the key to future HSC therapies. Obtaining purified HSCs is a major challenge, and purification in a clinical setting is expensive and difficult.

Another major barrier to progress in HSC research and transplantation therapy is that it has not been possible to culture HSCs in vitro (outside the body), although recent studies of mouse HSCs grown in combination with components of the bone marrow have offered some preliminary promise (Ema et al., 2000; Moore et al. 1997). This stubborn and not insignificant obstacle is faced by researchers with all types of adult stem cells. If it were possible to expand the numbers of stem cells by growing them in culture or to stimulate their expansion in vivo (in the living body), the prospects for patients in need of stem cell transplants would be significantly improved. However, as Ernest Beutler pointed out at the workshop, finding a way to get HSCs to proliferate is not enough. In the long run, it is necessary to understand not only what activates HSCs to self-renew, but also what controls their decisions to differentiate into the various components of the blood and prevents them from developing into leukemic cells (Saito et al., 2000).

During the past 2 years, scientific reports of stem cells in other organs of adult mice—including brain, muscle, skin, digestive system, cornea, retina, liver, and pancreas—have cast a new light on the body’s own capability to replenish its tissues (NIH, 2001). Their discovery has also fostered speculation that these cells exist in the adult human, that they have the characteristic of plasticity that enables them to change into precursors of cell types of other tissues, and that they will someday be used to produce the tissues for therapeutic use. The finding of stem cells in adult tissues, not all of which have been confirmed, offers a first glimpse at potential solutions to long-standing puzzles about why some human organs have a greater capacity for self-repair than others.

The idea of employing adult stem cells in certain therapeutic applications is appealing for several reasons. First, adult stem cells are naturally poised to generate a particular tissue, which might consist of several cell