Intrinsic control of stem cell fate.

Efforts to expand the number of stem cells ex vivo are based on the assumption that the self-renewal properties of stem cells are subject to extrinsic control. Although loss of stem cell properties ex vivo has been well-documented, conclusive evidence for a gain of stem cell function in vitro has not been forthcoming. In this short discussion paper, some issues related to the self-renewal of stem cells in relation to future experimental strategies are presented.