The research on teratocarcinomas, embryonal carcinoma cells and embryonic stem cells (ES cells) that has been carried out over the past 50 years has passed through several distinct phases (Timeline).

Leroy Stevens discovered that the teratocarcinomas in 129 strains of inbred mice arose from germ cells, those of the sperm and egg. Paradoxically, germ cells do not give rise to tumors, nor do ...

In other words, the stem cells of mouse teratocarcinomas are pluripotent. Like the cells of the early embryo, they can differentiate to form derivatives of all 3 primary layers.