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Fig. 2. FGF/ERK activity antagonizes the ability of tar*/acvr1b to
induce endoderm. Expression of CB187 (A-C) and
sox17 (D-F) in embryos at 80% epiboly. (A,D) Wild type.
(B,C,E,F) tar*/acvr1b RNA at 200 ng/µl was injected alone or in
combination with DN-FGFR1 RNA at 500 ng/µl in one animal pole blastomere at
the 64-cell stage embryo. Injection of tar*/acvr1b RNA alone is able
to induce ectopic CB187 (B) and sox17 (E) expression at the
animal pole (arrows). Co-injection with DN-FGFR1 abolishes the ability of
tar*/acvr1b to induce CB187 (C), whereas DN-FGFR1 slightly
increases the ability of tar*/acvr1b to induce sox17 (F).
(G) RT-PCR analysis of three whole embryos injected with
tar*/acvr1b RNA at 25 ng/µl. FGF signal inhibition by treatment
with 15 µM of SU5402 after the 1000-cell stage enhances cas and
sox17 responses. (H,I) tar*/acvr1b RNA at 1
ng/µl was injected alone or in combination with ERK2* RNA at 50 ng/µl
into one blastomere of a 16-cell stage embryo. In both cases a lineage tracer
(FLDX) was co-injected and later detected by immunostaining and sox17
expression was detected. (H) The clone of cells ectopically expressing
sox17 in H is congruent with the territory containing the lineage
tracer. (I) The size of the clone of cells ectopically expressing
sox17 is greatly decreased in the presence of ERK*. (A-F,H,I) Lateral
views.
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