For decades, stem cells have attracted the attention of medical researchers and others because they have the capacity to develop into specialized cells that make up a variety of organ and other tissues. These so-called "building blocks of nature" can literally transform into any other type of cell in the body, making them potentially invaluable in treating many diseases and injuries.
There are two basic kinds of stem cells: those found in certain adult tissues and those found in the cells of three- to five-day-old embryos. Adult stem cells, found in brain, bone marrow, muscle, skin, blood and liver tissue, can change into a limited number of cell types. The stem cells found in embryos, on the other hand, are pluripotent, that is, they have the unique ability to develop into any of the 220 cell types in the human body.
In addition to their versatility, embryonic stem cells are easier to grow in the laboratory than adult stem cells. Adult stem cell lines proliferate only for a limited time, while embryonic stem cells potentially can continue dividing forever.
The first and best-known success in adult stem cell research is the bone marrow transplant, in which stem cells from a donor's bone marrow are used to regenerate healthy bone marrow in patients with leukemia and other blood diseases. This therapy is still used today.
Embryonic, or pluripotent, stem cells offer even more dramatic prospects for new treatments and cures for conditions such as spinal cord injuries, blindness and juvenile diabetes, as well as Parkinson's disease, Lou Gehrig's disease and Alzheimer's disease. In addition, some scientists say research using pluripotent cells could ultimately result in treatments for certain types of cancer, new methods of rapidly testing drugs and a basic understanding of human development and genetic abnormalities.
Early Research and Controversy
In the early 1980s, scientists began studying embryonic stem cells, first in mice. But because mouse embryos develop very differently than human embryos, researchers soon sought a way to isolate human embryonic stem cells. In 1998, a team lead by James Thomson at the University of Wisconsin was the first to successfully harvest stem cells from human embryos donated by fertility clinics. Since then, scientists have developed some 400 stem cell lines using private and public funds.
Embryonic stem cells are harvested in two ways: from existing human embryos and from embryos that have been created using a cloning process known as somatic cell nuclear transfer (SCNT). In both cases, the embryo is ultimately destroyed, which opponents of embryonic stem cell research argue is immoral.
The SCNT process was developed by the Scottish scientist Ian Wilmut, who cloned Dolly the sheep in 1996. During the process, the nucleus of a human egg cell is removed and replaced with the nucleus of a donor's adult cell, which contains the person's DNA. The egg is then stimulated to begin subdividing, eventually growing into an embryo with stem cells that can be harvested. Many scientists consider SCNT very promising because it creates an embryo with stem cells that have the same DNA code as the person who donated the cell nucleus. These stem cells can then be used to create therapies that potentially would be compatible with that donor's immune system. In other words, if doctors grew new heart tissue to treat someone with heart disease, the use of the patient's DNA could greatly reduce the likelihood of the patient's body rejecting the new tissue.
A Potential Breakthrough
In 2007, Thomson and his team at the University of Wisconsin, along with scientists in Tokyo, created what appear to be pluripotent cells from adult human skin cells rather than from embryonic cells. The new technique coaxes skin cells to revert to an embryonic stem cell state by inserting a tiny DNA-containing virus.
Even though the new skin cell technique begins with an adult cell, the resulting stem cells appear to have the same makeup and properties as embryonic stem cells, potentially allowing scientists to create pluripotent cells without destroying embryos. If perfected, it also could eliminate the need to create embryos using SCNT. Some say this could eventually end the controversy surrounding stem cell research.
Like SCNT, the new skin cell technique would produce compatible stem cells. Some scientists say the skin cell technique may be more efficient than using embryos because it would eliminate the cumbersome process of acquiring donated eggs and embryos.
While the skin cell discovery has been hailed as a major breakthrough, many scientists caution that these new cells are not yet ready for therapeutic use. For instance, the virus used to coax regular cells into pluripotent stem cells could lead to cellular mutations that could, in turn, cause cancer in patients. Furthermore, Thomson and others contend that this possible breakthrough has by no means eliminated the need for continued research on embryonic stem cells, at least for now. Adult stem cells that have been altered to what appear to be pluripotent cells need to be tested in comparison with human embryonic cells, researchers say, to determine whether they do, indeed, have the same pluripotent qualities.
This report was written by Christine Vestal, Staff Writer, Stateline.org.