformation,thesecellsgeneratetheheartprimordion,
whereasinthetrunktheycontributetoawidevarietyof
organsofthe digestive,excretory and reproductive
sys-tems.Recentdataindicatesthatasignalfromthe
NMP-derived neural tubeis instrumentalto prevent the
pro-duction of heart lineages from the lateral mesoderm.
Interestingly, as for PS formation, Tet-mediated
epige-neticmodificationsseemtoplayanessentialroleinthis
process[47].Specifically,intheabsenceofTetactivity,
Wntsignallingbecomesover-activatedintheprogenitor
zone,blockingneuraldifferentiation ofNMPs,whichin
turnleadstoafailureinkeepingthecardiacfieldwithin
itsnormaldomain,extendingcaudallyintothe
prospec-tiveneckregion[47].Theseobservationsindicatethat thepositionofthefirstvertebraeandthecaudallimitof theheartlineagesarebothinter-relatedandlinkedtothe
emergenceofNMPs,whichoccursatanalmostinvariable
axialpositionindifferentvertebratespecies.Conversely,
activationoftrunklateralmesoderm,generallymarkedby
thepositionoftheforelimbbud,occursathighlyvariable
axial levels. The relativetimingof thesetwo processes
most likely will provide the basis for the broad neck
lengthdiversityamongvertebratespecies.
Asforthelasttransition,recentdataplacesGdf11 signal-lingatthetopoftheregulatoryhierarchycontrollingthe
shift from primary to secondary body formation. The
absence of Gdf11 alone or together with Gdf8 resulted
infetuseswithextendedtrunks[48,49],whereas
prema-tureactivationofGdf11signallingledtodramatic
short-eningsinthisregion,withanearlytransitioninto
second-ary body formation [50]. The axial level of Gdf11
expression directly correlates with the position of the
species[36,51],furthersupportingakeyroleforGdf11
signallinginthisprocess.Interestingly,Gdf11andNodal
signallingsharemanyfeatures,whichconstitutean
inter-esting parallelism between mechanisms initiating and
finishingPS-dependentvertebrate development.
None-theless,themechanismsoperatingdownstreamofGdf11
signallingareonly partiallyunderstood.One of thekey
componentsof thisprocess, theend oforgan-producing
progenitors, seems to depend on the Gdf11-mediated
activationof Isl1in theprogenitors fortheintermediate
and lateral mesoderm [50]. This activation triggers the
terminaldifferentiationofthoseprogenitorsbymeansof
inducingboththehindlimbfromitssomaticlayerandthe
cloaca-associatedmesodermfrom itssplanchnic
compo-nent.However,themechanismsregulatingNMP
reallo-cation from theepiblast into the tailbud remain to be
discovered.
Concluding
remarks
Holmdahl’sfirstproposalfortheexistenceofprimaryand
secondary body formation was solely based on
morphological traits. The age of genetics has mostly
proven him right; yet, it also revealed the enormous
complexity surrounding the development of the
verte-bratebody.Despiteitscontinuousnature,theprocessof
axisextensionseemstobeorganizedintodevelopmental
units,eachcontrolledbydistinctivemolecularnetworks
thatbroadlycorrespondtoeachbodyregion.Recentdata
has added new layers of complexity to these relevant
regulatory networks, including the discovery of acting
epigenetic mechanisms. A full understanding of these
processesand their mutualinteractions will furtherour
knowledgeofhowthevertebratebodyplanisshapedand
how modifications in these processes and interactions
produced the wide variety of body shapes displayed
amongvertebratespecies.
Acknowledgements
WorkinM.M.labwassupportedbygrantsPTDC/BEX-BID/0899/2014 (FCT,Portugal)andSCML-MC-60-2014(fromSantaCasadaMiserico´rdia deLisboa,Portugal).A.D.isrecipientofaPhDfellowshipPD/BD/128426/ 2017(FCT,Portugal).
