Figure 2.

The contribution of MSCs to the microenvironment of bone marrow stem cell niches. Several cell types contribute to the stem cell niches, especially the osteoblasts, MSCs and ECs. The MSCs are localized both close to the sinusoids where they seem to contribute to the regulation of cell trafficking across the vessel wall, and the extravascular space between the vessels and close to the osteoblasts in the endosteum where they also release extracellular matrix molecules. Thus, MSCs are important both for the extravascular microenvironment of the HSCs and for cell trafficking to and from this microenvironment. Monocytes/macrophages and sympathetic nerve fibres are also important for the supportive functions of the stem cell niches; these components are not shown in the figure.