Fig. 4 . Bmp4 mRNA is expressed in the tailbud and its derivatives. Whole-mount image (A) and transverse vibratome sections (B-D) of chick embryos processed for the detection of Pax2 (green) and Bmp4 (purple) mRNAs. At HH st14 (A), Pax2 mRNA is detected in the nephric duct (nd) and at the caudal-most aspect of the embryo. Bmp4 mRNA is abundantly expressed in the lateral plate (lp) and the tailbud (tb). By HH st21 (B), Bmp4 mRNA is abundantly expressed by mesenchyme surrounding the cloaca (c) and the lateral plate mesenchyme (lp), whereas Pax2 mRNA is expressed in the developing metanephros (boxed area). High-power images of developing metanephros (C,D) demonstrate that the UB (outlined by broken line) and nephrogenic mesenchyme (nm) surrounding the proximal UB tip express Pax2 mRNA. Bmp4 mRNA is expressed around the distal (dUB) but not the proximal UB (pUB). The pattern of Bmp4 expression in the developing chick urinary tract (C,D) is markedly similar, if not identical, to the pattern of Bmp4 expression in the developing murine urinary tract (E,F). Whole-mount image (E) of the urinary tract isolated from an E11.5 mouse embryo demonstrates that abundant levels of Bmp4 mRNA are expressed by mesenchyme surrounding the distal UB (dUB) and the cloaca (c). Bmp4 is not detected in the nephrogenic mesenchyme surrouding the proximal UB tip (pUB). (F) Similar results are seen in sections of E11.5 mouse metanephric kidney rudiments isolated from Bmp4^+/lacz mouse embryos.