What are adult stem cells and where are they found?
An adult stem cell is an undifferentiated cell that is found among specialized cells in a tissue or organ, can renew itself, and can differentiate to produce the major specialized cell types of that 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. Unlike embryonic stem cells, which are defined by their origin (the inner cell mass of a 4-day-old embryo), the origin of adult stem cells in most mature tissues is unknown. The history of research on adult stem cells began about 40 years ago. In the 1960s, researchers discovered that the bone marrow contains a very rare population of cells that could generate all of the cells of our immune system and all the red cells in our blood. Furthermore, if these “hematopoietic” stem cells were transplanted into mice that had already received a strong dose of radiation, they allowed the mice to survive an otherwise lethal procedure. These early basic observations
