Normal limb development in conditional mutants of Fgf4

Fibroblast growth factors (FGFs) mediate multiple developmental signals in vertebrates, Several of these factors are expressed in limb bud structures that direct patterning of the limb. FGF4 is produced in the apical ectoderm al ridge (AER) where it is hypothesized to provide mitogenic and morphogeni c signals to the underlying mesenchyme that regulate normal limb developmen t. Mutation of this gene in the germline of mice results in early embryonic lethality, preventing subsequent evaluation of Fgf4 function in the AER. A conditional mutant of Fgf4, based on site-specific Cre/loxP-mediated excis ion of the gene, allowed us to bypass embryonic lethality and directly test the role of FGF4 during limb development in living murine embryos, This co nditional mutation was designed so that concomitant with inactivation of th e Fgf4 gene by excision of all Fgf4-coding sequences, a reporter gene was a ctivated in Fgf4-expressing cells, allowing assessment of the site-specific recombination reaction. Although a large body of evidence led us to predic t that ablation of Fgf4 gene function in the AER of developing mice would r esult in abnormal limb outgrowth and patterning, we found that Fgf4 conditi onal mutants had normal limbs. Furthermore, expression patterns of Shh, Bmp 2, Fgf8 and Fgf10 were normal in the limb buds of the conditional mutants. These findings indicate that the previously proposed FGF4-SHH feedback loop is not essential for coordination of murine limb outgrowth and patterning. We suggest that some of the roles currently attributed to FGF4 during earl y vertebrate limb development may be performed by other AER factors in vivo .