The formationoftheanteroposterioraxisinmicerequiresa Wnt3-dependentsymmetry-breakingevent
that leadstotheformationoftheprimitivestreakandgastrulation. Wnt3 is expressedsequentiallyin
two distinctareasofthemouseembryobeforetheappearanceoftheprimitivestreak;firstinthe
posteriorvisceralendodermandsoonafterintheadjacentposteriorepiblast.Hence,althoughanaxial
requirementfor Wnt3 is wellestablished,itstemporalandtissuespecificrequirementsremainanopen
question.Here,wereporttheconditionalinactivationof Wnt3 in theepiblastofdevelopingmouse
embryos.Contrarytopreviousstudies,ourdatashowsthatembryoslacking Wnt3 specificallyinthe
epiblastareabletoinitiategastrulationandadvancetolateprimitivestreakstagesbutfailtothrive
and areresorbedbyE9.5.Atthemolecularlevel,weprovideevidencethat Wnt3 regulatesitsown
expressionandthatofotherprimitivestreakmarkersviaactivationofthecanonicalWntsignaling
pathway.