Gearhart harvested cells from the region that gives rise to the testes or ovaries. Such fetal stem cells appear to be as malleable as embryonic ones. One feature that all stem cells share is an urge to travel. Evan Snyder, a neurologist at Harvard Medical School in Boston, has found that stem cells are attracted to injured tissues, perhaps because of biological cues released by dying or diseased cells. Indeed, one of Snyder's lab colleagues found that a batch of stem cells had migrated from one side of a rat's brain to the other to infiltrate a tumor.
Stem cells are also highly responsive to their surroundings.
Researchers have taken adult stem cells from the brains of rats and put them in bone marrow and watched, in astonishment, as they spewed out blood cells. True, they did not form all the different blood-cell types, just a few. But until then no one had known that adult stem cells could adapt even that much to their environment. Not in words but in biological signals, like growth factors and surface proteins, and by simply touching them. David Anderson and his colleagues at Cal Tech have, for example, identified many environmental triggers that get the nervous system's stem cells to turn into neurons or into their supporting glial cells.