Fig. 6. Suppression of MAPK activation and FoxA expression by an ectoderm signal. (A,B) Activation of MAPK (ERK) in an unmanipulated control 44-cell-stage embryo (A) and a 44-cell-stage embryo with a-line cells removed at the early 32-cell stage (B). Anterior views. Arrowheads indicate nuclei of the four nerve cord lineage cells to be compared. Yellow arrowheads represent ectopic activation of MAPK in the medial nerve cord precursors. a, animal pole; v, vegetal pole. (A',B') Nuclear staining with DAPI to show the position of blastomeres and their nuclei. Midlines are indicated by broken lines. Pink and white letters below nuclei show blastomeres with and without activated MAPK in their nuclei, respectively. (C-F) Expression of Hr-FoxA (C,D) and Hr-Bra (E,F) in 110-cell embryos. Vegetal views. Anterior is up. (C) An unmanipulated 110-cell-stage embryo. (D) An embryo with a-line cells removed at the early 32-cell stage. Note that the uppermost layer of nerve cord cells is ectopically stained. Dots in the diagrams (C',D') signify the blastomeres that expressed FoxA. (E) A a4.2 blastomere on the left side was injected with Hr-FGF9/16/20 mRNA at the eight-cell stage. Red arrowheads indicate weak ectopic expression of Bra. (F) A4.1 and a4.2 blastomeres on the left side were injected with FoxA and FGF mRNAs at the eight-cell stage, respectively. Black arrowheads indicate nerve cord lineage cells that ectopically expressed Bra. The percentages shown in C-F represent the proportion of embryos that showed ectopic expression of FoxA (C,D) and Bra (E,F) in cells of nerve cord lineage. Scale bar: 100 µm. Ecto, ectoderm; En, endoderm; N, notochord; NC, nerve cord.