Embryonic to the effects that may be caused.

Embryonic Stem (ES) Cell testing is a potential alternative to animal testing. ES cells are simply undifferentiated stem cells that are derived from human embryos.  This type of testing has the potential to give more precise reactions of human cells to stimuli, as well as cut costs of testing significantly.  One study using ES cell lines that were derived from human embryos, tested this.  Five ES cell lines developed from five different embryos, resulted in cell lines that were similar to Rhesus monkeys.  This is significant because Rhesus monkeys are a common nonhuman primate used in animal testing, making these cells close to what was currently acceptable in research.  Each of the five ES cell lines were successfully cryopreserved and thawed, showing that they are capable of being saved if all are not used (Thompson, et al. 1998).  This is significant because the storage of these cell lines would be much lower than housing testing animals, which require housing in a safe environment, cleaning, food, water, and supervision.  Four out of five of the cell lines were able to undergo five to six months of undifferentiated proliferation, and the fifth cell line proliferated for eight.  These cell lines were observed to have no issues and had high telomerase activity, suggesting healthy long term replication.  On top of that, all of the cell lines maintained the potential of all three embryonic germ layers; endoderm, the inner layer, mesoderm, the middle layer, and the ectoderm, the outer layer, demonstrating the potential to differentiate into normal human cells (Thompson, et al. 1998).Since ES cells are human cells, they have the potential to offer insight into testing areas with high consequences such as birth defects, infertility, and pregnancy loss, without harming human or animal reproductivity.  Especially when concerned with reproductive issues, ES cells are much more accurate because mice embryos have significantly different structures to human embryos, so using mice embryos will not give a full understanding to the effects that may be caused. ES cells can also be beneficial in the developmental and functional areas of tissues that are different between mice and human, because they can be differentiated into different cell types. Screening processes of ES cells could also offer specific gene targets for drugs, the teratogenic properties and toxic properties of certain compounds because of the rapid development of new cells (Thompson, et al. 1998), meaning there can be multiple trials done in a short period because of the proliferation of new cells.  Some limitations to this type of testing are that since in this specific case, the ES cell lines were derived from individual colonies, as opposed to the cloning of a single cell, meaning there could be possible variation in the cell development potential (Thompson, et al. 1998), but it neither confirms nor denies this.  The other limitation is current regulations within the United States that could limit the use and research done on and with ES cells.