Cell lineage labels and region-specific markers in the analysis of inductive interactions

This paper reviews work with cell lineage labels and cell-type specific markers in the analysis of inductive interactions in early amphibian development. Our results provide clear evidence for the existence of three such interactions. Mesodermal induction occurs in the early blastula and results from the action of vegetal pole cells on the animal hemisphere. At least two mesodermal rudiments are formed, one dorsal and one ventral. During the next interaction, which we call dorsalization, the ventral mesodermal rudiment becomes subdivided into several territories under the influence of the dorsal marginal zone, or organizer. Finally, during gastrulation, the involuting organizer induces neural tissue from the overlying ectoderm. This interaction is called neural induction. Although these phenomena can readily be demonstrated under experimental conditions, direct evidence that they occur in normal development awaits an understanding of the molecular basis of induction.