Heart induction in wild-type and cardiac mutant axolotls (Ambystoma mexicanum)

We have re-examined some of the factors affecting the induction of heart-forming mesoderm in the axolotl. The formation of functional, rhythmically contracting myocardial tissue was used as an assay. We have found that heart-forming mesoderm is fully induced and capable of completing its developmental repertoire by the end of neurulation. As has been previously reported, pharyngeal endoderm appears to be the major inductor of heart mesoderm. Unlike previous workers, we have found that the inducing activity appears to be highly localized in the mid-ventral pharyngeal endoderm. The endoderm retains its inductive properties, and the mesoderm retains at least some capacity to respond, long after the heart-forming mesoderm is apparently fully induced. We have also found that RNA extracts from pharyngeal endoderm, which are capable of causing cardiac-lethal (c/c) mutant axolotl hearts to begin beating, are not capable of inducing early wild-type heart-forming mesoderm. Based on these results, we speculate that induction of heart-forming mesoderm is a two-step process. The first signal, occurring during neurulation, directs the mesoderm to begin differentiating into cardiomyocytes, and the second, beginning in mid- to late neurulation and continuing until just prior to the onset of heartbeat, causes myofibrillogenesis and the initiation of rhythmic contractions. The latter signal, which is lacking in c/c mutant embryos, appears to be necessary to override an inhibition present in the embryonic milieu.