Fig. 4. Tbx5 expression precedes Fgf8 expression in the limb field. Expression of Tbx5 (A-C) is shown at E8.5 (five-somite stage, A), E8.5 (eight-somite stage, B) and E9.25 (C). Expression of Fgf8 (D-F) is shown at E8.5 (eight-somite stage, D,E) and E9.25 (F). Yellow arrowheads indicate Tbx5 expression in the forelimb precursors, beginning at the eight-somite stage (B). This expression was readily detectable in all embryos examined at this stage and beyond (B,C). Red arrowheads indicate weak Fgf8 expression in the intermediate mesoderm, beginning at the eight-somite stage. Note that this weak expression was observable in only a few embryos at this stage (compare D with E), but was reproducibly detected at E9.25 and beyond (F).