Studies in mice showed that, when fetal cardiomyocytes were placed in myocardial scar tissue, new heart muscle cells formed and heart function improved. Similarly, in a rat model of myocardial infarction, embryonic stem cells injected into heart muscle became part of the myocardium and allowed it to contract more efficiently. Small-scale studies in humans after myocardial infarction suggest that transplanting stem cells may also be feasible and might improve heart function. Although human embryonic stem cells cause little immune reaction when transplanted into other humans, supplies of these cells are limited and their use has posed difficult ethical issues. A possible alternative is to collect and use stem cells in the patient's own bone marrow that have been stimulated to grow by using chemicals called growth factors. In an experimental mouse model, these chemically stimulated bone marrow stem cells, when injected directly into the heart muscle, migrated to the injured area and formed myocardial tissue. Heart function improved, and new vessels that brought more blood to the heart developed. Another possibility is to inject stem cells into the coronary arteries that supply blood to the myocardium. A few patients have had stem cells derived from bone marrow injected into damaged heart muscle at the time of bypass surgery. Blood flow to the myocardium improved in some of these patients.