The use of stem cells is one of the most promising, yet ethically controversial issues in modern medicine. The primordial cells with the potential to become any tissue in the body—including the almost impossible to regenerate nerve, vascular, and ligamentous tissues—have sparked policy debates around the world because one of the most promising human sources for stem cells is blastocysts and embryos—specifically, those obtained after in vitro fertilization and abortions. Many governments, including the U.S. government, highly restrict the use of embryonic stem cells in research. Some stem-cell treatments, particularly those from embryonic stem cells, also carry high risks to patients, who may experience side effects including anaphylactic shock or tumors. At the same time, many scientists have considered adult stem cells—those sourced from bone marrow and fat—to hold far less potential for medical purposes and to be extremely difficult to harvest and cultivate.