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During pregnancy, a fetus can be affected by exposure to a variety of chemicals and pharmaceuticals. These effects can occur through exposure of the mother and subsequent placental transport. Thus, it is important to assess embryotoxicity (developmental toxicity without the observation of maternal toxicity) and teratogenicity (irreversible structure developmental effects without the observation of maternal adversities) prior to the marketing of compounds. The complexity of the reproductive system and the vast number of tissue targets for the exogenic induction of malformations during embryonic development are the rationale underlying the toxicity testing of chemicals in highly standardized animal experiments such as screening tests or multigenerational studies, according to specific Organization for Economic Co-operation and Development (OECD) test guidelines. All of these tests provide information on the biological effects of industrial chemicals or the pharmacological side effects. These guidelines generally specify timeconsuming and expensive in vivo experiments, most of which are performed with mammalian species such as rats or rabbits. However, for both economical and ethical reasons, there is a great demand for alternatives to living mammals in the testing of chemical-induced adverse effects on reproduction and development. Over the past 30 years, various in vitro models have been developed to detect the teratogenic effects of chemicals. These test systems utilize either dissociated cells from the limb buds and brains of rat embryos (micromass test; (Flint & Orton 1984)) or whole embryos of rats (whole embryo culture test; (Freeman & Steele 1986)). Thus, these test systems must sacrifice living animals in order to obtain cells or embryos for each experiment. In recent years, stem cells have become important new tools for the development of in vitro model systems to test drugs and chemicals; they have also shown potential to predict or estimate toxicity. Among various stem cells, embryonic stem (ES) cells are the most valuable in developing in vitro model systems because they are able to self-renew and differentiate into every cell type of the mammalian organism; they therefore have higher plasticity than adult stem cells. The fact that stem cells are able to self-renew means that they can be continuously cultured in an undifferentiated stage, giving rise to more specialized cells such as heart, liver, bone marrow, blood vessels, pancreatic islets, or neuronal cells upon addition or removal of certain growth factors (Hoffman & Merrill 2007). In 1997, Spielmann et al. developed an in vitro model for the screening of embryotoxicity based on mouse (m) ES cells. This is termed the “embryonic stem cell test” (EST; (Spielmann et al. 1997)). The EST is based on the

Assessment of Embryotoxicity and Teratogenicity by the Embryonic Stem Cell Test