Fig. 1. FGF signalling is required for early EB differentiation. (A) EB differentiation is most sensitive to the Fgfr inhibitor SU5402 when added at the start or before (day –2) ES cell aggregation. (B) Western blot. -Fetoprotein expression shows the effect of SU5402 on visceral endoderm differentiation. Differentiation was defined as the percent of EBs exhibiting two epithelia and cavitation, as seen in plastic-embedded Toluidine Blue stained sections.