THE ROLE OF GROWTH-FACTORS IN EMBRYONIC INDUCTION IN XENOPUS-LAEVIS

Establishment of the body pattern in all animals, and especially in vertebrate embryos, depends on cell interactions. During the cleavage and blastula stages in amphibians, signal(s) from the vegetal region induce the equatorial region to become mesoderm. Two types of peptide growth factors have been shown by explant culture experiments to be active in mesoderm induction. First, there are several isoforms of fibroblast growth factor (FGF), including aFGF, bFGF, and hst/kFGF. FGF induces ventral, but not the most dorsal, levels of mesodermal tissue; bFGF and its mRNA, and an FGF receptor and its mRNA, are present in the embryo. Thus, FGF probably has a role in mesoderm induction, but is unlikely to be the sole inducing agent in vivo. Second, members of the transforming growth factor-beta (TGF-beta) family. TGF-beta-2 and TGF-beta-3 are active in induction, but the most powerful inducing factors are the distant relatives of TGF-beta named activin A and activin B, which are capable of inducing all types of mesoderm. An important question relates to the establishment of polarity during the induction of mesoderm. While all regions of the animal hemisphere of frog embryos are competent to respond to activins by mesoderm differentiation, only explants that include cells close to the equator form structures with some organization along dorsoventral and anteroposterior axes. These observations suggest that cells in the blastula animal hemisphere are already polarized to some extent, although inducers are required to make this polarity explicit. How do inducing factors affect the differentiation of the responsive tissue? One approach to this question has been to look for gene expression in response to induction, especially the activation of regulatory loci like homeobox genes. Several homeobox-containing genes including Mix.1, Xhox3, X1Hbox1, and X1Hbox6, goosecoid and members of a new class of genes named Xlim, are activated by inducing factors with different patterns of expression in the embryo. Differential expression of regulatory genes probably controls the formation of distinct tissues in an orderly pattern during embryogenesis.