Teratomas Obtained Through Extrauterine Growth of Seven-Day Mouse Embryos

Segments of 7-day mouse egg cylinders were transplanted under the kidney capsules of adult mice. Two-thirds of the transplants developed into teratomas consisting of well-differentiated adult tissues. One-third also displayed large undifferentiated areas, which were considered malignant when examined under the light micrsocope. A few large teratomas were successfully retransplanted for 5 generations. Microscopically, tumors obtained by retransplantation had the same appearance as the teratomas from which they stemmed. The tumors were examined ultrastructurally, The cytoplasm of undifferentiated cells had a simple architecture typical of embryonal carcinoma. It was replete with ribosomes but devoid of other organelles except a well-developed Golgi apparatus, a few mitochondria, and occasional tubules of rough endoplasmic reticulum. Many of these tumor cells contained somewhat more cytoplasmic organelles or displayed some signs of cytoplasmic specialization, such as myofibrils or secretory granules. Some of these more differentiated cells contained many autophagic vacuoles. It is concluded that most of the undifferentiated cells look like embryonal cells in the early stages of development. The retention of the embryonal character is considered the basis for continuous and progressive growth of embryo-derived malignant teratomas.