Why Two Stem Cell Niches

It is plausible that the vascular niche is the location for HSC differentiation and mobilization and that the osteoblastic niche provides a quiescent environment ideal for HSC maintenance. The vascular niche could be the alternative niche for mobilized stem cells, promoting proliferation and further differentiation or maturation and entry into the circulatory system. This description is attractive, considering the sinusoids in the vascular niche consist of a single layer of endothelial cells, designed for blood cell penetration. This would be an ideal location for HSCs to initiate a rapid response to hematopoietic stress or cytokine stimulation. Additionally, at least in mice, an extramedullary vascular niche also exists in the spleen or is established upon ablation of the endosteal microenvironment (Klassen 1972).

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