Fig. 2. Expression of positive and negative PE markers. (A-E) Whole-mount in situ hybridization of chick embryos at stage 18 (A), stage 16 (B), stage 17 (C,E) and stage 15 (D) with probes for Wt1 (A), capsulin (B), Tbx18 (C), Cfc1 (D) and Pax2 (E). Arrows, proepicardium (PE); arrowheads, intermediate mesoderm. (F) A transverse frozen section through the PE of a stage 17 embryo. The section was immunostained with the anti-Wt1 antibody (green) and was counterstained with DAPI (blue). (G) A sister section of that shown in F stained with the anti-Pax2 antibody (green) and DAPI (blue). Notice that Wt1 but not Pax2 is detectable in the PE, whereas similar levels of staining are seen in the intermediate mesoderm.