[Electron microscopic investigations on early stages of experimental heart tumours in rats (author's transl)].

The views on the histogenesis of experimental heart tumours in rats are controversal. Up to now electron microscopic investigations on early tumour stages are lacking. In this study ultrastructural findings of 4 early stages of heart neoplasms in the left ventricle of BD-IX rats are reported. Material and methods. BD-IX rats received repeated intravenous injections of methylnitrosourea in a dosage of 20 mg/kg body weight. After averagely 9 months the animals were killed by ether anesthesia. Sections of the heart were fixed in solution according to Karlsson and Schultz (1965). In order to discover early stages of tumours light microscopic examinations were carried out on heart sections. Afterwards the small tumour-containing tissue samples were postfixed in 1% OsO4, embedded in durcupan, and double contrasted with uranyl acetate and lead citrate. Results. Light microscopy (semithin sections): Beneath an endothelium-like cell layer an area of parallel to each other arranged spindle-shaped tumour cells with usually hyperchromatic nuclei was found. Some cells were larger and contained pale nuclei. Electron microscopy: The spindle-shaped neoplastic cells with irregular nuclei are loosely arranged. Most of the cells were surrounded by a basement membrane. In the extracellular space abundant thin cell processes and collagen fibers were demonstrable. The tumours contained characteristic bundles of cell processes as well as single processes which enveloped a virtual point of the space. Some large and widely undifferentiated tumour cells had stump-like thick processes. In the area of trabecular muscle neoplastic cells of an apparently mesenchymal origin were present. The ultrastructural findings are discussed regarding the histogenesis of the small heart tumours in the rats. They probably developed from neoplastic Schwann cells but an additional mesenchymal component seems to be engaged. Further studies are necessary to clarify the significance of both components and their relations in the experimental heart tumours.