Physiological migration of hematopoietic stem and progenitor cells.

Hematopoietic stem cells (HSCs) reside predominantly in bone marrow, but low numbers of HSCs are also found in peripheral blood. We examined the fate of blood-borne HSCs using genetically marked parabiotic mice, which are surgically conjoined and share a common circulation. Parabionts rapidly established stable, functional cross engraftment of partner-derived HSCs and maintained partner-derived hematopoiesis after surgical separation. Determination of the residence time of injected blood-borne progenitor cells suggests that circulating HSCs/progenitors are cleared quickly from the blood. These data demonstrate that HSCs rapidly and constitutively migrate through the blood and play a physiological role in, at least, the functional reengraftment of unconditioned bone marrow.