Figure 4.

Defective formation of the antero-posterior axis in early gastrulating Otx2^−/− embryos. Conversion of the proximal-distal axis into the antero-posterior axis begins before gastrulation. Cells of the distal visceral endoderm undergo an oriented movement toward the future anterior pole of the embryo as illustrated by the Hex expression domain (shown in red). Conversely, cells of the extraembryonic endoderm expressing cystatin B and tag 123 appear to converge to the future posterior pole (shown in yellow). Black arrows symbolize this movement. In WT embryos, the anterior pole is also marked by the expression of Dkk-1. In the ectoderm layer, Fgf-15 expression forms a gradient distributed along the proximal-distal axis before gastrulation, then along the antero-posterior axis at 6.5 dpc. In Otx2^−/− embryos, the oriented movement of the cells from the visceral endoderm is abolished, resulting in the ectopic localization of the Hex expression domain as well as the accumulation of the cystatin B and tag 123-expressing cells at the embryonic–extraembryonic junction. Formation of the head organizer is also impaired, as assessed by the loss of expression of the head inductor Dkk-1. In addition, the ectodermal layer is affected, as shown by the absence of Fgf-15 expression. Hence, Otx2 is required for global cellular movements in the visceral endoderm, as well as for the proper orientation of the antero-posterior axis before gastrulation. Extra, extraembryonic region; Emb, embryonic region; A, anterior; P, posterior; Pr, proximal; D, distal. Embryos at the top are pregastrulating embryos. Embryos at the bottom are 6.5 dpc embryos.