Figure1
Head Neck Trunk Tail
Primary body Secondary body
NMP-indep. NMP-dependent
Tail bud Gdf11(+) PS Nodal(-) Epiblast Oct4(+) Gdf11(-)
PS Nodal(+) Tet Wnt3 Wnt3a Cdx T(Brachyury) ~E 7.5 ~E 8.5 ~E 9.5 ~E 10.5 PS PS TB
Current Opinion in Cell Biology
Schematicrepresentationofthedifferentsequentialstagesinvolvedintheformationofthevertebratebody.Thehead,neckandtrunk representtheprimarybody,associatedwiththepresenceoftheprimitivestreak(PS),whereasthetailrepresentsthesecondarybody, relyingonthetailbud(TB).Notallprimarybodydependsonthesamemechanisms.Inthehead,thePSrequiresNodalandWnt3butin thetrunkthePSisindependentofthesesignals,becomingdependentonOct4.Theneck,trunkandtailallrequireNMPsfortheir development,whichcoincideswiththerequirementofT,Wnt3aandtheCdxfamilyfortheirdevelopment.Thedrawingsrepresentmouse embryosatstagesrelatedtotheproductionofhead,neck,trunkandtailstructures.Theredarrowindicatesthedirectionoftheaxial growth.
References
and
recommended
reading
Papersofparticularinterest,publishedwithintheperiodofreview, havebeenhighlightedas
ofspecialinterest ofoutstandinginterest
1. WilsonV,Olivera-Martı´nezI,StoreyKG:Stemcells,signalsand vertebratebodyaxisextension.Development2009, 136:1591-1604.
2. BentonMJ:VertebratePalaeontology.BlackwellScienceLtd; 2005.
3. CracraftJ,DonoghueMJ:AssemblingtheTreeofLife. Oxford: OxfordUniversityPress;2004.
4. Be´naze´rafB,Pourquie´ O:Formationandsegmentationofthe vertebratebodyaxis.AnnuRevCellDevBiol2013,29:1-26.
5. HolmdahlDE:ExperimentelleUntersuchungenuberdieLage derGrenzeprimarerundsekundarerKorperentwicklungbeim Huhn.AnatAnz1925,59:393-396.
6. ArnoldSJ,RobertsonEJ:Makingacommitment:celllineage allocationandaxispatterningintheearlymouseembryo.Nat RevMolCellBiol2009,10:91-103.
7. Perea-GomezAetal.:Nodalantagonistsintheanteriorvisceral endodermpreventtheformationofmultipleprimitivestreaks. DevCell2002,3:745-756.
8. VoiculescuO,BodensteinL,JunIL,SternCD:Localcell interactionsandself-amplifyingindividualcellingression driveamniotegastrulation.Elife2014,2014:1-26.
9.
Ramkumartheepithelial-to-mesenchymalNetal.:Crumbs2promotestransitioncellatingressiongastrulation.duringNat CellBiol2016,18:1281-1291.
TheauthorsreportthatCrumbs2isrequiredforcellstocompleteEMT duringgastrulationandthat,togetherwithmyosinIIB,playsakeyrole definingwhichcellswillingressinthePS.
10.
DaigastrulationH-Qetal.:byTET-mediatedregulatingLefty-NodalDNAdemethylationsignalling.Naturecontrols2016, 538:1-20.
Thisstudydemonstratesthatkeysignallingpathwaysduring gastru-lation are controlled by DNAmethylation of specific promoters.In particular, the authors show that the TET family of dioxygenases, throughDNAdemethylation,playsafundamentalroleregulatingPS formation.
11. KumarA,LualdiM,LewandoskiM,KuehnMR:Broad mesodermalandendodermaldeletionofNodalat postgastrulationstagesresultssolelyinleft/rightaxial defects.DevDyn2008,237:3591-3601.
12. HenriqueD,AbranchesE,VerrierL,StoreyKG:
Neuromesodermalprogenitorsandthemakingofthespinal cord.Development2015,142:2864-2875.
13. CambrayN,WilsonV:Axialprogenitorswithextensivepotency arelocalisedtothemousechordoneuralhinge.Development 2002,129:4855-4866.
14. CambrayN,WilsonV:Twodistinctsourcesforapopulationof maturingaxialprogenitors.Development2007,134:2829-2840.
15. TzouanacouE,WegenerA,WymeerschFJ,WilsonV,NicolasJF: Redefiningtheprogressionoflineagesegregationsduring mammalianembryogenesisbyclonalanalysis.DevCell2009, 17:365-376.
16.
