Cell migration and chick limb development: chemotactic action of FGF-4 and the AER

Experiments have been carried out to investigate the role of the apical ectodermal ridge (AER) and FGF-4 on the control of cell migration during limb bud morphogenesis. By coupling DiI cell labeling with ectopic implantation of FGF-4 microcarrier beads we have found that FGF-4 acts as a potent and specific chemoattractive agent for mesenchymal cells of the limb bud. The response to FGF-4 is dose dependent in both the number of cells stimulated to migrate and the distance migrated. The cell migration response to FGF-4 appears to be independent of the known inductive activity of FGF-4 on Shh gene expression. We investigated the role of the AER in controlling cell migration by characterizing the migration pattern of DiI-labeled subapical cells during normal limb outgrowth and following partial AER removal. Subapical cells within 75 micrometer of the AER migrate to make contact with the AER and are found intermingled with nonlabeled cells. Thus, the progress zone is dynamic with cells constantly altering their neighbor relationships during limb outgrowth. AER removal studies show that cell migration is AER dependent and that subapical cells redirect their path of migration toward a functional AER. These studies indicate that the AER has a chemoattractive function and regulates patterns of cell migration during limb outgrowth. Our results suggest that the chemoattractive activity of the AER is mediated in part by the production of FGF-4.