Quiescence/Proliferation Issue and Stem Cell Niche

The dominant opinion today is that the hematopoietic stem cells are quiescent and that this quiescence is related to their position in hypoxic niches and their “hypoxic” phenotype. This simplified perception is not compatible with the phenomenon of self-renewal in steady state, which is not possible without cell division. This “shortened” statement derived from the experimental results on phenotypically defined populations still functionally heterogeneous and not composed exclusively of stem cells, demands more critique and a more flexible interpretation. In addition, there is a nonnegligible amount of results and arguments showing that at least a fraction of the stem cells can proliferate in low O 2 niches not only after an insult but also in steady state. Furthermore, this low-energy, slow proliferation (conditioned by glycolytic metabolic type) may condition self-renewal (cell division without commitment and differentiation).