Versatility of stem and progenitor cells and the instructive actions of cytokines on hematopoiesis

For many years, developing hematopoietic cells have been strictly compartmentalized into a rare population of multi-potent self-renewing hematopoietic stem cells (HSC), multi-potent hematopoietic progenitor cells (MPP) that are undergoing commitment to particular lineage fates, and recognizable precursor cells that mature towards functional blood and immune cells. A single route to each end-cell type is prescribed in the "classical'' model for the architecture of hematopoiesis. Recent findings have led to the viewpoint that HSCs and MPPs are more versatile than previously thought. Underlying this are multiple routes to a particular fate and cells having clandestine fate options even when they have progressed some way along a pathway. The primary role of cytokines during hematopoiesis has long been seen to be regulation of the survival and proliferation of developing hematopoietic cells. Some cytokines now clearly have instructive actions on cell-fate decisions. All this leads to a new way of viewing hematopoiesis whereby versatile HSC and MPP are directed towards lineage outcomes via cytokine regulated cell-fate decisions. This means greater flexibility to the shaping of hematopoiesis.