Complementary functions of Otx2 and Cripto in initial patterning of mouse epiblast

The development of the mammalian antero-posterior (A-P) axis is proposed to be established by distinct anterior and posterior signaling centers, anter ior visceral endoderm and primitive streak, respectively. Knock-out studies in mice have shown that Otx2 and Cripto have crucial roles in the generati on and/or functions of these anterior and posterior centers, respectively. In both Otx2 and Cripto single mutants, the initial formation of the A-P ax is takes place in a proximal-distal (P-D) orientation, but subsequent axis rotation fails to occur. To examine the developmental consequences of the l ack of these two genes, we have analyzed the Otx2(-/-);Cripto(-/-) double h omozygous mutant phenotype. In the double mutants, the expression of the A- P axis markers Cer-1, Lim1, and Wnt3 was not induced, while expression of F gf8 and T was expanded throughout the epiblast, indicating that the double mutants could not form the A-P axis even in its initial P-D orientation. In addition, the double mutants displayed defects in differentiation of the v isceral endoderm overlying the epiblast, as well as in the extraembryonic e ctoderm. Furthermore, differentiation of neuroectoderm was accelerated as j udged by the reduction of Oct4 expression and emergence of Sox1 and Gbx2 ex pression in the double mutant epiblast. The resulting ectoderm only display ed characteristics of anterior hindbrain, implicating it as a ground state in the mammalian body plan. Our results indicate that complementary functio ns of Otx2 and Cripto are essential for initial patterning of the A-P axis in the mouse embryo. (C) 2001 Academic Press.