Here is a diagram I drew for my presentation on the paper “Sphingosine-1-phosphate mobilizes osteoclast precursors and regulates bone homeostasis” by Ishii et al., 2009.
The femur above is just to illustrate where the diagram below takes place, that being where the blood vessel, marrow and bone interact.
Below, you can see what this system might look like in one of the test subjects (in this case, mice) from the paper. Monocytes fuse to form osteoclast precursors. These precursors are either drawn into the blood stream by the higher S1P concentrations therein, or they actively bind to RANKL in the marrow, which differentiates them into osteoclasts.
Other things from the paper included in this diagram are fingolimod (FTY720), a drug which increases the attractive force of S1P on the osteoclast precursors, and the antibody-fluorescent protein CX3CR1-EGFP.