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VASCULAR BIOLOGY
1Department of Pathology, University of Washington; 2Division of Pathology and Laboratory Medicine, Veterans Administration Puget Sound Health Care System, Seattle, WA; and 3Laboratory of Neurobiology and Neuroregenerative Therapy, "Carlo Besta" Institute, Milan, Italy
Submitted 21 April 2005 ; accepted in final form 28 July 2005
Pericytes play an important role in modulating angiogenesis, but the origin of these cells is poorly understood. To evaluate whether the mature vessel wall contains pericyte progenitor cells, nonendothelial mesenchymal cells isolated from the rat aorta were cultured in a serum-free medium optimized for stem cells. This method led to the isolation of anchorage-independent cells that proliferated slowly in suspension, forming spheroidal colonies. This process required basic fibroblast growth factor (bFGF) in the culture medium, because bFGF withdrawal caused the cells to attach to the culture dish and irreversibly lose their capacity to grow in suspension. Immunocytochemistry and RT-PCR analysis revealed the expression of the precursor cell markers CD34 and Tie-2 and the absence of endothelial cell markers (CD31 and endothelial nitric oxide synthase, eNOS) and smooth muscle cell markers (
-smooth muscle actin,
-SMA). In addition, spheroid-forming cells were positive for NG2, nestin, PDGF receptor (PDGFR)-
, and PDGFR-
. Upon exposure to serum, these cells lost CD34 expression, acquired
-SMA, and attached to the culture dish. Returning these cells to serum-free medium failed to restore their original spheroid phenotype, suggesting terminal differentiation. When embedded in collagen gels, spheroid-forming cells rapidly migrated in response to PDGF-BB and became dendritic. Spheroid-forming cells cocultured in collagen with angiogenic outgrowths of rat aorta or isolated endothelial cells transformed into pericytes. These results demonstrate that the rat aorta contains primitive mesenchymal cells capable of pericyte differentiation. These immature cells may represent an important source of pericytes during angiogenesis in physiological and pathological processes. They may also provide a convenient supply of mural cells for vascular bioengineering applications.
angiogenesis; stem cells; smooth muscle; mural cells; collagen
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