Fig. 6. Redundant BMP signaling induces posterior endoderm formation. (A-D) Cell divisions were permanently arrested at the 110-cell stage by cytochalasin B, and whole embryos were cultured until hatching stage. Then expression of ALP (A,B) and myosin (C,D) was detected. Vegetal views. Arrows indicate the posterior endoderm blastomeres. Treatment with MEK inhibitor did not affect ALP expression in the posterior endoderm blastomeres. Arrowheads indicate presumptive mesenchyme blastomeres that ectopically express muscle myosin. (E,F) The anterior-vegetal A4.1 blastomeres were removed from embryos ( A4.1) with a fine glass needle at the eight-cell stage. The embryos were cultured without cleavage-arrest until hatching stage, and ALP expression was detected. Without A4.1 blastomeres, embryos became sensitive to MEK inhibitor (F). (G-I) B4.1 partial embryos treated with DMSO (G), MEK inhibitor (H), and both MEK inhibitor and BMP4 protein (I). In I, BMP4 restored ALP expression. Scale bars: 100 µm.