- The phenotype of marrow hematopoietic stem cells is determined by cell-cycle state and microvesicle entry into the stem cells. The stem cell population is continually changing based on cell-cycle transit and can only be defined on a population basis. Purification of marrow stem cells only addresses the heterogeneity of these populations. When whole marrow is studied, the long-term repopulating stem cells are in active cell cycle. However, with some variability, when highly purified stem cells are studied, the cells appear to be dormant.
- Long-term engrafting marrow hematopoietic stem cells have been considered to be a quiescent stem cell in G0. However, there are contradictory reports on this point in the literature, showing marked variability of results over time and between mice. Furthermore, there are circadian rhythms for stem cells and progenitors. In general, most studies have not taken stochastic variability or circadian rhythms into account. In addition, stem cell purification has represented the present gold standard in stem cell research.
- Allogeneic stem cell transplantation is curative for certain cancers, but the high doses of chemotherapy and radiotherapy may lead to toxicity. This review summarizes the field of cellular immune therapy using very-low–dose conditioning for refractory cancers.
- Characterization of a cord blood derived unrestricted somatic stem cell (USSC) with capacity to differentiate into hematopoietic and nonhematopoietic tissues in the absence of cell fusion has highlighted the great potential of stem cell plasticity. A great variety of stem cell types have been defined and even the most pure marrow stem cells are highly heterogeneous. Data suggest that stem cells may exist in a continuum with continually and reversibly changing phenotype. These cells also possess a capacity to produce lung, liver, skin, and skeletal muscle under conditions of tissue injury.
- In many ways, the homing of hematopoietic stem cells to bone marrow and other tissues defines these cells and their immediate and long-term fates Once homed, an inevitable series of proliferative and differentiative events presumptively follows. These comments, of course, hold for both homing to marrow, or alternatively, to other nonmarrow tissues. In this review, we will specifically focus on homing and engraftment to bone marrow because this is the best-studied and clinically applicable system.