Endothelial progenitor cells (EPCs) represent a cell population in the peripheral circulation being capable of differentiation into endothelial cells.Accumulating evidence suggests that EPCs play a critical role in neoangiogenesis and endothelial maintenance and repair, and EPC-based cell therapy is emerging as a promising novel therapeutic approach for various cardiovascular disorders.It is most commonly accepted that EPCs in circulation are originated from bone marrow (BM) ,and that vascular injury triggers the mobilization of EPCs from the BM to peripheral blood (PB).Accordingly, both BM and PB are widely used for the isolation and culture of EPCs.Although both BM-and PB-derived EPCs (BM-EPCs and PB-EPCs, respectively) have been proved to incorporate into the vasculature of experimentally induced wounds or ischemic areas, the therapeutic potential of BM-EPCs remains conflicting.The present study was conducted to compare the phenotypes and in vitro angiogenic capacity of EPCs derived from BM and PB from an avian species, fast-growing meat-type chicken (broiler).Mononuclear cells were isolated from broiler BM and PB (BMMNCs and PBMNCs) and cultured in EGM-2 medium.Cells at days 7 to 14 were used for experiments.The expression of progenitor and endothelial markers, the number of DilacLDL/lectin dual-positive cells, and adipogenic and osteogenic differentiation were determined.The migration and in vitro angiogenic ability between BMMNC-and PBMNC-derived cells were compared.PBMNCs developed typical EPC appearances, with initial spindle shape followed by a cobblestone appearance, whereas BMMNC-derived cells retained their constitutive spindle-like morphology throughout the experiment.Cells derived from both sources expressed CD133, CD31 and VEGFR-2, while PBMNCs-derived cells had decreased CD133 expression.Nevertheless, the number of DllacLDL/lectin dual-positive cells was not different between groups.Adipogenic and osteogenic lineages were verified in BMMNC-but not in PBMNC-derived cells.PBMNC-derived cells formed tubular networks on Matrigel.However, BMMNC-derived cells formed few tube-like structures which were not morphologically comparable to those developed by their counterparts.Our results suggest that the so called "EPCs" derived from BMMNCs are not "true" EPCs, supporting previous findings on mammals that BM may not serve as an optimal isolation source of EPCs.