A few small studies have also been carried out in humans, usually in patients who are undergoing open heart-surgery. Several of these have demonstrated that stem cells that are injected into the circulations or directly into the injured heart tissue appear to improve cardiac functions and/or induce the formation of new capillaries. The mechanism of these repair remains controversial, and the stem cells likely regenerate heart tissue through several pathways. However, the stem cell populations that have been tested in these experiments vary widely, as do the conditions of their purification and application. Although much more research is needed to assess the safety and improve the efficacy of this approach, these preliminary clinical experiments show how stem cells may one day be used to repair damaged heart tissue, thereby reducing the burden of cardiovascular disease.