cells (Thompson et al., 1998). The experiments showed the capability of ESCs to produce a variety of tissues, but the results also highlight the complexity of the biological “program” of tissue development that can unfold in different biological environments. These results also emphasize the abnormal, potentially neoplastic potential of ESCs when placed into unnatural environments.
Major questions remain about the genetic or environmental factors in the body that control the fate of ESCs and about the importance of different factors during various stages of cell differentiation. Even on the basis of the limited findings, however, the ability to grow human ESCs in vitro and to have them differentiate in the laboratory makes them an important and unique tool with which to conduct the basic research that is critical for the foundation of future regenerative therapies. It has been possible, for example, to create a lineage of mouse ESCs that generate neural cell precursors (Li et al., 1998). Studies of the genes turned on and off as cells begin to differentiate, which are already under way with ESCs, will permit a better understanding of the genetic controls important in tissue differentiation (Duncan et al., 1998). In vitro studies of ESCs also provide an opportunity to explore the role of biochemicals produced in the normal cellular environment that induce stem cells to differentiate, to migrate to a site needing repair, and to assimilate into tissues (Schuldiner et al., 2000).
At the workshop, James Thomson and Thomas Okarma suggested that human ESCs will someday provide a potentially unlimited source of cells, differentiated in vitro, for transplantation therapies involving the liver, nervous system, and pancreas. Irving Weissman alluded to the possible use of ESCs to enhance the success of whole-organ transplantation. If HSCs derived from human ESCs could be successfully transplanted into the blood system of a transplant recipient (by using immunosuppressive drugs), any further implant tissue (say kidney or pancreas)