A cell that is not differentiated itself but can undergo unlimited division to form other cells, which either remain as stem cells or differentiate to form specialized cells. For example, stem cells in the bone marrow divide to produce daughter cells that differentiate into various types of immune cell (e.g. monocytes, lymphocytes, mast cells). Also, stem cells in the intestine continually divide to replace cells sloughed off the gut lining. Embryonic stem cells, such as those taken from an early human embryo, are capable of differentiating into all of the various tissue cells found in a fully developed individual—they are described as totipotent. Stem cells that can differentiate into a more limited range of tissues are described as pluripotent. Cultures of such cells have the potential to provide replacement tissues and organs for medical use, including transplantation and the treatment of various diseases, such as type 1 diabetes, Parkinson’s disease, and Huntington’s disease. However, ethical concerns have led to tight controls on research using human embryonic stem cells in many countries, including the USA and UK. Such objections can be overcome by techniques that turn tissues cells into stem cells. For example, adult skin cells can be harvested from a patient, cultured, and genetically manipulated so that they are effectively dedifferentiated. One technique uses a retroviral cloning vector to introduce copies of crucial stem cell regulatory genes into cultured skin fibroblasts. The fibroblasts are deprogrammed by these ‘master genes’ to form induced pluripotent stem cells (iPS cells). These can then be induced to differentiate into various tissues, with the great advantage that they are accepted as ‘self’ tissue by the patient’s immune system. Meristem tissue from plants contains stem cells from which new plantlets can be grown using the technique of micropropagation. Commercial cloning of plants also makes use of the totipotent callus tissue that forms over wounds. See also haemopoietic tissue; meristem.
http://www.stem-cell-forum.net/ Website of the International Stem Cell Forum, which promotes good practice and progress in stem cell research