How are embryonic stem cell lines made (in the lab)?

Embryonic stem cells are usually derived from the inner cell mass of preimplantation embryos, corresponding to 5-9 days after fertilization in humans and 3-4 days in mice. Embryos used to generate human ESCs come from several sources. The first human ESCs were derived from donated embryos left after in vitro fertilization (IVF). IVF embryos analyzed by preimplantation genetic diagnosis can also be used to generate ESCs. An alteration of this technique allows generation of ESCs from single cells removed from embryos in a process similar to preimplantation genetic testing. ESCs can be derived from eggs that have been parthenogenetically activated; that is, the eggs are induced to divide without being fertilized by sperm. Somatic cell nuclear transfer (SCNT) can be used to produce embryos from somatic or adult cells using donated enucleated eggs, and then ESCs can be generated from the resulting embryos.