Unlimited stem cells are currently obtained with patients' permission from leftover three-day old embryos that would otherwise be discarded from fertility clinics. These embryos are created and exist entirely outside the body. Unlimited stem cells can develop into any kind of cell type or tissue in the body.
Although a patient may need only a few embryos, in the process of creating them, several more are made. The ones which are not implanted in the uterus are eventually discarded. Each year it is estimated that couples undergo 120,000 cycles of fertility treatment using in vitro (outside the body) fertilization (IVF), and from these cycles 7,500 embryos are discarded. Each of these embryos can be used to create a large number of unlimited stem cells.
Limited stem cells are rare cells that can be found in only some developed organs or tissues. Limited stem cells have been successfully used for some time now in bone marrow transplants (either with bone marrow stem cells or umbilical cord blood stem cells) and skin and hair transplants.
There is also another potential way to get embryonic stem cells using an unfertilized egg and an adult cell from the patient. However, this is still being studied (known as somatic cell nuclear transfer or therapeutic cloning).
There are two types of stem cells.