Mesoderm induction in amphibians and chick.

ABSTRACT

Induction is a process in which the developmental pathway of one cell is controlled by signals emitted from another. Mesoderm induction is the first inductive interaction in the Xenopus enbryo and probably occurs in all vertebrates. It is a very important event as it is implicated in the regulation of morphogenesis. Nieuwkoop first demonstrated the importance of vegetal endoderm in inducing the mesoderm. Slack and co-workers incorporated the information obtained from experimental embryology in a "three signal" model for mesoderm induction in amphibians (signals arising from ventral vegetal hemisphere, dorsal vegetal hemisphere and the organizer). More recent research has resulted in the detection of mesoderm inducing factors which are members of FGF and TGF--β families. Activin, a member of the TGF-ß family, has been shown to induce differential gene expression and cell differentiation in a concentrationdependent manner giving credence to the theory of morphogen gradients. Study of mesoderm induction in the chick embryo is much more difficult due to several reasons. Novel experimental approaches, however, have been used which point to the role of activin and FGF in chick mesoderm induction. The demonstration of mesoderm inducing activity of activin and FGF in other groups of vertebrates, particularly the chick embryo brings out the possibility of a universal mechanism of mesoderm induction being operative in all the vertebrates.