Stem cell motility, directed movement, and homing are important new fields in hematopoietic stem cell biology. Homing is the first step in long-term engraftment, the definining characteristic of hematopoietic stem cells.

Experiments were designed to define a variety of the adhesion receptors and ligands that mediate cell-to matrix and cell-to-interactions. (These include selectins, integrins, the Ig family, and "others".)

Stem cells migrate by homing in on their "niches". Niches are microenvironments made up of the external signals that collectively control migration and cell fate. The hematopoietic stem cells homes in on its niche, the target recipient marrow stromal cells, through interactions with: extracellular matrix (ECM) proteins, proteoglycans, and growth factors.

These studies demonstrate that hematopoietic stem cells are not the dormant, sluggish cells one imagines. Rather they are highly active and functional cells. They migrate to multiple sites in the body and are easily mobilized with infection, cytokine, or cytotoxic treatment. Stem cells possess the capacity to travel in the blood stream, rapidly traverse marrow endothelial barriers, enter the marrow cavity, set up residence in marrow niches, and differentiate and proliferate to produce all host lymphohematopoietic cells from a single cell. (69)