Teratocarcinoma: neoplastic lessons about normal embryogenesis.

Germ cell tumors of the testis and the ovary have been studied extensively in humans and experimental animals. Murine teratocarcinomas proved to be one of the best experimental models for elucidating the histogenesis of these tumors and the nature of their undifferentiated stem cells. These spontaneous and experimentally induced tumors, especially those produced from early postimplantation stage embryos, provided a wealth of data about the differentiation of tumor stem cells and the regulation of their growth. This made it possible to draw parallels between the teratocarcinoma cells and their normal equivalents in the embryo. Cumulative data indicate that neoplastic development of murine embryonic cells is just one of the possible ontogenic pathways these cells can take while proliferating in various developmental fields. The malignancy of teratocarcinoma stem cells is determined genetically but can be regulated epigenetically. Development of stem cells in murine teratocarcinomas parallels events in the normal embryo, suggesting that events in the tumor have their normal regulatory counterparts in the embryo proper. The study of early embryos has provided data relevant for oncology, while the study of murine teratocarcinoma helped elucidate some basic developmental events occurring normally in the embryo.