Stem cells are characterized by the ability to self-renew, or divide without senescing, and to differentiate into specialized somatic cells. Multiple types of stem cells have been identified, including embryonic and adult stem cells. Embryonic stem cells are pluripotent and can differentiate into all cell types of the mature organism. In contrast, adult (somatic) stem cells, including hematopoietic, muscle, cardiac, neural, and mesenchymal stem cells, are thought to have limited potency, and their differentiated derivatives are generally thought to populate only the tissue of residence. Stem cells reside within distinct microenvironments, or niches. Adhesion molecule expression mediates the retention of stem cells within the niche and influences self-renewal potential.
More recent advances in stem cell biology have identified the transcription factors that are sufficient to reprogram somatic cells back to an undifferentiated state. These induced pluripotent stem cells, and other stem cell types, have potential uses for understanding multipotency and self-renewal, developing models of human diseases for drug and toxicology studies, and applying cell-replacement strategies for regenerative medicine. In addition, the cellular origins of some cancers are hypothesized to depend on genetic alterations in cancer stem cells that allow for uncontrolled proliferation.