Stem cells.

Stem cells derived from adult and embryonic sources have great therapeutic potential, but much research is still needed before their clinical use becomes commonplace. There is debate about whether adult stem cells can be used instead of those derived from embryos. Rationalisation is needed but can be exercised only once the various cells have been carefully compared and contrasted under appropriate experimental conditions. Some characteristics that might help resolve the issue of cell source can already be applied to the debate. Accessibility is important; some adult cells, such as neural stem cells, are difficult to obtain, at least from living donors. Other factors include the frequency and abundance of adult stem cells and their numbers and potency, which might decline with age or be affected by disease. For embryonic stem cells, ethical concerns have been raised, and the proposed practice of therapeutic cloning tends to be misrepresented in the lay media. For both adult and embryonic stem cells, stability, potential to transmit harmful pathogens or genetic mutations, and risk of forming unwanted tissues or even teratocarcinomas have yet to be fully assessed.