Requirement for Wnt3 in vertebrate axis formation

Several studies have implicated Wnt signalling in primary axis formation du ring vertebrate embryogenesis, yet no Wnt protein has been shown to be esse ntial for this process(1-3), In the mouse, primitive streak formation is th e first overt morphological sign of the anterior-posterior axis, Here we sh ow that Wnt3 is expressed before gastrulation in the proximal epiblast of t he egg cylinder, then is restricted to the posterior proximal epiblast and its associated visceral endoderm and subsequently to the primitive streak a nd mesoderm, Wnt3(-/-) mice develop a normal egg cylinder but do not form a primitive streak, mesoderm or The epiblast continues to proliferate in an undifferentiated state that lacks anterior-posterior neural patterning, but anterior visceral endoderm markers are expressed and correctly positioned. Our results suggest that regional patterning of the visceral endoderm is i ndependent of primitive streak formation, but the subsequent establishment of anterior-posterior neural pattern in the ectoderm is dependent on deriva tives of the primitive streak. These studies provide genetic proof for the requirement of Wnt3 in primary axis formation in the mouse.