A ROLE FOR FGF-8 IN THE INITIATION AND MAINTENANCE OF VERTEBRATE LIMBBUD OUTGROWTH

Background: The outgrowth of the vertebrate limb bud is the result of a reciprocal interaction between the mesenchyme and a specialized regi on of the ectoderm, the apical ectodermal ridge (AER), which overlies it. Signals emanating from the AER act to maintain the underlying mese nchyme, called the progress zone, in a highly proliferative and undiff erentiated state. Removal of the AER results in the cessation of limb bud growth, thus causing limb truncation. The best candidates for this AER-derived signal are members of the fibroblast growth factor (FGF) family, in particular FGF-4, which can maintain limb bud outgrowth fol lowing removal of the AER. However, FGF-4 is only expressed after cons iderable outgrowth has occurred and a well-developed limb bud has form ed, and then only in the posterior part of the AER. Likewise, the othe r FGFs studied to date are not candidates for this activity. Results: We report evidence that a recently identified member of this family, F GF-8, is expressed in the ectoderm of the prospective limb territory p rior to morphological outgrowth of the limb bud in both mouse and chic k. Thereafter, expression is maintained throughout the AER during limb development. We have produced and purified the FGF-8 protein, and sho wn that it will substitute for the AER in maintaining limb bud outgrow th in mouse embryos from which the AER has been surgically removed. FG F-8 does not, however, maintain expression of the sonic hedgehog gene. Conclusions: These results indicate that FGF-8 is an AER-derived mito gen that stimulates limb bud outgrowth. Moreover, our data suggest tha t FGF-8 may also be an ectodermally derived mitogen that stimulates th e onset of limb bud outgrowth (budding) in the absence of a morphologi cal AER, and indicate the possible involvement of FGF-8 in the establi shment of the limb field.