What is an embryo in the context of stem cell research?
The embryos used for stem cell research are about the size of the period at the end of this sentence. They are derived from four- and five-day-old embryos created through in vitro fertilization. Embryos at this stage, called blastocysts, are spheres containing about 100 cells. They have no nervous system, no heart, and no specialized tissues. Many of the cells are still undifferentiated, meaning they can become any type of cell in the human body.
