Injection of increasing amounts of rarabAvp mRNA increasingly activates reporter expression (A-D), but inhibits endogenous target gene expression (E-L). (A, E, I) Uninjected control embryos. (B, F, J) In embryos injected with 50 pg of rarabAvp mRNA, modest levels of the reporter are induced (B), as well as ectopic expression of dhrs3a (arrowhead in F) and hoxb5b (arrowhead in J). (C, G, K) In embryos injected with 100 pg of rarabAvp mRNA, higher levels of the reporter are induced (C). Although ectopic activation of dhrs3a and hoxb5b is induced (arrowheads in G and K), their endogenous expression is also inhibited (arrows in G and K). (D, H, L) In embryos injected with 200 pg of rarabAvp mRNA, the highest levels of the reporter are induced (D), while endogenous dhrs3a and hoxb5b expression are inhibited and ectopic expression is not induced (arrowheads in H and L). All images are lateral views with anterior up. Fgfr2a is an Fgf receptor required for the generation of type 3 lymphoid cells. Its loss also causes loss of the endoderm and the mesenchyme. Because Fgfr2a is required for the generation of type 3 lymphoid cells, but is not required for their maintenance, it was predicted that ectopic expression of Fgf ligands would cause ectopic type 3 lymphoid cells to form. This was tested by injecting Fgf2aMO from 2 to 4 somites into 8 somite stage embryos (AB), then explanting the embryos at 11 somites. (M-R) Non-injected control embryos (M, N). (O-R) Embryos injected with 2 ng of Fgf2aMO at 8 somites exhibit ectopic expression of tlr7a (O) and tlr8a (P) in the ectoderm, but no ectopic expression of gata3b (Q), dhrs3a (R), or hoxb5b (S).