Growth factors as inducing agents in early Xenopus development.

Factors from two growth factor families have been identified as having mesoderm inducing activity. These include activin and TGF beta 2 from the TGF beta superfamily, and all members of the fibroblast growth factor (FGF) family. When isolated ectoderm explants are treated with any of these factors, a proportion of their cells are caused to differentiate into mesodermal tissue types instead of epidermis. There are several differences in the biological activities which can be broadly summarized by saying that activin yields dorsal type inductions and FGF ventral type inductions. Both bFGF and an FGF receptor have been detected in Xenopus blastulae, but it has not been shown that bFGF is normally secreted from vegetal cells. Various TGF beta-like mRNAs have also been detected and it is expected that an activin-like molecule will prove to be responsible for induction of the dorsal mesoderm in vivo.