The promise of embryonic stem cells in curing diseases is their ability to differentiate into almost every other kind of body cell. The medical hope is that human embryonic stem cells can do the same in adult human beings, and thereby create healthy cells to replace diseased ones.
The ability to isolate and grow embryonic stem cells is a relatively recent one, developed by Dr. James Thomson in 1998. This discovery opened the doors of embryonic stem cell research.
The first known human clinical trial using human embryonic stem cells was approved by the FDA in January 2009. Geron Corporation will investigate the ability of human embryonic stem cells to regrow and remyelinate nerve cells, which could restore normal organ and tissue function in people who have been paralysed as a result of acute spinal cord injury. The subject pool is limited to those who have complete thoracic spinal cord injuries: paraplegics with no motor or sensory function in the abdominal muscles and below. At the current level of medical knowledge, fewer than 5% of people paralysed with this kind of spinal cord injury ever recover any degree of locomotion. If human embryonic stem cells perform as hoped, at least some of these people might be able to walk again.
If human embryonic stem cells can regrow nervous tissue, what can't they do? Diabetics might be given human embryonic stem cells to regrow healthy pancreatic tissue which can produce insulin normally. Many types of cancers might be healed if human embryonic stem cells could specialise and replace the malignant cells. Entire organs might be grown in the lab or even in the patient's own body, replacing the need for organ transplantation and avoiding much of the risk of tissue rejection. Will we someday treat stomach and colon cancers with a yogourt cocktail of prebiotic stem cells?
Yet obtaining human embryonic stem cells is ethically problematic. At our current level of medicine, obtaining embryonic stem cells for research requires killing an embryo. The current source for such embryos are the embryos left over after successful IVF treatment and released for research by the parents. The only other options for these embryos is to be donated to other infertile couples or to be discarded entirely.
Two other options exist. Previously donated embryos have been coaxed into dividing and growing in the laboratory, creating pluripotent stem cell lines. The potential lifespan of such cultures is not known. The other option is to draw pluripotent cells from older foetuses, potentially as a side benefit of in utero surgery to treat a congenital condition before birth. Both of these options have ethical issues, but they may be less harsh than direct embryo harvesting.
With the development of preimplantation genetic diagnosis (PGD), another option has opened up. In PGD, a single cell is removed from an 8-cell embryo (fertilised in vitro) to examine it for the presence or absence of a particular severe genetic disease such as Tay-Sachs syndrome. If the cell is found to be healthy, the now 7-cell embryo is allowed to develop normally. It is possible that in future, we could similarly remove a single cell from one or more embryos and coax it into indefinite pluripotential division in the lab.
One alternative is to use adult stem cells instead. There are not many options. Most adult stem cells are multipotent rather than pluripotent, so their ability to specialise is limited to only a few cell types.
The best known type of adult stem cell are those in bone marrow (haematopoietic stem cells), which have been used successfully for bone marrow transplants in leukemia patients for over 40 years. A few limited studies have examined the use of other types of adult stem cells in treating diabetes and advanced kidney cancer, but the subject pool has been so small that no conclusive results can be made. Current active studies using adult stem cells can be found at ClinicalTrials.gov.
Adult stem cells can also be found in umbilical cord blood. Obviously this is a limited time offer, so an increasing number of parents are choosing to freeze and store this blood when their children are born, just in case. As investments in the future go, this one has better odds than most.
In late 2007, Shinya Yamanaka et al were able to revert adult human skin cells into a stem cell-like state. These types of stem cells are called induced pluripotent stem cells (iPSCs). This research should still be treated with caution, since it is not yet known if iPSCs and human embryonic stem cells differ in clinically significant ways.