Ultrastructure of terminally differentiated adult rat cardiac muscle cells in culture.

Ventricular cardiac muscle cells isolated from adult rats were maintained in culture for 28 to 60 days and examined by transmission electron microscopy. In order to elucidate the detailed ultrastructure of the cultured myocytes, several different electron-dense stains were used. These included tannic acid, osmium ferrocyanide, osmium tetroxide (applied as a primary fixative), and lanthanum chloride, as well as more widely used stains such as osmium tetroxide, uranyl acetate, and lead citrate. Our results show that, compared to cultured neonatal rat myocytes, cultured myocytes derived from adult rats more closely resemble in vivo adult ventricular cells. The cultured adult myocytes contained typically distributed organelles such as nuclei, mitochondria, and elements of the sarcoplasmic reticulum. Myofilaments were well organized, and typical intercalated disks were observed between adjacent cells. Unlike cultured neonatal myocytes, the adult cells contained numerous residual bodies and a relatively well developed transverse tubular system. The transverse tubular system was identified by its continuity with the extracellular space (as indicated by the penetration of electron-dense extracellular tracers), location at or near the Z line, large lumenal diameter, and frequent participation in couplings with elements of the sarcoplasmic reticulum.