Until recently, adults stem cells, defined by their self-renewal and differentiation abilities, were thought to be tissue-specific. This concept has been challenged by bone marrow transplantation experiments in mice, demonstrating generation of cells of different phenotype after transplantation of marrow or muscle cells. The term "plasticity" has been coined to explain this phenomenon which could be due to the persistence in adult tissues, of stem cells with multidifferentiation ability or to the "transdifferentiation" ability of some adult cells committed to differentiation, under the influence of unknown environmental cues. The relationship of the cells at the origin of the stem cells plasticity with a new type of mesodermal cell designed under the term of "multipotent adult progenitor cell" (MAPC) remains to be determined. The discovery of this latter is a major advance in this field as the MAPC have isolated from the adult bone marrow and presents certain characteristics of embryonic stem cells with the demonstration of their totipotency towards many tissues, including hematopoiesis. The discovery of the adult stem cell plasticity phenomenon in general, represents a major change in our concepts of stem and developmental biology and possibly the basis for the development of future cell therapy protocols.