[Plasticity of hematopoietic stem cells]

Stem cells are capable of differentiating not only into mature cells of their tissue of origin but also into any cell type. This process is called transdifferentiation or plasticity. Concerning the plasticity of hematopoietic stem cells, four mechanisms have been proposed: transdifferentiation from pluripotent stem cell; indirect transdifferentiation or dedifferentiation; direct transdifferentiation; and fusion with nonhematopoietic cells. In animals, hematopoietic stem cells are capable of transdifferentiating into brain cells (neural, astrocyte and oligodendrocyte), skeletal and cardiac muscle cells, alveolar cells, liver and kidney as well as pancreatic cells. Marrow mesenchymal or stromal stem cell can initiate regeneration of cardiac and skeletal muscle cells, while brain stem cells differentiate into hematopoietic and skeletal muscle cells. Preliminary clinical results are presented. However, progress will have to be made on multiple fronts. It is critical that researchers optimize and standardize experiments. It is also important that developing clinical applications can occur concurrently with studies to elucidate the underlying cellular mechanism. Clinical findings will provide information on the underlying mechanism of plasticity and the most applicable model systems.