FGF Can Induce Outgrowth of Somatic Mesoderm both Inside and Outside of Limb-Forming Regions

In the vertebrate embryo, only somatopleural cells in the limb-forming region are released from the mesodermal layer and undergo outgrowth from the embryonic body to form the limb bud. Molecular signals which regulate limb bud induction are unknown to date. In the present study we examined the ability of fibroblast growth factor (FGF) to induce limb bud formation in chicken embryos. A replication-defective retrovirus encoding FGF type 4 with a reporter, bacterial beta-galactosidase, was microinjected into lateral plate mesoderm inside and outside limb-forming regions. Effects of the ectopic and precocious expression of FGF were assessed at various stages after infection. Here we report that somatic mesodermal cells in both flanks and limb-forming regions can respond to FGF and induce limb bud-like outgrowth. The supernumerary limb bud induced within a limb-forming region differentiated into extralimb structures. These results strongly suggest potential roles of FGF signaling for induction of limb bud formation.