Embryonic induction.

Inductive interactions between tissue components in proximity constitute a universal guiding principle for synchronized development during embryogenesis. Such sequential morphogenetic events involve both specific, determinative "instructions" and less specific, supporting or "permissive" influences acting upon predetermined target cells. Transmission of these intercellular messages may be mediated by diffusible signal substances, by morphogenetically active interfacial materials,or via actual cell contacts. Inductive interactions can be upset experimentally by exposure to various exogenous agents known to be potential teratogens, and several mutant strains of animals are known in which a genetic defect is manifested as a malformation through a defective interactive process. Hence, such inductive interactions should be considered likely targets for both genetic and exogenous factors in teratogenesis.