The role of the dorsal lip in the induction of heart mesoderm in Xenopus laevis

We have examined the tissue interactions responsible for the expression of heart-forming potency during gastrulation. By comparing the specification of different regions of the marginal zone, we show that heart-forming potency is expressed only in explants containing both the dorsal lip of the blastopore and deep mesoderm between 30 degrees and 45 degrees lateral to the dorsal midline. Embryos from which both of these 30 degrees-45 degrees dorsolateral regions have been removed undergo heart formation in two thirds of cases, as long as the dorsal lip is left intact. If the dorsal lip is removed along with the 30 degrees-45 degrees regions, heart formation does not occur. These results indicate that the dorsolateral deep mesoderm must interact with the dorsal lip in order to express heart-forming potency. Transplantation of the dorsal lip into the ventral marginal zone of host embryos results in the formation of a secondary axis; in over half of cases, this secondary axis includes a heart derived from the host mesoderm. These findings suggest that the establishment of heart mesoderm is initiated by a dorsalizing signal from the dorsal lip of the blastopore.