Fig. 9. Model illustrating the origin of the Sox3 null PA2 phenotype. The position of future PP1 is determined by a focalised endodermal outpocketing at 8.0-8.5 dpc (A). Studies in urochordates (Manni et al., 2002) have shown that ectodermal and endodermal cell interdigitations then result in the opening of the pouch, surrounded by a newly formed epithelium, the pouch margin. At this stage SOX3 is restricted to the posterior pouch margin and ncc are migrating in PA2. In the absence of Sox3 (B), the endoderm contacts the ectoderm over a much wider region, resulting in the formation of an enlarged pouch when the perforation occurs, and also an enlarged pouch margin. As a consequence, ncc migration is impaired.