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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.
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