Maturation of embryonic stem cells into endothelial cells in an in vitro model of vasculogenesis

A primitive vascular plexus is formed through coordinated regulation of dif ferentiation, proliferation, migration, and cell-cell adhesion of endotheli al cell (EC) progenitors. In this study, a culture system was devised to in vestigate the behavior of purified EC progenitors in vitro. Because Flk-1() cells derived from ES cells did not initially express other EC markers, t hey were sorted and used as EC progenitors. Their in vitro differentiation into ECs, via vascular endothelial-cadherin (VE-cadherin)(+) platelet-endot helial cell adhesion molecule-1 (PECAM-1)(+) CD34(-) to VE-cadherin(+) PECA M-1(+) CD34(+) stage, occurred without exogenous factors, whereas their pro liferation, particularly at low cell density, required OP9 feeder cells. On OP9 feeder layer, EC progenitors gave rise to sheet-like clusters of Flk-1 (+) cells, with VE-cadherin concentrated at the cell-cell junction. The gro wth was suppressed by Flt-1-IgG1 chimeric protein and dependent on vascular endothelial growth factor (VEGF) but not placenta growth factor (PIGF). Fu rther addition of VEGF resulted in cell dispersion, indicating the role of VEGF in the migration of ECs as well as their proliferation. Cell-cell adhe sion of ECs in this culture system was mediated by VE-cadherin. Thus, the c ulture system described here is useful in dissecting the cellular events of EC progenitors that occur during vasculogenesis and in investigating the m olecular mechanisms underlying these processes. (C) 1999 by The American So ciety of Hematology.