Ultrastructure of the cardiac muscle of early human embryo.

In order to compare the ultrastructural difference between cardiac myocytes of the 54 day gestational age and that of the 114 day gestational age, the human fetal hearts were studied by means of electron microscopy. In the 54 day heart, although the nexus was developed already, contractile elements were immature and there was no close association between myofibril and mitochondria, and also sarcoplasmic reticulum was not accomplished. On the contrary, in the 114 day heart, the contractile elements, mitochondria and sarcoplasmic reticulum were partially developed fairly well in myocytes.