Dormant Emergency Stem Cells
An intriguing discovery from a cancer research group that I suspect has more promise for the field of regenerative medicine: "Up to now, scientists have assumed that adult stem cells have a low division rate. According to theory, they thus protect their DNA from mutations ... [Researchers] have now discovered a group of stem cells in mouse bone marrow that remain in a kind of dormancy [and] divide only about five times throughout the life of a mouse. Translated to humans, this would correspond to only one cell division in 18 years. ... In contrast, stem cells of the larger group, the 'active' stem cells, divide continuously about once a month. However, in an emergency such as an injury of the bone marrow or if the messenger substance G-CSF is released, the dormant cell population awakes. Once awakened, it shows the highest potential for self-renewal ever to be observed in stem cells. If transplanted into irradiated mice, these cells replace the destroyed bone marrow and restore the whole [blood] system. It is possible to isolate new dormant stem cells from the transplanted animals and these cells are able to replace bone marrow again - this can be done several times in a row. The situation is different with 'active' stem cells, where bone marrow replacement can successfully be carried out only once."
Link: http://www.eurekalert.org/pub_releases/2008-12/haog-dsc120408.php