Mobilization of hematopoietic stem cells from the bone marrow niche to the blood compartment

The vast majority of hematopoietic stem cells (HSCs) reside in specialized niches within the bone marrow during steady state, maintaining lifelong blood cell production. A small number of HSCs normally traffic throughout the body; however, exogenous stimuli can enhance their release from the niche and entry into the peripheral circulation. This process, termed mobilization, has become the primary means to acquire a stem cell graft for hematopoietic transplant at most transplant centers. Currently, the preferred method of HSC mobilization for subsequent transplantation is treatment of the donor with granulocyte colony-stimulating factor. The mobilizing effect of granulocyte colony-stimulating factor is not completely understood, but recent studies suggest that its capacity to mobilize HSCs, at least in part, is a consequence of alterations to the hematopoietic niche. The present article reviews some of the key mechanisms mediating HSC mobilization, highlighting recent advances and controversies in the field.