FGF7 and FGF10 directly induce the apical ectodermal ridge in chick embryos

During vertebrate limb development, the apical ectodermal ridge (AFR) plays a vital role in both limb initiation and distal outgrowth of the limb bud. In the early chick embryo the prelimb bud mesoderm induces the AER in the overlying ectoderm. However, the direct inducer of the AER remains unknown. Here we report that FGF7 and FGF10, members of the fibroblast growth facto r family, are the best candidates for the direct inducer of the AER. FGF7 i nduces an ectopic AER in the flank ectoderm of the chick embryo in a differ ent manner from FGF1, -2, and -4 and activates the expression of Fgf8, an A ER marker gene, in a cultured flank ectoderm without the mesoderm. Remarkab ly, FGF7 and FGF10 applied in the back induced an ectopic AER in the dorsal median ectoderm. Our results suggest that FGF7 and FGF10 directly induce t he AER in the ectoderm both of the flank and of the dorsal midline and that these two regions have the competence for AER induction. Formation of the AER of the dorsal median ectoderm in the chick embryo is likely to appear a s a vestige of the dorsal fin of the ancestors. (C) 1999 Academic Press.