RÉSUMÉ
@#Traditionally, stem cells in adult tissues were believed to be restricted in their differentiation potential and limited to generate the types of cell present in the tissue in which the stem cell resides. Recently, however, many studies indicate that stem cells may display a broader differentiation potential, termed plasticity or transdifferentiation, than what was previously thought. But the validity of this understanding has been challenged by some investigators. In this review, the authors present some standpoints on stem cell plasticity and discuss some phenomena of stem cell plasticity observed on animal models those have been generally accepted currently.
RÉSUMÉ
The naturally occurring stem cell migratory patterns, the availability of expanding homing and engraftment sites, and the presence of tissue/organ-specific signals in the developing mammalian fetus provide the ideal setting for stem cells to exhibit their full biological potential. These characteristics combined with the relative immunological naivete of the early gestational age fetus that permits the engraftment and long- term persistence of allogeneic and xenogeneic donor stem cells make it possible to use the developing fetus to assess the in vivo potential of a variety of stem cells. We have taken advantage of these permissive characteristics of the fetus to develop a large animal model of human hematopoiesis in sheep that permits not only the long-term engraftment of human hematopoietic stem cell/progenitor cells and their differentiation into the full range of lymphohematopoietic elements, but also the relatively robust expression of their potential to contribute to the formation of non-hematopoietic tissues.