Hematopoietic stem/progenitor cell mobilization. A continuing quest for etiologic mechanisms.

The physiologic egress of mature hemopoietic cells and of hemopoietic stem/progenitor cells from bone marrow to the circulation are poorly understood processes. Likewise, the mechanism of their enforced emigration or mobilization through the use of several agents has not been unraveled. Although mobilization is suspected to be a multi-step process, involving sequential and/or overlapping changes in adhesion and migratory capacity, a model of molecular hierarchy, like the one governing the extravasation of mature leukocytes to tissues of inflammation, has not been worked out. Understanding the in vivo mechanism of mobilization has been a challenge. Signals emanating from both stromal cells and from hemopoietic cells are likely involved. However, dissecting out their roles, specificity, and interactions has been difficult. Nevertheless insightful information is rapidly emerging, especially with the current availability of many mouse models bearing targeted disruptions of cytoadhesion or signaling molecules.