Fig. 5. Model of visceral endoderm cell movements and accompanying changes in gene expression patterns associated with the anteroposterior axis formation between E5.5 and E6.5 [based on data from Beddington and Robertson (Beddington and Robertson, 1999) and Kimura et al. (Kimura et al., 2001)]. Visceral endoderm (specifically distal and later AVE, expressing Hex and Cer-like, in red) and extra-embryonic ectoderm (specifically distal, expressing BMP4, orange arrows) are implicated in axis definition in the adjacent epiblast. Axis specification involves anterior movement of the visceral endoderm (red arrow) from the distal tip of the egg cylinder and the posterior shift (white arrow) of gene expression (for example, brachyury and Wnt3) from the initially proximal epiblast towards the posterior by E6.25 (in turquoise). The anterior displacement of the visceral endoderm cells is proposed to be important in shifting the expression of genes in the proximal epiblast towards the posterior by inhibiting their expression in the future anterior. White ‘T’ arrows represent putative posterior repressing activity [demonstrated at E6.5 (Kimura et al., 2000), but still hypothetical for earlier stages].