BIOLOGICAL AND HISTOPATHOLOGICAL CHARACTERISTICS OF EXPERIMENTAL YOLK SAC TUMORS DEVELOPED BY FETECTOMY AND THEIR USEFULNESS IN REDIOTHERAPEUTIC EXPERIMENTS

Abstract

Human teratoid tumors are considered to arise in the process of dedifferentiation of primitive germ cells. Some experiments on yolk sac tumors in rats induced by fetectomy have been reported. By almost the same fetectomy methods, a slow growing yolk sac tumor with a distant metastatic potential could be established in rats. This paper reports a study of the biological and histopathological characteristics and usefulness of radiotherapeutic experiments on these induced tumors. In an experiment on induction, tumors were induced in inbred Wistar rats (Wistar Mishima, WM/Ms), but not in closed colony rats (Wistar and Donryu strain). The induction of these tumors was completely inhibited by 200 cGy irradiation after fetectomy. Therefore, the radiation sensitivity of the tumor origin was considered to be extremely high. These tumors induced in inbred strain rats (WM/Ms) were transplantable to closed colony strain rats (Wistar). In an experiment on implantation, WM/Ms and Wistar rats were used. One biological characteristic of these tumors may be slow growth with potential for distant metastasis. At autopsy, remarkable lymphnode metastases were observed and hematogenous metastases were also seen in the lung, kidney and liver. The histopathological features of the tumor were almost the same as those reported in other strains of rats. Typical findings included abundant intracellular PAS positive substance that appeared to be laminated in electron microscopy of primary developed, implanted and metastatic sites. The take rate of the tumor, and the survival rate of rats with the tumor after implantation of tumor cells irradiated in vitro depended on the radiation dose. The BrdUrd labeling index of irradiated tumors likewise depended on the radiation dose. Radiotherapeutic experiments on hyperfractionation, or combinations of biological response modifiers, usually needed long observation to check for radiation injury in normal tissue and prolongation of the life span. Because few animals with experimental tumors have long survival time and few of those tumors have metastatic potential, this slow growing tumor with a lymphnode metastatic potential may be useful in experimental models requiring long term observation.