So far, most of the successful experiments in rats with stem cells have used the embryonic variety. In August, Ira Black at the Robert Wood Johnson Medical School in New Jersey reported that he had converted bone marrow stem cells into the precursors of neurons. This makes adult stem cells more attractive, since marrow is easier to harvest. But Black says we will never know what biochemical cues give cells their identity unless researchers can study embryonic stem cells-the most undifferentiated cells of all. Limiting research to adult stem cells would leave scientists groping in the dark.
Neuroscientists still face formidable challenges.
Many cells die, for example, and some go to the wrong place or become the wrong kind of cell. Rothstein says we still don't know how long implanted stem cells survive, or whether some will become tumours due to their extraordinary ability to keep dividing. It isn't even clear when to implant such cells. These questions can only be answered by testing stem cells over and over in the lab-and embryonic cells are the only type suitable, say researchers. Ironing out these kinks is a key issue in developing therapies for neurological diseases.