Physiological migration of hematopoietic stem and progenitor Celts

Hematopoietic stem cells (HSCs) reside predominantly in bone marrow, but lo w numbers of HSCs are also found in peripheral blood. We examined the fate of blood-borne HSCs using genetically marked parabiotic mice, which are sur gically conjoined and share a common circulation. Parabionts rapidly establ ished stable, functional cross engraftment of partner-derived HSCs and main tained partner-derived hematopoiesis after surgical separation. Determinati on of the residence time of injected blood-borne progenitor cells suggests that circulating HSCs/progenitors: are cleared quickly from the blood, Thes e data demonstrate that HSCs rapidly and constitutively migrate through the blood and play a physiological role in, at least, the functional reengraft ment of unconditioned bone marrow.