stem cell
a cell that is itself undifferentiated but can divide to produce one or more types of specialized tissue cells (e.g., blood cells, nerve cells). Because of this ability, stem cells act as a kind of continual repair system for the living organism by replenishing specialized cells. Stem cells found in embryos (embryonic stem cells) are produced in humans during the blastocyst stage of development and are capable of forming any type of tissue cell; stem cells that occur in adults (adult stem cells) are more limited in the range of cell types they can produce. Because of this difference, many researchers believe that human embryonic stem cells hold greater potential for the development of therapeutic treatments than do adult stem cells. Nonetheless, the use of adult stem cells to treat some diseases has been common since the 1960s (e.g., the use of bone marrow stem cells in the treatment of leukemia and lymphoma), and research continues on the
potential use of both types of stem cells as renewable sources of replacement cells and tissues to treat a range of conditions, such as cardiovascular disease, Alzheimer’s disease, Parkinson’s disease, spinal cord injury, diabetes, and arthritis. There are ethical concerns about human embryonic stem cell research, mainly because harvesting such cells involves destroying the embryo and thus, for some, raises questions about the rights of the unborn.