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When human trials finally begin, there's no method for precisely determining whether the transplanted stem cells are functioning correctly. "If we transplanted cells to regenerate a pancreas," says Owen Witte, director of UCLA's Institute for Stem Cell Biology and Medicine, "we can measure in your blood if you're producing insulin, but we can't see whether the cells have grown or evaluate whether they might grow into a tumor." So scientists are seeking to develop marking systems that let them trace a transplant's performance.
THE PROMISE AND PITFALLS OF ADULT CELLS
Even as scientists press ahead with embryo research, exciting news has come from the least controversial sources: the stem cells in umbilical-cord blood and placentas, and even in fully formed adult organs. While not as flexible as embryonic cells, cord and placental cells have proved more valuable than scientists initially hoped. Although about 90% of cord-blood stem cells are precursors for blood and immune cells, the remaining 10% give rise to liver, heart-muscle and brain cells and more. Over the past five years, cord-blood transplants have become an increasingly popular alternative to bone-marrow transplants for blood disorders, particularly when a bone-marrow match can't be found.
If you want to lean out over the edges of science and marvel at what is now possible, visit Dr. Joanne Kurtzberg's program at Duke University Medical Center. Children with blood diseases that were almost certainly fatal a decade ago have got cord-blood transplants that essentially cure them. Now she and her team are taking a more targeted approach by attempting to differentiate cord-blood cells to address heart, brain and liver defects. "I think cord-blood cells have a lot of promise for tissue repair and regeneration," says Kurtzberg. "But I think it will take 10 to 20 years."
Less plastic than cord-blood cells are adult stem cells, which until recently researchers thought couldn't do much more than regenerate cell types that reflected the stem cells' origin--blood and immune cells from bone marrow, for example. Even so, some scientists believe adult stem cells may prove to be a powerful source of therapies. "In some cases, you may not want to go all the way back to embryonic stem cells," says Kurtzberg. "You may want something more specific or less likely to stray. You wouldn't want to put a cell in the brain and find out later that it turned into bone."
Researchers in Thailand have taken stem cells from the blood of cardiac patients, grown the cells in a lab and reinjected them into patients' hearts, where they set about repairing damage. Two UCLA researchers last week published a study demonstrating that they could transform adult stem cells from fat tissue into smooth-muscle cells, which assist in the function of numerous organs. Welcome as the advances are, the subject of adult stem cells is highly political and invites a conflation of real hopes and false ones. "There are papers that have claimed broad uses for certain adult stem cells, and some people say that is sufficient cause to not work on embryonic stem cells," Witte says. "Many of those claims were overblown."