Model of BM proliferative response to G-CSF treatment. In response to G-CSF, dormant HSCs are mobilized to the circulation without proliferation. These cells will accumulate in the spleen and return to the BM with time. The remaining cells in the BM after G-CSF treatment can be organized based on CD41 expression, which positively correlates with divisional history. Proliferative response to G-CSF is exclusively associated with CD41 expressing cells, and the CD41^Hi population rapidly matures into megakaryocytes. Label dilution is illustrated by the various shades of green in the nuclei of the cells. Circular arrows represent cell proliferation.