Fig. 6. Developmentally and spatially regulated inductive activity and competence. Embryos implanted with an inducer tissue (A,G,M,S) were cultured for 20 hours (B,H,N,T) and processed for immunohistochemistry. Transverse sections were cut and immunostained with anti-Wt1 (C,I,O,U) or Pax2 (E,K,Q,W) antibody (green), and with the QCPN antibody (red) and DAPI (blue). D,F,J,L,P,R,V,X are high-magnification images of the boxed regions in C,E,I,K,O,Q,U,W, respectively. Asterisks indicate bona fide expression of Wt1 or Pax2 in the intermediate mesoderm and dorsal mesothelium. nt, neural tube. (A-L) Inductive capacity of non-liver endodermal organs. (A-F) Embryo implanted with the lung bud. Ectopic induction of Wt1 expression is not detectable. (G-L) Embryo implanted with the stomach, showing a weak ectopic signal for Wt1 in a small number of cells in the host-derived tissue (arrowheads). No ectopic signal for Pax2 was detectable (E,F,K,L). (M-X) Developmetntally regulated inductive activity of the liver bud and of mesodermal competence to respond to this activity. (M-R) Young chick host (stage 11^-) implanted with quail liver bud. Induction of Wt1 expression is not evident. (S-X) A stage 13 embryo implanted with an older (stage 21-22) liver bud. A low level of ectopic Wt1 expression in mesodermal tissue adjacent to the implant (arrowheads in V) was observed. No ectopic signal for Pax2 was detectable (Q,R,W,X).