title: Circulating hematopoietic stem cells do not efficiently home to bone marrow during homeostasis.
abstract: OBJECTIVE
  Hematopoietic stem cells (HSC), normally resident in bone marrow, can be detected in the murine and human circulation. It is thought that HSC move in and out of bone marrow daily and that returning HSC are generally equivalent to their bone marrow counterparts in phenotype and function. However, large numbers of mononuclear blood cells are required to rescue animals from lethal irradiation, indicating either that the prevalence of circulating HSC is low, or they are inherently deficient in their repopulating ability. Accordingly, recent data suggest that circulating HSC may be unable to stably engraft WBM under homeostatic conditions. The purpose of this study was to explore these dynamics in detail using parabiosis and bone marrow transplantation.
  
  
  METHODS
  The WBM and skeletal muscle HSC stem cell compartments of parabiosed CD45 congenic mice were analyzed functionally (via bone marrow transplantation) and phenotypically (via flow cytometry) for circulating stem cells at specific time points postparabiosis and after surgical separation.
  
  
  RESULTS
  Surprisingly, we find that stem cells trafficking out of bone marrow and into the circulation do not stably return to bone marrow, although long-lived lymphoid precursors do stably re-engraft. Circulating HSC do, however, take up residence in skeletal muscle, wherein they account for HSC activity.
  
  
  CONCLUSIONS
  Circulating HSC are not in flux with the bone marrow HSC and can persist in peripheral tissues.
authors.affiliation: Center for Cell and Gene Therapy, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: Shannon
authors.initials: S
authors.lastname: McKinney-Freeman
authors.forename: Margaret A
authors.initials: MA
authors.lastname: Goodell
mesh_terms: D000818:Animals; D001854:Bone Marrow Cells / Q000502:physiology*; D016026:Bone Marrow Transplantation; D002465:Cell Movement; D005260:Female; D006412:Hematopoietic Stem Cells / Q000502:physiology*; D006706:Homeostasis; D051379:Mice; D008810:Mice, Inbred C57BL; D018482:Muscle, Skeletal / Q000166:cytology; D010227:Parabiosis