Stem cell research involves the study of stem cells, which are special human cells that have the ability to develop into many different cell types in the body during early life and growth. Stem cells can also renew themselves, and they have the potential to repair damaged tissues. There are two main types of stem cells: embryonic and adult. Embryonic stem cells come from unused embryos that result from in vitro fertilization procedures and are pluripotent, meaning they can turn into more than one type of cell. Adult stem cells are found in small amounts in the body and can be found in tissues such as bone marrow, blood, and fat. Stem cell research has the potential to dramatically alter approaches to understanding and treating diseases, and to alleviate suffering. Researchers study stem cells to learn if these basic building blocks of life can help unlock new medical treatments. Stem cell therapy, also known as regenerative medicine, promotes the repair response of diseased, dysfunctional, or injured tissue using stem cells or their derivatives. Stem cells may be used to help create new tissue, and healthcare providers may be able to treat people with chronic heart disease by growing healthy heart muscle cells in a lab and transplanting them into damaged hearts. Stem cell research is a controversial topic, particularly when it involves embryonic stem cells, as there are ethical considerations surrounding the use of human embryos. However, not all stem cell research involves human embryos, and there are many less controversial sources of acquiring stem cells, such as using cells from the umbilical cord, breast milk, and bone marrow.