ABSTRACT
The basic structural unit of a blood vessel is an endothelial cell, which forms the lumen of the vessel and is in contact with the blood, and a pericyte or smooth muscle cell, positioned on the abluminal surface of the endothelium. Here, we will focus on human cells isolated from blood that can form the endothelial lining of vessels in vivo1-4-we call these cells endothelial progenitor cells (EPCs); some groups use the term endothelial colony-forming cell, abbreviated ECFC, to emphasize the clonogenic potential of these cells5-7 while others use the term blood outgrowth endothelial cells (BOECs).8 EPCs are distinct from blood-and bone marrow-derived cells that are referred to in the literature as early EPCs, CFU-EC (colony-forming unit-endothelial cell, or angiogenic monocytes).9,10 Cells designated “early EPCs” have been shown to augment capillary tube formation11 and angiogenesis in vivo12 but do not directly participate in forming vessel structures. Of note, a murine equivalent of EPCs or ECFCs-highly proliferative, clonogenic cells with vessel building activity in vivo-has been dif—cult to obtain. One report describes murine BOECs, but the proliferative potential appeared to be modest.13 The obscurity of the murine counterpart to human EPCs has likely contributed to some of the controversy on the identity of EPCs. The perivascular component of the vessel wall, pericytes and smooth muscle cells, has been studied intensively to determine origins, phenotypic plasticity, and roles in blood vessel formation and stability.14-16 Here, we will focus on human mesenchymal stem/progenitor cells that have been shown to differentiate into perivascular cells and to support formation of functional blood vessels.4,17,18
Endothelial cells are identi—ed by expression of endothelial markers and by functional endothelial properties that can be assayed in vitro and ultimately in vivo. The most reliable cellular markers are CD31, also known as PECAM-1 (plateletendothelial cell adhesion molecule-1), CD144, also known as VE-cadherin (vascular endothelial-cadherin) or cadherin-5, and von Willebrand Factor (vWF), sometimes referred to as Factor VIII-related antigen. However, there are caveats that must be kept in mind when relying on the presence of these markers to assign an endothelial phenotype. CD31 is expressed on nearly all continuous endothelium, but its expression is variable in fenestrated endothelium such as in the liver sinusoidal endothelial cells.19 A more pressing issue is that CD31 is expressed on myeloid lineage cells, in particular monocytes, albeit at about 1/10 the level of CD31 present on the surface of endothelial cells. VE-cadherin is also expressed broadly in the adult vasculature and has excellent utility as an endothelial marker, although VE-cadherin transcriptional activity has been detected in hematopoietic cells20 and in hemogenic endothelial cells.21,22 vWF is a highly speci—c marker when localized within rod-shaped organelles known as Weibel-Palade bodies, which are unique to endothelial cells.23,24 However, vWF is also present in platelets and can be deposited along the endothelium when platelets are activated. Less reliable markers are TIE2, CD34, vascular
endothelial growth factor receptors (VEGFRs) and CD105 (also known as endoglin or SH2). These cell surface membrane proteins are expressed to varying degrees in hematopoietic cells,6,25 such that cultures of adherent cells from blood or bone marrow are often mistakenly identi—ed as EPCs based on expression of these markers.26 In addition, CD105 is expressed on mesenchymal stem cells (MSCs), and CD146, originally identi—ed on human umbilical vein endothelial cells (HUVECs) using the monoclonal antibody P1H12,27 has been shown to be a striking marker of human pericytes in vivo.28 Thus, the detection of one or more of these markers is certainly consistent with an endothelial phenotype but does provide suf—cient information to designate the cells as endothelial.
