NEW PERSPECTIVES ON THE ROLE OF THE FIBROBLAST GROWTH-FACTOR FAMILY IN AMPHIBIAN DEVELOPMENT

It has been known for several years that the fibroblast growth factors (FGFs) have potent mesoderm-inducing activity. As a result they have been considered good candidates for one of the endogenous vegetally lo calized mesoderm-inducing signals in the amphibian Xenopus laevis. In this review the properties of the FGFs and their expression patterns i n Xenopus are described. Recent work is discussed which reveals a clos e link between FGF signalling and regulation of the Xenopus brachyury (Xbra) gene. These data are used to build a model of FGF function whic h is quite different from what was originally conceived. Present evide nce supports the view that during blastula stages the FGFs do not act as vegetally localized inducing signals. Instead, they are required in the animal hemisphere as competence factors, which provide a low leve l stimulation of the tyrosine kinase signal transduction pathway. FGF activity is necessary for the full range of responses to the vegetal i nducing signals, including the activation of Xbra transcription in the marginal zone of the late blastula. Xbra is able to activate the zygo tic transcription of eFGF, which suggests that there is a period of au tocatalytic activation of eFGF and Xbra transcription within the formi ng mesoderm of the marginal zone. FGF activity continues to be require d to maintain the expression of a sub-set of mesodermal genes, includi ng Xbra, in the blastopore region and possibly also in the notochord t hrough gastrula and neurula stages. In addition a role for the FGFs in anteroposterior specification and development of the myogenic lineage s is discussed.