Objective To investigate the effect of brain microvascular endothelial cells (BMECs) on mesenchymal stem cells (MSCs) expressing fetal liver kinase-1(Flk-1) and von Willebrand factor (vWF) under normal and hypoxic conditions.   Methods Human bone marrow MSCs and rat BMECs were isolated

cultured and identified. We used direct and indirect co-culture to induce the differentiation of MSCs. The expression of Flk-1 and vWF after induction was observed and analyzed by flow cytometry and immunofluorescence staining.  Results Undifferentiated MSCs did not express Flk-1 or vWF. In indirect co-culture group

indirect co-culture under hypoxic conditions induced (7.58±0.58)% (n=6

P=0.034) MSCs to express Flk-1 but not vWF

while in normoxic indirect co-culture

MSCs expressed neither. In direct co-culture group

after 5-day direct co-culture with BMECs under normoxia or hypoxia

MSCs expressing Flk-1 accounted for (13.76±1.67)% (n=6

P0.01) and (23.64±2.50)% (n=6

P0.01) of the total co-cultured mixed cells

and the vWF staining was still negative in the normoxic co-culture system

while some Flk-1 positive cells began to co-express vWF simultaneously in the hypoxic co-culture system.  Conclusion BMECs induce MSCs to differentiate into endothelial cells through direct cell contact. By direct co-culture in hypoxic conditions

BMECs induce more MSCs to differentiate into endothelial cells.