Recent studies have clearly identified that hematopoietic stem cells (HSC) are part of a hierarchical organization defined by cells able to initiate long-term repopulation (LT-HSC) of injured BM followed by populations with transient repopulation ability. As HSCs are able to supply blood cells for the lifespan of the individual, LT-HSCs undergo an aging process. The consequences of aging are an increase in the stem cell pool size, increased self-renewal and production of cells with expression of myeloid-specific genes and genes commonly expressed in myeloid leukemia. The age-related increased incidence of myeloproliferative disorders is therefore not surprising. Marrow cells circulate and migrate to a number of different organs. Previously assumed tissue and organ specificities appear to be less stringent and primitive cells residing in one organ may contribute to the structural and functional regeneration of other organs. It is intriguing to speculate that cells and cytokines used in appropriate concentrations may be able to repair damaged organs by using the defective organ as a scaffold, thus reducing the need for the transplantation of solid organs.