This article talks about new findings that injected stem cells live only a number of hours, due to being out of the influence of blood vessel-forming cells. If these cells are used with the stem cells, however, they last for a few weeks!
http://www.indiatimes.com/technology/science/new-technique-to-make-stem-cell-therapy-reality-158676.html
http://www.rdmag.com/news/2014/07/research-team-pursues-techniques-improve-elusive-stem-cell-therapy
http://www.indiatimes.com/technology/science/new-technique-to-make-stem-cell-therapy-reality-158676.html
http://www.rdmag.com/news/2014/07/research-team-pursues-techniques-improve-elusive-stem-cell-therapy