Morphological studies on cardiac lethal mutant salamander hearts in organ cultures

Abstract

Abstract Recessive mutant gene c in Ambystoma mexicanum results in an absence of embryonic heart function. When mutant hearts are transplanted into normal embryos, the defect is corrected and the mutant hearts beat. In reciprocal transplants normal hearts do not beat. These experiments suggest that there is either a lack of induction from surrounding tissues or an inhibitory influence of some kind. We undertook the present study to determine whether the influence in vivo might be an inhibitory one which could be corrected simply by removing the heart from the embryo and growing it in culture. Hearts from normal and mutant siblings from stages 35–40 were explanted into hangingdrop cultures in Holtfreter's solution. Normal hearts continued to beat strongly while mutant hearts failed to beat. After five to seven days, cultured hearts were compared to in vivo hearts at the same period of development using light and electron microscopy. The myocardium of normal hearts is composed of highly differentiated muscle cells with numerous well‐organized myofibrils. The mutant myocardial cells contain a few filaments and an occasional Z body, however, sarcomere organization is not apparent. In general, no major differences in morphology or function could be found between the cultured and in vivo hearts at the same period of development. Thus, the cultures closely mimick the in vivo situation and present evidence against an inhibitory process causing the defect.