Regulation and function of SF/HGF during migration of limb muscle precursor cells in chicken

Limb muscles of vertebrates are derived from migratory dermomyotomal cells which emanate from a limited number of somites located adjacent to the developing limb buds. We have generated additional limb buds in chicken embryos by implantation of FGF-beads into the interlimb region in order to analyze whether these somites can be programmed to supply ectopic limbs with myogenic precursor cells. We show that migrating myogenic precursor cells are released from somites at the level of the newly formed limb, even when cell migration into the natural limb has been completed. The implantation of FGF beads in the lateral plate mesoderm rapidly induces SF/HGF expression. FGF beads implanted between HH stages 10 and 12 inhibit limb bud formation or shift the normal limb position. When an additional FGF bead was implanted at the original limb position at HH stage 15, SF/HGF expression was transiently induced to low levels without inducing a new limb. This demonstrates that the initial induction of SF/HGF by FGF does not require limb formation. Expression of SF/HGF during early limb bud stages was found in the entire developing bud and the adjacent lateral plate mesoderm with direct contacts to the lateral edge of the dermomyotome. Later, the SF/HGF expression domain retracts to a distal region below the apical ectodermal ridge. To investigate the role of SF/HGF in the migratory process, we implanted beads soaked in SF/HGF-alone or together with FGF into different locations of the developing chick embryo. In the experiments SF/HGF caused delamination of migratory cells from the dermomyotomal epithelium but no chemotactic attraction of migrating cells toward the SF/HGF source.