Potential adhesion mechanisms for localisation of haemopoietic progenitors to bone marrow stroma

Haemopoiesis occurs in intimate physical association with the stromal elements of the bone marrow. Current evidence supports the hypothesis that the restriction of primitive haemopoietic progenitor cells (HPC) to the bone marrow involves developmentally regulated adhesive interactions between HPC and the stromal cell microenvironment. This review examines the expression and function of cell adhesion molecules (CAM) on human HPC and marrow stromal cells. These data demonstrate that a broad range of CAMs representing at least three adhesion molecule superfamilies (integrins, selectins, immunoglobulin gene superfamily) participate in these adhesive interactions. We discuss the potential contribution of these various adhesion molecules to homing of HPC to the bone marrow, their retention within the extravascular haemopoietic compartment and their egress into the peripheral circulation. It is likely that each process is mediated not by a single binding event but requires the coordinated participation of multiple receptor-ligand pairs.