To assess in vitro angiogenesis, cellular co-culture assays have been utilized to study adherence, spreading, differentiation and proliferation, and migration of endothelial cells. Formation of tubule or capillary-like networks is influenced by the presence of vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) but other factors provided by cell sources and/or direct contact with multiple cell types may facilitate this formation. The hypothesis of this study is that umbilical cord blood (UCB)-derived endothelial precursor cells (EPCs) may influence the formation of human umbilical vein endothelial cell (HUVEC) tubule structures during angiogenesis.

Methods: UCB-derived EPCs were isolated from CD133negative cells after a 7-day culture on human fibronectin in EGM-2 media. Tubule formation was evaluated (passage 1–2, 20 x 103 or 2 x 103 cells) by adding HUVECs without or with EPCs to cultures of human bone marrow-derived mesenchymal stromal cells (MSCs) under normoxic (20%) conditions (37°C, 5% CO2, containing VEGF, epidermal growth factor, FGF, insulin-like growth factor, heparin, hydrocortisone, and ascorbic acid in EGM-2 medium) for a 2-week period. HUVECs were added to cultures without or with labeling with Vybrant® CM-DiI which allows the temporal observation of tubule formation progress and cellular incorporation. Final tubule formation was confirmed using a primary anti-CD31 (PECAM) antibody followed by a FITC-conjugated secondary antibody for signal amplification.

Results: After 2–4 days, linear aggregates of labeled HUVECs (2-D arrangement) were observed. After 14 days, there was remodeling of HUVECs into the development of a 3D network of linear and branched tubule structures. EPCs facilitated the formation of tubules affecting both the extent of tubule formation and also enhanced proliferation of HUVEC cells. A minority (< 5%) of EPCs were incorporated into developing tubules (estimated using CM-Dil-labeled EPCs). To quantify tubule formation, digital pictures of representative areas of culture wells (2–4/well) were acquired. Using Image Pro Plus software, tubules were quantified using multi-parameter analysis with respect to length, area, and perimeter. The presence of EPCs (equal to the number of added HUVECs) significantly enhanced all parameters. In comparison to control samples, the presence of EPCs increased the area, perimeter and size by 15.2-fold, 3.4-fold, and 3.2-fold, respectively. Confocal microscopy revealed that the co-cultures formed anatamoses, indicating the formation of a connected network.

Conclusions: Taken together, these results suggest that the presence of cord blood-derived EPCs facilitate tubule formation and development via a heterotypic cell-cell interaction without integrating into the angiogenic structures. Further studies will evaluate the secretion of cytokines and growth factors.

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