Embryonic stem cells come from embryos that are three to five days old. At this stage, an embryo is called a blastocyst and has about 150 cells. Embryonic stem can divide into more stem cells or become any type of cell in the human body. Use in people has been limited to eye-related disorders such as macular degeneration.
Adult stem cells are found in small numbers in most adult tissues, such as bone marrow or fat. Compared with embryonic stem cells, adult stem cells are more limited in their ability to become different cells in the body. Until recently, researchers thought adult stem cells could create only similar types of cells. For instance, it was believed that stem cells residing in the bone marrow could give rise only to blood cells. Continuing research results suggest that adult stem cells may be able to create unrelated types of cells.