Fig. 6. Local administration of rBMP4 induces thickening of the amnion in wild-type embryos. (A,B). Lateral view of wild-type embryos that were either control injected (A) or injected with rBMP4 (B) in the exocoelom at the EHF stage (E7.5), and cultured in vitro until the 5S stage. The amnion (arrowhead) developed a local thickening in the rBMP4-injected embryo (dashed circle). Aggregates were always much smaller than in Smad5 mutants, and difficult to see through the yolk sac. (C-F) Sections through control (C) and rBMP4-injected (D-H) embryos. The appearance and size of the amnion thickening (dashed circle) varied significantly between embryos. Thickening of the amnion was only scored when observed in more than five successive sections. The embryos in C-E were injected at the HF stage; D and E are sections from the same embryo, illustrating that amnion thickening can develop at different locations. The embryos in F-H were injected at the NP stage. The embryo in H has an unusually large yolk sac. Al, allantois; Fg, foregut; He, heart; Nt, neural tissue; Ys, yolk sac. Scale bars: 100 µm.