Studies of heart development in normal and cardiac lethal mutant axolotls: a review

The morphology of developing hearts in axolotls, Ambystoma mexicanum, has been studied by scanning electron microscopy in order to provide a chronology of morphogenesis that can be correlated with ongoing biochemical and immunocytochemical studies. In addition to normal embryos we have studied homozygous recessive cardiac lethal mutant axolotls. The mutant myocardium undergoes aberrant sarcomere development and lacks a normal heartbeat. Morphogenesis of mutant hearts appears to be nearly normal with respect to myocardial cell shape changes, epicardial formation, and the distribution of extracellular matrix fibrils in the cardiac jelly. This suggests that the deficient arrangement of contractile proteins in mutant myocardial cells does not prevent the normal organization or function of cytoskeletal isoforms of these proteins in the developing myocardium and epicardium. The implications of biochemical and morphological investigations of axolotl hearts are considered in the context of the entire developmental history of the cardiogenic mesoderm.