Homing and mobilization in the stem cell niche.

All mature blood cells are derived from the haemopoietic stem cell (HSC). In common with all other haemopoietic cells, stem cells are mobile, and it is this property of mobility that has allowed bone marrow transplantation to become a routine clinical option. Successful transplantation requires haemopoietic stem cells to home to the bone marrow, leave the peripheral circulation and become stabilized in regulatory niches in the extravascular space of the bone marrow cavity. This homing and tethering process is reversible - haemopoietic stem cells can be released from their bone marrow tethering through changes in molecular interactions, which are also important in homing following transplantation. The molecular mechanisms regulating this two-way flow of stem cells are beginning to be elucidated, and much recent data has emerged that sheds light on the processes and molecules involved in these complex physiological events. This article reviews current knowledge of the adhesive, homing and proliferative influences acting on HSCs and progenitor cells.