Embryonic stem cells come from embryos created for in vitro fertilization. For the procedure, doctors create a number of fertilized embryos, halting their development 4 or 5 days past fertilization at the blastocyst stage. After successful fertilization, families can donate these extra embryos for stem cell harvesting. The process destroys the blastocysts as the undifferentiated stem cells are harvested. These are only collected from embryos that would otherwise be discarded.
The controversy over using embryonic stem cells has led some countries to pass legislation barring or restricting their use in medicine and science. To avoid this problem, scientists have developed techniques of inducing other cells to behave like undifferentiated stem cells. These induced pluripotent stem cells are not as versatile as true embryonic stem cells, but they offer other advantages. For instance, stem cells could be harvested directly from a patient using this method, requiring no donor material whatsoever.
Embryonic stem cells offer significant medical potential due to their ability to transform into any cell required by the body. Stem cells have shown potential for replacing damaged organs, generating new islets inside the pancreas for diabetes sufferers, or creating new bone marrow cells to treat leukemia. They may also be able to regenerate nerves, which the body normally cannot repair.