WymeerschprogenitortypesFJetal.:inthePosition-dependentprimitivestreak.Elifeplasticity2016,of5:1-28.distinct
Inthisstudytheauthorsassesstherelationship,potencyandplasticityof differentpopulationsofaxialprogenitorsintheepiblastandPS, suggest-ingthatthechoiceofneuroormesodermaldifferentiationofNMPsis dictatedbytheirpositioninthesetissues.
17. GontLK,SteinbeisserH,BlumbergB,deRobertisEM:Tail formationasacontinuationofgastrulation:themultiplecell populationsoftheXenopustailbudderivefromthelate blastoporelip.Development1993,119:991-1004.
18. CatalaM,TeilletMA,LeDouarinNM:Organizationand developmentofthetailbudanalyzedwiththequail-chick chimaerasystem.MechDev1995,51:51-65.
19. TakadaSetal.:Wnt-3aregulatessomiteandtailbudformation inthemouseembryo.GenesDev1994,8:174-189.
20. MartinBL,KimelmanD:CanonicalWntsignalingdynamically controlsmultiplestemcellfatedecisionsduringvertebrate bodyformation.DevCell2012,22:223-232.
21.
KochcontrolFettheal.:fateAntagonisticchoiceofneuro-mesodermalactivitiesofSox2progenitors.andBrachyuryDev Cell2017,42:514-526.
Thisstudyprovidesaglobalanalysisofthekeymolecularinteractions thatcontrolNMPmaintenanceanddifferentiationduringvertebratetrunk development.
22.
mesodermGoutiMetal.:specificationAgeneregulatoryduringvertebratenetworkbalancestrunk neuraland development.DevCell2017,41:1-19.
ThisstudyprovidesthemolecularfingerprintoftrunkNMPsatasinglecell resolutionandproposeatranscriptionalregulatorynetworkcontrolling NMPdifferentiationthatinvolvesopposingWntandretinoicacidsignals.
23. GoutiMetal.:Invitrogenerationofneuromesodermal progenitorsrevealsdistinctrolesforWntsignallinginthe specificationofspinalcordandparaxialmesodermidentity. PLoSBiol2014,12http://dx.doi.org/10.1371/journal.
pbio.1001937.
24. TsakiridisAetal.:DistinctWnt-drivenprimitivestreak-like populationsreflectinvivolineageprecursors.Development 2014,141:1209-1221.
25. JurbergAD,AiresR,No´voaA,RowlandJE,MalloM: Compartment-dependentactivitiesofWnt3a/b-catenin signalingduringvertebrateaxialextension.DevBiol2014, 394:253-263.
26.
JavalinoveltransientAetal.:Co-expressionneuromesodermofTbx6progenitorandSox2cellidentifiesstate. a Development2017,144:4522-4529.
Thisstudyidentifiedacellpoolco-expressingTbx6andSox2inthetail budthatconstituteanewspecificsubdomainofNMPsduringposterior bodyformation.
27. GarriockRJetal.:Lineagetracingofneuromesodermal progenitorsrevealsnovelWnt-dependentrolesintrunk progenitorcellmaintenanceanddifferentiation.Development 2015,142:1628-1638.
28. BouldinCMetal.:WntsignalingandTbx16formabistable switchtocommitbipotentialprogenitorstomesoderm. Development2015,142:2499-2507.
29. GotoH,KimmeySC,RowRH,MatusDQ,MartinBL:FGFand canonicalWntsignalingcooperatetoinduceparaxial mesodermfromtailbudneuromesodermalprogenitors throughregulationofatwo-stepepithelialtomesenchymal transition.Development2017,144:1412-1424.
30. NaicheLa,HolderN,LewandoskiM:FGF4andFGF8comprise thewavefrontactivitythatcontrolssomitogenesis.ProcNatl AcadSciUSA2011,108:4018-4023.
31. HerrmannBG,LabeitS,PoustkaA,KingTR,LehrachH:Cloning oftheTgenerequiredinmesodermformationinthemouse. Nature1990,343:617-622.
32.
inAmintheSembryonicetal.:CdxandaxialTBrachyuryprogenitorco-activateniche.CellgrowthRep2016,signaling 17:3165-3177.
TheauthorsreportthatTandCdx2directlycooperatetoactivateWntand Fgfgeneregulatorynetworksessentialfortrunkelongation.
