Loren Field, Ph.D., professor of medicine and of pediatrics at the Indiana University School of Medicine and senior author of one of the Nature papers says "these studies demonstrate that the stem cells tested do not form new heart muscle when injected into damaged organs. This suggests that the functional benefit seen in clinical trials may arise from other mechanisms (for example increased blood vessel formation), and raises the possibility that there may be alternative and perhaps more efficacious ways to accomplish this."
Both research teams injected bone-marrow-derived hematopoetic stem cells into the damaged hearts of living mice and used marker proteins to monitor the injected cells. They report that although some of the transplanted cells appeared to survive, they did not appear to differentiate into new heart muscle cells. Instead they matured into cells of the traditional blood lineage.
Dr. Field's study was funded by the National Institutes of Health.