What Stem Cells Can Do—and Can't

Stem cell promise new disease treatments but haven't yet delivered.

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All stem cells can replicate over and over, thanks to an enzyme that slows their aging. This enzyme, telomerase, is what makes cancer cells proliferate, too. Stem cells can also make new cells programmed to do specific jobs. Bone marrow stem cells, for example, make red blood cells, which carry oxygen through the body.

In the past decade, researchers have discovered different forms of stem cells, all of which hold promise as tools for developing new treatments for diseases such as diabetes, heart disease, Parkinson's, and Alzheimer's. The FDA just approved the first clinical trial of embryonic stem cells in humans. The trial involves an attempt to repair severed spinal cords in paraplegics.

Here are the three types of stem cells, and their pluses and minuses in medical applications:

Embryonic stem cells are pluripotent. In other words, they can become almost any specialized cell or body tissue. Derived from human embryos left over from in vitro fertilization, they give researchers flexibility to control the growth of cells and tissues needed for treatment. When transplanted, however, donor ESCs could still be rejected by a patient, who in most cases would be a nonrelative. And some people object to using cells from human embryos for research, making embryonic stem cells probably the most controversial topic in biomedicine.

Adult stem cells, like those in bone marrow that form red and white blood cells, are usually limited to forming specialized cells. Adult stem cells in the skin and digestive system are mostly multipotent. These stem cells could be used to regenerate many, but not all, tissues in the body.

Induced pluripotent cells, which closely mimic embryonic stem cells, were first made by scientists from adult skin cells in 2007. These hybrid species make use of a patient's own cells and could skirt the ethical problem of using embryonic stem cells altogether. They also would erase the problem of rejection of donor cells. For now, however, iPSs aren't safe for treatment because the process of making them includes the addition of certain cancer-causing agents.