33. SavoryJGetal.:Cdx2regulationofposteriordevelopment throughnon-Hoxtargets.Development2009,136:4099-4110.
34. YoungTetal.:CdxandHoxgenesdifferentiallyregulate posterioraxialgrowthinmammalianembryos.DevCell2009, 17:516-526.
35. DeVealeBetal.:Oct4isrequiredE7.5forproliferationinthe primitivestreak.PLoSGenet2013,9:e1003957.
36.
diversity.AiresRetal.:DevOct4Cellis2016,akey38:262-274.regulatorofvertebratetrunklength
ThisstudyshowsthatOct4andGdf11areatthetopoftheregulatory networkscontrollingvertebratetrunklength.Italsoprovidesevidence suggesting that the characteristic long trunks of snakes result from persistentOct4expressionduringdevelopment.
37. HuangR,ZhiQ,PatelK,WiltingJ,ChristB:Contributionofsingle somitestotheskeletonandmusclesoftheoccipitaland cervicalregionsinavianembryos.AnatEmbryol(Berl)2000, 202:375-383.
38. OkaCetal.:DisruptionofthemouseRBP-Jkappageneresults inearlyembryonicdeath.Development1995,121:3291-3301.
39. SagaY,HataN,KosekiH,TaketoMM:Mesp2:anovelmouse geneexpressedinthepresegmentedmesodermandessential forsegmentationinitiation.GenesDev1997,11:1827-1839.
40. ZhangN,GridleyT:Defectsinsomiteformationinlunatic fringe-deficientmice.Nature1998,394:374-377.
41. EvrardYA,LunY,AulehlaA,GanL,JohnsonRL:Lunaticfringeis anessentialmediatorofsomitesegmentationandpatterning. Nature1998,394:377-381.
42. ShifleyETetal.:Oscillatorylunaticfringeactivityiscrucialfor segmentationoftheanteriorbutnotposteriorskeleton. Development2008,135:899-908.
43. WilliamsDR,ShifleyET,LatherJD,ColeSE:Posteriorskeletal developmentandthesegmentationclockperiodaresensitive toLfngdosageduringsomitogenesis.DevBiol2014, 388:159-169.
44. CasacaA,NovoaA,MalloM:Hoxb6caninterferewith somitogenesisintheposteriorembryothroughamechanism
independentofitsrib-promotingactivity.Development2016, 143:437-448.
45. QianL,MahaffeyJP,AlcornHL,AndersonKV:Tissue-specific rolesofAxin2intheinhibitionandactivationofWntsignaling inthemouseembryo.ProcNatlAcadSciUSA2011, 108:8692-8697.
46. SavoryJGA,MansfieldM,RijliFM,LohnesD:Cdxmediates neuraltubeclosurethroughtranscriptionalregulationofthe planarcellpolaritygenePtk7.Development2011, 138:1361-1370.
47.
LineuroectodermalXetal.:Tetproteinsandmesodermalinfluencethefatebalancechoicebetweenbyinhibiting Wntsignaling.ProcNatlAcadSci2016,113:E8267-E8276.
TheauthorsshowthatthecontrolofWntsignalinglevelsbyTet-mediated DNAdemethylationplaysafundamental roleintheregulationoffate selectionbyNMPs,eventuallyimpactingthedelimitationofthedomain takingcardiogenicdifferentiationroutes.
48. McPherronAC,LawleAM,LeeS-J:Regulationofanterior/ posteriorpatterningoftheaxialskeletonbygrowth/ differentiationfactor11.NatGenet1999,22:260-264.
49. McPherronAC,HuynhTV,LeeS-J:Redundancyofmyostatin andgrowth/differentiationfactor11function.BMCDevBiol 2009,9:24.
50. JurbergAD,AiresR,Varela-LasherasI,No´voaA,MalloM: Switchingaxialprogenitorsfromproducingtrunktotail tissuesinvertebrateembryos.DevCell2013,25:451-462.
51. MatsubaraYetal.:Anatomicalintegrationofthesacral– hindlimbunitcoordinatedbyGDF11underliesvariationin hindlimbpositioningintetrapods.NatEcolEvol2017, 1:1392-1399.
