Developmental potential of hematopoietic and neural stem cells: unique or all the same?

Like many other animals, mammals develop from fertilized oocytes - the ultimate stem cells. As embryogenesis proceeds, most cells lose developmental potential and eventually become restricted to a specific cell lineage. The result is the formation of a complete and structured mature organism with complex organs composed of a great variety of mature, mostly mitotically quiescent effector cells. However, along the way, some exceptional cells, known as somatic stem cells (SSCs) are set aside and maintain a high proliferation and tissue-specific differentiation potential. SSCs, in contrast to embryonic stem (ES) cells, which are able to give rise to all cell types of the body, have been regarded as being more limited in their differentiation potential in the sense that they were thought to be committed exclusively to their tissue of origin. However, recent studies have demonstrated that somatic stem cells from a given tissue can also contribute to heterologous tissues and thus show a broad nontissue restricted differentiation potential. The question arises: how plastic are somatic stem cells? To provide a tentative answer, we describe and review here recent investigations into the developmental potentials of two somatic stem cell types, namely hematopoietic and neural stem cells.