Fig. 5. Bmp7 signalling via Acvr1 cooperates with Bmp2/4 signalling to maintain pluripotency. (A) Western blot showing pSmad1/5/8 expression (upper) and loading control (lower) after Bmp4 and Bmp7 stimulation of control and Bmpr1a^-/- ES cells. (B) Upregulation of Acvr1 is observed in Bmpr1a^-/- cells by RT-PCR. (C) Ventral view of a medium-contribution chimera generated with Bmpr1a^-/- lacZ ES cells. Red arrow, allantois. (D-F) Transverse sections of a medium-contribution chimera. Black arrows mark (D) the surface ectoderm, (E) the endoderm-derived gut and (F) the mesodermally derived notochord. (G,H) Bmpr1a^-/- embryos display self-renewal defects when cultured in serum-free conditions and Bmp4. (I,J) The self-renewal defects are rescued when Bmpr1a^-/- embryos are cultured in serum-free conditions plus Bmp7.