Fig. 3. Wnt1, but not Wnt3a, is an immediate downstream target of BMP signaling in the dorsal neural tube. (A-F) Expression of Wnt1 mRNA. (A) An embryo aged 20 somites with decreasing to no signal opposite the segmental plate mesoderm. Rostral is towards the top. (B) Graft of CHO-noggin cells inhibits Wnt1 transcription 10 hours later; (C,D) cross-sections of marked regions in B at grafted and intact levels, respectively. Asterisk (^*) marks grafted cells. (E) Unilateral noggin electroporation (EP, ^*electroporated hemi tube) inhibits Wnt1 transcription 10 hours later. (F) A BMP4-coated bead implanted on the neural tube at the segmental plate level induces ectopic and premature Wnt1 expression (arrowhead). (G-I) Expression of Wnt3a mRNA. (G,H) Whole-mount in situ hybridization of 15 and 21 somite stage (SS) embryos, respectively. Rostral is towards the left. There is a rostrocaudal gradient of expression along the neural tube in the younger embryo with no signal adjacent to the segmental plate mesoderm, but uniform rostrocaudal signal at 21 somites and onwards (not shown). (I) Noggin overexpression (^*) has no effect on Wnt3a mRNA following 10 hours incubation. Scale bar: 40 µm in C,D,I.