Via PubMed, a study suggesting that old stem cells can be harmful: "Local injection of hematopoietic stem cell enriched cells, including mouse lin(-) cells, accelerates vascularization in animal injury models, apparently by release of angiogenic factors. Locally injected lin(-) cells from non-diabetic mice dramatically improve, but those from obese diabetic mice inhibit vascular growth in obese diabetic mouse skin wounds. ... Our data suggest that bone marrow-derived cells may be poor candidates for therapeutic use in older patients, and could actually harm such patients." Is this a function of the cells themselves, or a function of the environment they find themselves in? More research is needed, especially given the use of diabetes as a proxy for aging in this study.