Objective     To study the effect of direct cell-cell contact on differentiation of mesenchymal stem cells(MSC) into vascular endothelial cells(VEC) when MSC were co-cultured with human umbilical vein endothelial cells（HUVEC）in vitro.      Methods     MSC and HUVEC were cultured and identified respectively

and then co-cultured. VEGF levels were measured in experimental group and control group by ELISA assays 48 h after culture. Expression of fetal liver kinase-1 (FLK-1) and von Willebrand factor (vWF) in MSC and HUVEC 5 d after co-cultured

as well as phagocytic power of Dil-ac-LDL

were detected and analyzed by immunofluorescence cytology. Changes in ultra-microstructure of MSC were observed under a transmission electron microscope.     Results     The VEGF level was significantly higher in experimental group than in control group. FLK-1 protein and vWF protein were expressed in MSC. Some MSC could phagocytize Dil-ac-LDL. Characteristic differentiation of MSC was observed under transmission electron microscope with a lower karyoplasmic ratio

rich organelle

and cell fusion.     Conclusion     HUVEC can induce differentiation of directly co-culture-induced MSC into endothelial cells through direct cell-cell contact

which may be closely related to VEGF secretion due to direct cell-cell contact and cell fusion.