Fibroblast growth factors (FGFs) are believed to be vital for limb outgrowth and patterning during embryonic development.
Although the effect of FGFs on the formation of the skeletal elements has been studied in detail, their effect on the development of the limb musculature is still uncertain. In this study, we used Blindwell chemotactic chambers to examine the effect of FGF-2 and FGF-4 on the motility of myogenic cells obtained from the proximal region of day 11.5 mouse forelimbs. The limb myogenic cells were found to be chemotactically attracted to FGF-2 and FGF-4 at 1-50 ng/ml. Both FGFs increased myogenic cell migration in a dose-dependent manner, with maximal responses attained at 10-50 ng/ml for FGF-2 and at 10 ng/ml for FGF-4; however, FGF-2 was found to be a more potent chemoattractant than FGF-4. It was possible to inhibit the myogenic cells' response to FGF-2 and FGF-4 by the addition of the appropriate neutralizing antibody. The effects of FGF-2 on cell migration were further investigated by loading this cytokine into Affi-Gel blue beads and transplanting them into day 11.5 forelimb buds. The results showed that FGF-2 attracted DiIlabelled proximal cells to migrate toward the implanted beads and that the migration was more extensive than that observed in the absence of FGF-2. A checkerboard assay was performed in which various concentrations of FGF-2 and FGF-4 were introduced to both the upper and lower wells of the Blindwell chambers. The results indicated that both FGF isoforms can stimulate chemokinesis as well as chemotaxis in myogenic cells. In addition, the effect of FGF-2 and FGF-4 on other aspects of skeletal muscle development was investigated. FGF-2 at 0.1-10 ng/ml stimulated a significant increase in the number of myocytes expressing sarcomeric myosin on examination after 48 hr in culture, but the effect of FGF-4 was negligible at all concentrations analyzed; however, both FGF-2 and FGF-4 inhibited myocyte fusion compared with the spontaneous fusion observed in control cultures. Finally, we used in situ hybridization and immunohistochemical techniques to determine the distribution of myogenic cells and FGF-2 protein in the day 11.5 mouse forelimbs.