Identification of cardiac myocytes in vivo and in vitro by the presence of glycogen and myofibrils

Abstract

On the basis of cell shape, refractility under phase optics, spontaneous beating and nucleolar number, two cell types have been distinguished in embryonic heart cell cultures: M cells (myoblastlike) and F cells (fibroblast-like). However, by different criteria, more than two cell types have been found in the heart in vivo. In the present study, heart cell types are redefined by properties which can be used for identifying cells both in vitro and in vivo. Trypsin-dissociated cells from 7-day chick hearts were cultured for 24 h. PAS staining indicated that all M (thick, refractile) cells contain glycogen (gly +), while most F (spread) cells do not (gly −). Under the electronmicroscope, only gly + cells contain myofibrils; most of these cells beat spontaneously in culture. Gly − cells do not beat. Gly + cells with myofibrils as well as gly − cells are found in the inoculum and in the heart in vivo. The ultrastructure of gly + cells in vitro and of muscle cells in the myocardium in vivo is similar. Therefore, it is concluded that gly + cells in culture are derived from the myocardium (muscle) of the heart. Gly − cells are apparently derived from the epicardium, the endocardium and the endothelial lining of blood vessels.