Bone-marrow-derived endothelial progenitor cells (BM-EPCs) contribute to postnatal neovascularization and therefore are of great interest for cell therapies to treat ischemic diseases. However, their origin and characteristics are still in controversy. In this paper, we identified the origin/lineage of the BM-EPCs that were isolated from bone marrow mononuclear cells and differentiated with the induction of bone-marrow endothelial-cellconditioned medium (ECCM). BM-EPCs were characterized in terms of phenotype, lineage potential, and their functional properties. Endothelial cell colonies derived from BM-EPC were cultured with ECCM for 3 months. Cultured EPC colony cells expressed endothelial cell markers and formed the capillary-like network in vitro. EPC colony cells expressed differential proliferative capacity; some of the colonies exhibited a high proliferative potential (HPP) capacity up to 20 population doublings. More importantly, these HPP-EPCs expressed hematopoietic marker CD45, exhibited endocytic activities, and preserved some of the myeloid cell activity. In addition, the HPP-EPCs secrete various growth factors including VEGF and GM-CSF into the culture medium. The results demonstrate that these EPCs were primarily derived from hematopoietic origin of early precursor cells and maintained high proliferative potential capacity, a feature with a significant potential in the application of cell therapy in ischemic diseases. 1. Introduction Bone marrow mononuclear cells (BMMNCs) contain endothelial progenitor cells (EPCs) valuable in cell therapy to enhance postischemic neovascularization. It has been shown that BMMNCs, which can be easily prepared from bone marrow extraction, have dramatic effects in the formation of neovascularization in ischemic diseases or tumor model [1–4]. In spite of extensive studies on EPC [3, 5–7], it is still quite challenging to induce all the potential EPCs from BMMNC or bone marrow into functional EPC due to the following reasons: (1) hierarchy of hematopoietic progenitors in bone marrow; (2) EPCs exist as a distinct cell type residing in the bone marrow stroma; (3) imperfect induction conditions in vitro. Extensive literature clearly documented the presence of EPCs in the bone marrow; it is important to elucidate the origin of the BM-EPC and continuously to improve strategies for isolating and characterizing the valuable EPC from BMMNC, thereby ultimately utilizing these EPCs for therapeutic purposes in ischemic and tumor diseases. We have established and characterized a BMEC line in our lab [8]. BMEC-CM derived
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