In medical terms, stem cells are "undifferentiated" cells that have the ability to reproduce themselves. These stem cells can be found in the umbilical cord blood, and in the umbilical cord tissue. While cord blood stem cells currently have the ability to help treat certain diseases, cord tissue is still being researched, however, these stem cells give rise to multiple specialized cell types that can make up the heart, lung, skin, and other tissues. In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury or disease.