Will Stem Cells Finally Deliver?
Major breakthroughs are propelling the field. Science becomes medicine.
Many diseases involve the death of cells that the body cannot naturally replace. Sometimes cell death comes suddenly, as in a heart attack. Other times it is slow and inexorable, as in Alzheimer's disease. The great promise of stem cells—the body's equivalent of renewable energy—is that they could be coaxed into becoming and then replacing cells lost to disease.
But daunting scientific challenges, ethical concerns and even politics have slowed progress for more than a decade. In the past two years, however, a series of remarkable breakthroughs has advanced the field: suddenly, it appears possible to create cells with all the potential of embryonic stem cells without using embryos, eliminating most of the ethical concerns surrounding stem-cell research.
Embryonic stem cells have two extraordinary properties that make them, potentially, the most medically useful. First, they are "pluripotent," with the capacity to become any type of specialized cell in the body—a heart-muscle cell that pumps blood, an acid-producing cell in the stomach, a cell in the retina of your eye that sees light, or a brain cell that stores memories. Second, embryonic stem cells can keep dividing and making unlimited copies of themselves—an important property, since huge numbers of new cells may be needed to replace cells lost to disease.Scientists have also been studying adult stem cells, work that doesn't raise the ethical questions posed by em-bryonic-stem-cell research because it doesn't involve the use of human embryos. Bone marrow and organs like the heart and liver all naturally contain adult stem cells. These cells have the potential to develop into most of the cells in their specific organ. Adult stem cells help replace specialized cells that have been killed, since most specialized cells cannot naturally reproduce themselves. However, adult stem cells in most organs cannot naturally repair the massive injury caused by many diseases, although scientists are working on ways to change that. Also, adult stem cells are not pluripotent: unlike embryonic stem cells, they cannot be turned into any cell in the body.
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