Embryonic stem cells are derived from the inner cells of a human blastocysts , a very early human embryo. At the blastocyst stage, five to 10 days after fertilization, the embryo is a cluster of 100-200 cells (it is sometimes called a pre-implantation embryo because in a pregnancy, a blastocyst has not yet implanted in the uterus).
Blastocysts are sometimes donated for research purposes by couples who have undergone in vitro fertilization (IVF) to become pregnant and have more embryos than they need. Scientists also work with blastocysts that are judged to be of insufficient quality to use in IVF and would otherwise be discarded (the Daley Lab has successfully derived high-quality human embryonic stem cell lines from this source). Once embryonic stem cells have been isolated, the blastocyst is no longer viable.
Blastocysts can also potentially be created from a donor egg, through the process known as somatic cell nuclear transfer, or through parthenogenesis. To date, these procedures have only led to the successful creation of embryonic stem cells in animals. Human blastocysts created through these procedures are only allowed to develop for a few days and are never implanted into a woman’s womb, in accordance with regulatory guidelines that strictly forbid cloning human beings for reproductive purposes.