FGF Signaling Restricts the Primary Blood Islands to Ventral Mesoderm

[摘要] Accordingtothethree-signalmodelofmesodermpatterninginXenopus,allmesoderm,withtheexceptionoftheSpemannorganizer,isoriginallyspecifiedasventraltype,suchaslateralplateandprimarybloodislands.ItisproposedthatthebloodislandsbecomerestrictedtotheventralmostmesodermbecausetheyarenotexposedtotheBMP-inhibitingactivityoftheSpemannorganizer.Wepresentevidenceherethat,contrarytopredictionsofthismodel,thebloodislandsremainventrallyrestrictedevenintheabsenceofSpemannorganizersignaling.WefurtherobservedthatinhibitionofFGFsignalingwithadominantnegativereceptorresultedintheexpansionofthebloodisland-formingterritorywithaconcomitantlossofsomite.TherequirementforFGFsignalinginspecifyingsomiteversusbloodislandterritorieswasobservedasearlyasmidgastrulation.ThenonoverlappingexpressiondomainsofXnr-2andXbrainthegastrulamarginalzoneappeartomarkpresumptivebloodislandandsomite,respectively.InhibitionofFGFsignalingwithdominantnegativereceptorleadstoanexpansionofXnr-2expressionandtoacorrespondingreductioninXbraexpression.Ontheotherhand,wefoundnoevidencethatmanipulationofBMPsignaling,eitherpositivelyornegatively,alteredtheexpressiondomainsofXnr-2andXbra.TheseresultssuggestthatFGFsignaling,ratherthanBMP-inhibitingactivity,isessentialforrestrictionoftheventralbloodislandstoventralmesoderm.