Stem cell research: paths to cancer therapies and regenerative medicine.

Most tissues in complex metazoans contain a rare subset of cells that, at the single-cell level, can self-renew and also give rise to mature daughter cells. Such stem cells likely in development build tissues and are retained in adult life to regenerate them. Cancers and leukemias are apparently not an exception: rare leukemia stem cells and cancer stem cells have been isolated that contain all of the tumorigenicity of the whole tumor, and it is their properties that will guide future therapies. None of this was apparent just 20 years ago, yet this kind of stem cell thinking already provides new perspectives in medical science and could usher in new therapies. Today, political, religious, and ethical issues surround embryonic stem cell and patient-specific pluripotent stem cell research and are center stage in the attempts by governments to ban these fields for discovery and potential therapies. These interventions require physicians and physician-scientists to determine for themselves whether patient welfare or personal ethics will dominate in their practices, and whether all aspects of stem cell research can be pursued in a safe and regulated fashion.