Figure 6.

Graphic of the in vitro HSC/MSC co-culture system. (A) Migration towards and retention of HSC to MSC is mediated by SDF1/CXCR4 and adhesion molecules such as integrins. (B) The MSC layer serves in vitro as a boundary between two distinct compartments, i.e. the MSC surface mediating cell proliferation and a niche-like compartment beneath the layer. Here beneath the layer, phase-dim (PD) cells show a delayed cell-cycle activity and a more immature phenotype. In contrast, phase-bright (PB) HSC on the MSC surface revealed significantly more proliferation activity. We assume that cells from the MSC surface are released into the supernatant, the third microenvironment (non-adherent cells, NA) in the co-culture system. The graphic illustrates the dynamic interplay of HSC in the three compartments.