Morphological features of the developing human fetal cardiac muscle cells in culture was observed with scanning electron microscope. Myocardial tissue masses were isolated from three human fetus of 10, 13 and 16 week's gestation. They were cultured in vitro. New cardiac muscle cells grew and detached from the tissue masses. In the early stage (about one week) the cardiac muscle cells which detached from the tissue masses and grew on the glass slide were spherical or short spindle in shape. There were various processes on the surface of the cells. In the middle stage (about two to four weeks) the cells became irregular in shape. The processes extended from the surface of the cells and formed the intercellular connection. In the later stage (more than one month) part of the cells in culture developed and appeared rod shape. The intercellular connection was composed of the intercalated disc-like structure, the rod-shaped cells joined together and formed branched structures. In the development of human fetus in culture a continuous process of the formation of the intercalated disc-like structure was observed. It was found that the developed cells were gradually arranged in bundles running parallel to their long axis. This kind of structure looked like normal mature morphological feature of cardiac muscle cells. It is therefore considered that the cardiac muscle cells in culture may have possibility to differentiate into mature cardiac muscle.

Myocardial tissue masses were isolated from two human fetus of 10 and 16 week's gestation respectively and they were cultured under the conditions for cultivating the myocardial cells of the rodents. Spontaneous beating occured. In one case the frequency of beating was 21～27 beats/min, and the beating lasted for 30 days until the culture was terminated artifitially. In another case a cell mass continued to beat as long as 75 days with a frequency of 200～290 beats/min, and then stopped spontaneously. The differences in thickness and shapes of the tissue masses would influence the modes of growth after culturing. Thicker tissue mass grew more slowly and the fibroblasts and myocardial cells grew and spreaded out radially, or in band form. In tissue mass with a thin border the marginal myocardial cells bulged out in globular shape in the early stage and then separated from the cell mass and continued to grow. In thin layered tissue mass, because of the rapid overgrowth of the surrounding fibroblasts, the myocardial tissue would grow and overlap them and increased steadily in thickness. This procedure is simple and relatively stable, and possible to retain the long-lasting beating of the myocardial tissus, but it is difficult to obtain a pure myocardial cell culture for cell counts and other quantitative studies.