A role for bone marrow-derived cells in the vasculature of noninjured CNS.

The contribution of hematopoietic cells to the formation of blood vessels is currently the focus of intense scrutiny. Bone marrow-derived endothelial progenitor cells are thought to generate endothelial cells in many tissues, including myocardium, muscle, and certain tumors. In the central nervous system (CNS), however, the possible role of bone marrow-derived angiocompetent cells remains unclear. Here we have investigated the long-term involvement of bone marrow-derived cells in the maintenance of endothelial structures in the brain, spinal cord, and retina. Using hematopoietic chimeras stably expressing green fluorescent protein (GFP) in bone marrow-derived tissues, we found large numbers of hematopoietic cells closely associated with vessels in the CNS. None of these cells, however, showed an endothelial phenotype. They were positive for monocytic and microglial surface markers and demonstrated active phagocytosis of neighboring endothelial elements. Bone marrow-derived, vasculature-associated cells in the noninjured adult CNS are distinct from endothelial cells, but play an active role in vascular structures.