MESODERM INDUCTION IN AMPHIBIANS AND CHICK

Induction is a process in which the developmental pathway of one cell is controlled by signals emitted from another, Mesoderm induction is t he first inductive interaction in the Xenopus embryo and probably occu rs in all vertebrates. It is a very important event as it is implicate d 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 embry ology in a ''three signal'' model for mesoderm induction in amphibians (signals arising from ventral vegetal hemisphere, dorsal vegetal hemi sphere and the organizer). More recent research has resulted in the de tection of mesoderm inducing factors which are members of FGF and TGF- beta families. Activin, a member of the TGF-beta family, has been show n to induce differential gene expression and cell differentiation in a concentration-dependent manner giving credence to the theory of morph ogen gradients. Study of mesoderm induction in the chick embryo is muc h more difficult due to several reasons. Novel experimental approaches , however, have been used which point to the role of activin and FGF i n chick mesoderm induction. The demonstration of mesoderm inducing act ivity of activin and FGF in other groups of vertebrates, particularly the chick embryo brings out the possibility of a universal mechanism o f mesoderm induction being operative in all the vertebrates.