Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, which multiply by cell division to replenish dying cells and regenerate damaged tissue. Adult stem cells support tissue and organ turnover throughout life by maintaining a delicate balance between apoptosis and tissue-specific cellular proliferation and differentiation. Adult stem cells reside in a specific area of each tissue (called a "stem cell niche") and serve as long-term reservoirs of quiescent cells that can be called upon (activated) to repair tissues or organs by complex signaling pathways mediated by their resident niche. Unlike embryonic stem cells, adult stem cells are not pluripotent; they are developmentally restricted multipotent cells. These cells exhibit three major properties: they are long lived, capable of self-renewal and can differentiate to yield some or all of the major specialized cell types of the tissue or organ from which they are derived. In contrast to embryonic stem cells, which are derived from the early gestation material, adult stem cells are present throughout the adult organism lifespan.
Since their initial discovery more than 50 years ago by James Till and Ernest McCulloch, adult stem cells have been found in many more tissues than initially anticipated, including brain, bone marrow, fat, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium and testis. These cells could be used for patient-specific transplantation to treat a wide variety of injuries and diseases. Adult stem cells could also be used to study unknown biological events related to tissue and organ pathology, to model human diseases and to aid pharmacogenomic studies and drug testing.