Stem cell biology and the plasticity polemic.

Characterization of a cord blood derived unrestricted somatic stem cell (USSC) with capacity to differentiate into hematopoietic and nonhematopoietic tissues in the absence of cell fusion has highlighted the great potential of stem cell plasticity. A great variety of stem cell types have been defined and even the most pure marrow stem cells are highly heterogeneous. Data suggest that stem cells may exist in a continuum with continually and reversibly changing phenotype. These cells also possess a capacity to produce lung, liver, skin, and skeletal muscle under conditions of tissue injury. Arguments raised against the significance of adult marrow to nonmarrow conversions including the importance of cell fusion appear fallacious. We are at the beginning of an exciting and burgeoning field of research with great clinical potential.