Fig. 4. wt1 is expressed in the putative PGC attraction center of the anterior trunk at early somitogenesis. All pictures shown are dorsal views of flatmounts of whole-mount in situ stained embryos with wt1 in blue in A-C, nos1 in blue in D and other markers in red (see Materials and Methods). (A) At the two-somite stage, wt1 is exclusively expressed in the lateral mesoderm of the anterior trunk with a slight extension into the head, as seen in comparison with myoD in red, which stains the adaxial cells and the forming somites. Expression is confined to the mesoderm, as can be seen in side view (data not shown). Prior to the one-somite stage, no expression can be detected in whole-mount in situ staining. (B) By the six-somite stage, wt1 expression has extended posteriorly to the fifth somite. (C) Beginning at around the 10-somite-stage, wt1 expression starts to extend into the lateral mesoderm of the head and into the anterior halves of the first four somites, which do not express myoD. At about this stage, the PGCs start to migrate posteriorly. (D) The PGC clusters, which form between the one- and six-somite stages, are located within the wt1-expressing region, as seen by comparison with B and shown here for the eight-somite-stage.
