Fig. 4. Loss of expression of early neural markers from embryos treated with MEK inhibitor from the eight-cell stage onwards. The developmental stage is indicated on the right of the panels. All panels show vegetal views, anterior upwards. (A-D) Ci-otx in situ hybridisation. (A,C) Control embryos. (B,D) +PD184352. Ci-otx expression is lost from a6.5 at the 44-cell stage (0% expression, n=269) (B) and from a-line neural cells at the 64-cell stage (0%, n=208) (D). Ci-otx expression is also lost in b6.5 derivatives (dorsal nerve cord, muscle, endoderm fates) and B-line endoderm. The latter may represent the loss of posterior endoderm reported previously (Kim and Nishida, 2001). The same results were observed with U0126. (E,F) Ci-ETR-1 in situ hybridisation. (E) Control embryo. (F) PD184352-treated embryos, expression of Ci-ETR-1 is lost in a-line cells, but persists in A-line (100%, n=30).