Stem cells occur naturally in the body, but adult stem cells have limited differentiation capacity. Embryonic stem cells, on the other hand, can be maintained in a pluripotent state in a laboratory setting, allowing scientists to use them in many ways. Furthermore, adult precursor cells can be made to revert to a pluripotent stem cell (one that can give rise to any body cell, but not reproductive cells) state through the application of specific stimuli. An induced pluripotent stem cell (iPS) is an adult cell that has been transformed into a pluripotent stem cell (able to give rise to any other body cell type) through the application of specific stimuli. These cells have important possible applications for therapeutic techniques.
For example, macular degeneration is a disease in which the retina deteriorates, causing blurred vision and eventual blindness. It is currently incurable, but iPS therapy may offer hope. The disease arises from the poor functioning and death of cells in the retina. If these cells can be replaced, vision can be restored. Early experiments with embryonic stem cells failed, mostly because of immune rejection of the foreign cells. However, iPS cells derived from cells taken from the patient's body will not trigger an immune rejection because they contain the same genome as the patient. Research into iPS therapy, for this and other diseases, is ongoing, and much is yet to be discovered.
Another promising technique that can lead to therapeutic benefits is cloning. Reproductive cloning is the formation of an animal that is a genetic copy of its genetic donor by transplanting the nucleus of a somatic cell into an egg that has had its nucleus removed. Reproductive cloning of animals has many applications in biomedical research, development of pharmaceuticals, harvesting organs for donation, agriculture, and possibly the food supply. However, mammals are historically difficult to effectively clone, and reproductive cloning of humans is ethically questionable and not currently legal in any nation.There is another type of cloning that can be used for therapeutic purposes. Therapeutic cloning is the transfer of the nucleus of a somatic cell into an egg cell that has had its nucleus removed to form an embryo, from which embryonic stem cells can be derived.The cells can be maintained in a pluripotent state in vitro (in a test tube or petri dish), allowing for a line of embryonic stem cells to be generated for any living person, even long after the cells have matured.