BM stromal cell–derived molecules that could transduce survival/growth, differentiation, or chemotactic signals to CLP, early-B, or pro-B cells.

HSPG indicates heparan sulfate proteoglycan. The dashed arrow indicates that the membrane-bound form of Flt3 ligand is cleaved at the stromal cell surface. The question mark indicates the unknown growth factor that could bind to HSPG. IL-7 and the unknown growth factor are shown binding directly to their cognate receptors, or binding HSPG followed by binding to their cognate receptors.