Fig. 6. Schematic representation of developmental steps in mouse embryos that depend on ß-catenin. (A) Scheme of wildtype, Cripto, Wnt3 and ß-catenin mutant phenotypes. Blue, embryonic ectoderm; green, extra-embryonic ectoderm; yellow, visceral endoderm; orange, signalling centre in visceral endoderm; red, primitive streak; A, anterior; P, posterior. (B) Schematic representation of putative signalling events. At E6.0, ß-catenin regulates expression of Cripto, which is required for anteroposterior axis positioning. Components upstream of ß-catenin are unknown in this pathway. At E6.5, Wnt3 controls ß-catenin, which directly or indirectly regulates the expression of downstream genes (such as brachyury, Nanog and others) and leads to primitive streak and mesoderm formation. Red arrows indicate the order of signal transduction events, the broken arrow indicates crosstalk between the ß-catenin and Nodal signalling pathways via Cripto. Continuous and broken horizontal lines separate the extracellular, cytoplasmic and nuclear compartments.