Many components of the developmentally important Wnt/β-catenin signalling pathway are expressed in mouse pre-implantation embryos. However,on p. , Kemler and colleagues report that this pathway does not regulate pre-implantation development. Furthermore, the control of β-catenin levels in pre-implantation embryos and in some cells of post-implantation embryos does not involve the normal β-catenin degradation pathway. The researchers use a Cre-loxP strategy to derive embryos expressing β-catenin that lack the signals needed for its degradation. The pre-implantation development of these embryos is normal and the stabilised β-catenin fails to travel to the nucleus to activate Wnt target genes. The extra-embryonic region of post-implantation embryos also develops normally, but embryonic endoderm cells prematurely express Wnt target genes and undergo a premature epithelial-mesenchymal transition. Future work will determine how pre-implantation and extra-embryonic cells, but not those of the post-implantation embryo, control the Wnt/β-catenin signalling pathway.
