An Electron Microscopic Study on the Development of the Human Embryonic Heart

Part I Myofibril The development of the human embryo heart, especially the mode of development of myofibrils were studied by means of the electron microscope, and the following results were obtained. The myofilaments were found already in the cytoplasma of the CR length 20 mm myocytes. There was the finding that the myofilaments might be produced from the polysomes. Initially the myofilaments were distributed at random in the cytoplasma, and then were gathered and formed into bundles (myofibrils). The myofibrils three-dimentionally the network structure. Those myofibrils seemed like to lined up gradually into certain direction as the myocyte matured, and showed the striation as following order; the Z-line, A band and H disc. In cross section, the myofilaments showed hexagonal alignments, and these figures were the same of those of adult. In the case of the 3 month human fetal cardiac myocytes, the basic structure of myofibrils were established in part, and assumed to able to contract. In the case of the 7 month human fetal cardiac myocytes, the almost all of cytoplasm were occupied with the myofibrils which seemed like that of adult heart, These findings might be correlated with the fact that the immatured baby at this stage in able to survive under the careful treatment. Part II Mitochondria and Glycogen The developmental changes of the human embryo heart, in special reference of the mitochondria and glycogen were studied by means of the electron microscope, and the following results were obtained. A few mitochondria were distributed in the cytoplasm of the CR 20 mm embryo myocytes, and the cristae mitochondriales were developed poorly in this stage. In the case of the CR 25 mm length fetus myocytes, the number of mitochondria increased and the cristae mitochondriales developed more. Since some mitochondria distributed close to the myofibrils, it might be thought that the cardiac muscle would be already able to contract in this stage. As the embryo matured, the distribution of glycogen in the cytoplasm decreased. The particle of glycogen was the β-type. In the point of view of the developmental process of mitochondria, myofibril and glycogen, it might be considered that the ultrastructure of these organella would be accomplished partially in the cytoplasm of the CR 25 mm length myocytes.