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t m c » p u l s e | j a n / f e b 2 0 1 5 20 An Introduction What are stem cells? Stem cells have the remarkable potential to develop into many differ- ent cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell or red blood cell. What makes stem cells unique? Stem cells differ from other kinds of cells in the body. All stem cells, regardless of their source, have three general properties: they are capable of dividing and renewing themselves for long periods; they are unspecialized; and they can give rise to specialized cell types. What are embryonic stem cells? Embryonic stem cells, as their name suggests, are derived from embryos. Most embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro—in an in vitro fertilization clinic—and then donated for research purposes with informed consent of the donors. They are not derived from eggs fertilized in a woman's body. What are adult stem cells? An adult stem cell is thought to be an undifferentiated cell, found among differentiated cells in a tissue or organ. The adult stem cell can renew itself and can differentiate to yield some or all of the major specialized cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skele- tal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis. They are thought to reside in a specific area of each tissue (called a "stem cell niche"). Stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain tissues, or by disease or tissue injury. What are the similarities and differences between embryonic and adult stem cells? Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for cell-based regenerative therapies. One major difference between adult and embryonic stem cells is their different abilities in the number and type of differenti- ated cell types they can become. Embryonic stem cells can become all cell types of the body because they are pluripotent, while adult stem cells are thought to be limited to differentiating into different cell types of their tissue of origin. Adult stem cells, and tissues derived from them, are currently believed less likely to initiate rejection after transplantation. A single cell can replicate itself or differentiate into other cell types. There are multiple ways that these advances in regenerative medicine can change the land- scape of health sciences. I see regenerative med- icine providing hope for people who haven't had hope for years... DORIS A. TAYLOR, PH.D. Director of Regenerative Medicine Research at Texas Heart Institute