Fig. 1. ERK is activated transiently in the PMC precursors a few hours after zygotic expression of ets1. (A-E) In situ hybridization with an ets1 probe. (B-D) Starting at the early blastula stage up to the mesenchyme blastula stage, ets1 expression is restricted to the PMCs (arrow in D). (E) At the end of gastrulation, ets1 transcripts are also enriched in the SMCs (arrow). (F-N) Whole-mount immunolocalization of activated ERK using an anti-MAPK-P antibody. (A) Sixty-cell stage, (F) early blastula, (B,C) prehatching blastula, (G,H) hatching blastula, (I,J) early mesenchyme blastula, (D,K) mesenchyme blastula, (L,M) early gastrula (arrows in M indicate staining in the ectoderm adjacent to the PMC clusters) and (E,N,O) late gastrula. (O) Control embryo stained with the secondary antibody alone. All the embryos are viewed from the side with the animal pole at the top except when mentioned: av, animal view; vv, vegetal view.