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The Journal of the Royal Institute of Thailand Vol. 27 No. 1 Jan.-Mar. 2002 ‡´≈≈å µâ 𰔇𑠥 Òı¯ Abstract Stem Cells Athasit Vejjajiva, Surapol Issaragrisil Fellow, the Academy of Science, the Royal Institute, Thailand Stem cells are defined as clonogenic self-renewing progenitor cells that can give rise to one or more differentiated cell types. In vertebrates, there are two groups of stem cells. The first comprises embryonic stem cells which are derived from the inner cell mass of the blastocyst and are capable of generating all differentiated cells types in the body. Embryonic stem cells generate the second group, the adult stem cells in children and adults, which were long believed to be organ-or tissue-specific. Adult stem cells are mainly present in the bone marrow, skin and intestines throughout life. How- ever, recent studies have provided evidence of stem cells in other tissues such as the brain, skeletal muscles, cardiac muscle and the breasts. Furthermore, adult stem cells from one tissue or organ can be induced to differentiate into cells of other organs, either in vitro or after transplantation in vivo. The ability of stem cells to produce selectively one or more differentiated cell types will be of profound clinical importance, as it would enable the specific replacement of damaged or dysfunc- tional cell types within the body, potentially curing several diseases, including neurodegenerative disorders, heart diseases, diabetes mellitus and hematological disorders. Embryonic stem cells are capable of unlimited self-renewal, but ethical problems are still a major concern. Adult stem cells are more practical for use but they are less proliferative and are more difficult to expand. However, rigorous scientific studies to determine the productivity of these stem cell behaviors, and the fre- quency and characteristics of the cells that exhibit them are badly needed prior to their use for thera- peutic purposes in humans. Key words : stem cell, embryonic stem cell, adult stem cell