Adult Stem Cell Plasticity Defined

Introduction Investigators have postulated since the turn of the century that some cells in the body appear to participate in the formation of new cells to replace injured, aged, or infected cells in tissues and organs. The concept that certain specialized cells within an organ (stem cells) could give rise to mature functioning cells in that same organ has been demonstrated most clearly in the hematopoietic system. Evidence for stem cell function in the hematopoietic system has come from controlled laboratory experiments and as a consequence of human exposure to massive irradiation during armed conflict. World War II was brought to an end dramatically in August 1945 with the dropping of atomic bombs on the cities of Hiroshima and Nagasaki, Japan. In the wake of the devastation and human suffering of these events, several hundred thousand Japanese civilians and soldiers were exposed to lethal and sublethal doses of irradiation. Examination of the affected people included, but was not limited to, the ablative effects of irradiation on the body’s hematopoietic system. Observation and treatment of these patients led to several scientific reports confirming the postulated existence of stem cells that are responsible for the continuous production of all circulating blood cells. Subsequent studies performed on laboratory rodents detailed the detrimental effects of irradiation on the hematopoietic system as well as the ability of bone marrow transplantation to correct such anomalies. Hematopoietic repopulation was established by transplanting clonogenic marrow cells, which could be tracked to the spleen of the irradiated recipient following infusion. Thus, a single stem cell (clone) from the bone marrow could give rise to thousands of mature blood cells inside the splenic microenvironment within 8 days following transplantation. Over the past 30 years, additional animal and human studies have demonstrated the feasibility of successful transplantation of marrow stem cells into recipients, with reconstitution of the host blood cells with donor type cells and amelioration of congenital or acquired blood disorders. Today we recognize that numerous adult stem cells operate in various capacities in postnatal humans, regulating