Appendix M. 
375 
transcription factor, since GATA-2 knockout embryonic stem cells 
have a complete block in definitive hematopoiesis and seeding of the 
fetal liver and bone marrow®^ Moreover, the earliest precursor of 
both hematopoietic and endothelial cell lineage to have diverged 
from embryonic ventral endothelium has been shown to express 
VEGF receptors as well as GATA-2 and alpha4-integrins 
Subsequent to capillary tube formation, the newly-created 
vasculogenic vessels undergo sprouting, tapering, remodelling, and 
regression under the direction of VEGF, angiopoietins, and other 
factors, a process termed angiogenesis. The final component 
required for definitive vascular network formation to sustain 
embryonic organogenesis is influx of mesenchymal lineage cells to 
form the vascular supporting structures such as smooth muscle cells 
and pericytes. 
Characterization Of Endothelial Progenitors, Or Angioblasts, In 
Human Adult Bone Marrow. In studies using various animal models 
of peripheral ischemia a number of groups have shown the potential 
of adult bone marrow-derived elements to induce neovascularization 
of ischemic tissues In the most successful of these bone 
marrow-derived cells injected directly into the thighs of rats who had 
undergone ligation of the left femoral artery and vein induced 
neovascularization and augmented blood flow in the ischemic limb 
as documented by laser doppler and immunohistochemical analyses. 
Although the nature of the bone marrow-derived endothelial 
progenitors was not precisely identified in these studies, the 
cumulative reports indicated that this site may be an important 
source of endothelial progenitors which could be useful for 
augmenting collateral vessel growth in ischemic tissues, a process 
termed therapeutic angiogenesis. 
In more recent studies, our group has identified such 
endothelial progenitors in human adult bone marrow By 
employing both in vitro and in vivo experimental models we have 
sought to precisely identify the surface characteristics and biological 
properties of these bone marrow-derived endothelial progenitor cells. 
Following G-CSF treatment, mobilized mononuclear cells were 
harvested and CD34+ cells were separated using anti-CD34 mAb 
coupled to magnetic beads. 90-95% of CD34+ cells co-expressed the 
hematopoietic lineage marker CD45, 60-80% co-expressed the stem 
cell factor receptor GDI 17, and <1% co-expressed the 
monocyte/macrophage lineage marker CD14. By quadruple 
parameter analysis, the VEGFR-2 positive cells within the 
CD34+CD117^^^^^ subset displayed phenotypic characteristics of 
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