Science News is running a good overview article on stem cell research: its branches, possibilities and history. "The promise [of stem cells] has not been exaggerated. What's been lost in discussion is how long it will take and how difficult it will be for stem cells to live up to their billing ... The challenge of getting stem cells to differentiate reliably [in the lab] is to provide the cells with an identical replica of the chemical signals that the body naturally uses to sway adult and embryonic cells to differentiate. 'The problem is, we don't know what [those signals] are yet. ... Luckily, some adult stem cells save researchers the task of sorting cells or guessing chemical signals. ... For decades, researchers have taken advantage of this quality in treating leukemia patients with bone marrow stem cells.'"

Link: http://www.sciencenews.org/articles/20050402/bob10.asp