Fig. 3. Reversal of FGF signal direction causes reversal of mesenchyme/muscle asymmetric division. Expression of mesenchyme marker (Mch-3) in cleavage-arrested 110-cell embryos (A-G), and of muscle actin in 110-cell-stage embryos (A'-G'). All photos are vegetal pole views; anterior is up. Only the left side of each embryo was manipulated. Diagrams of the experimental designs are shown at the top of the panels. (A,A') The A4.1 and B4.1 blastomeres on the left side were injected with Hr-FGF9/16/20 MO at the eight-cell stage. (B,B') A b4.2 blastomere was injected with FGF mRNA at the eight-cell stage. (C,C') Endodermtransplanted embryos. Donor endoderm cells were injected with FGF MO (MOEn-T) at the eight-cell stage. (D-G,D'-G') b-line ectoderm-transplanted embryos. b-line donor cells were injected with either the FGF mRNA (D,E, mRNA-b4.2-T) or venus YFP mRNA (G, YFP-b4.2-T), and were transplanted. The host embryos had FGF MO (D,E, MO-A4.1-B4.1) or control MO (G, cMO-A4.1-B4.1) injected. (E,E') A subset of the b4.2 descendant cells injected with FGF mRNA (arrowhead in E') were transplanted. In E', the midline of the embryo is indicated by a broken line. (E'') Schematic representation of the results in E and E'. Normal positions of mesenchyme (green) and muscle (red) fates (right half) and altered positions of the fates on the manipulated side (left half) are shown. (F,F') b-line cells were transplanted without injection as a control. The percentage in E represents the proportion of embryos in which muscle-lineage but not mesenchymelineage cells expressed the Mch-3 antigen. The percentage in E' indicates the proportion of embryos in which mesenchyme-lineage but not musclelineage cells expressed actin. Scale bar: 100 µm.