Fig. 4. Effects of MEK and FGFR inhibitor on formation of the posterior endoderm. (A,B) B4.1 blastomeres were isolated at the eight-cell stage and cultured as partial embryos until hatching stage. They expressed both ALP and myosin according to expectation from the cell lineage. Scale bar: 100 µm. (C) B4.1 partial embryos were fixed at the 64-cell stage. Muscle actin gene is expressed in the nuclei of two blastomeres. (D-F) Treatment with a MEK inhibitor, U0126, resulted in loss of endoderm differentiation. Every constituent cell of the partial embryos expressed myosin protein. Actin mRNA is detectable in six blastomeres. (G) MEK inhibitor did not affect endoderm formation in A4.1 isolates. (H-J) Treatment with FGFR inhibitor, SU5402, also resulted in loss of endoderm differentiation. Every constituent cell expressed myosin protein. Actin expression is detectable in six blastomeres. (K) FGFR inhibitor did not affect endoderm formation in A4.1 isolates. (L) Period of sensitivity to MEK inhibitor. B4.1 blastomeres were isolated at the eight-cell stage, then treatment with MEK inhibitor was initiated at various stages. The period of sensitivity ends at the 64-cell stage. A4.1 partial embryos were not sensitive to MEK inhibitor. n, number of partial embryos examined.

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